// Copyright (c) 2016-2021 Fabian Schuiki

//! Expressions

#![deny(missing_docs)]

use std::collections::{HashMap, HashSet};
use std::marker::PhantomData;

use num::{BigInt, BigRational};

use crate::common::errors::*;
use crate::common::name::Name;
use crate::common::score::Result;
use crate::common::source::*;
use crate::common::util::*;
use crate::common::SessionContext;

use crate::add_ctx::AddContext;
use crate::arenas::Alloc;
use crate::hir;
use crate::konst::ConstInt;
use crate::op::*;
use crate::scope2::{Def2, ScopeData, TypeVariantDef};
use crate::score::*;
use crate::syntax::ast::{self, Dir};
use crate::syntax::lexer::token::{Exponent, ExponentSign, Literal};
use crate::ty::*;

/// A term.
///
/// This is a generalization of expressions and names in VHDL. It allows various
/// parts of the AST to be mapped to a common data structure, and then remapped
/// to the desired output structure. This is useful for example if the parser
/// processes a subtype indication as an expression.
#[derive(Debug, PartialEq, Eq)]
pub enum Term<'t> {
    /// A term of the form `null`.
    Null,
    /// A term of the form `open`.
    Open,
    /// A term of the form `others`.
    Others,
    /// A term of the form `default`.
    Default,
    /// An integer literal.
    IntLit(BigInt),
    /// A floating point literal.
    FloatLit(BigRational),
    /// A physical literal.
    PhysLit(BigInt, Spanned<EitherUnit<'t>>),
    /// A bit string literal.
    StrLit(Name),
    /// An unresolved name.
    Unresolved(ResolvableName),
    /// A term that refers to a definition.
    Ident(Spanned<Def>),
    /// A term that refers to a definition.
    Ident2(Spanned<Def2<'t>>),
    /// A term that refers to a type or subtype definition.
    TypeMark(Spanned<TypeMarkRef>),
    /// A term that refers to an enum variant.
    Enum(Vec<Spanned<EnumRef>>),
    /// A term that refers to an enum variant.
    Enum2(Vec<Spanned<Def2<'t>>>),
    /// A term of the form `T.<name>`.
    Select(Subterm<'t>, Spanned<ResolvableName>),
    /// A term of the form `T.all`.
    SelectAll(Subterm<'t>),
    /// A term of the form `T (to|downto) T`.
    Range(Spanned<Dir>, Subterm<'t>, Subterm<'t>),
    /// A term of the form `T range T`.
    RangeSuffix(Subterm<'t>, Subterm<'t>),
    /// A term of the form `T range <>`.
    UnboundedRange(Subterm<'t>),
    /// A term of the form `[T] <type_mark> [T]`. The first optional subterm is
    /// the resolution indication, the second is the constraint.
    SubtypeInd(
        Spanned<TypeMarkRef>,
        Option<Subterm<'t>>,
        Option<Subterm<'t>>,
    ),
    /// A term of the form `(T) T`.
    PrefixParen(Subterm<'t>, Subterm<'t>),
    /// A term of the form `T (T)`.
    SuffixParen(Subterm<'t>, Subterm<'t>),
    /// A term of the form `(T,T,…)`.
    Paren(Vec<Spanned<Term<'t>>>),
    /// A term of the form `(T|T|… => T, T|T|… => T, …)`.
    Aggregate(Vec<(Vec<Spanned<Term<'t>>>, Spanned<Term<'t>>)>),
    /// A term of the form `op T`.
    Unary(Spanned<UnaryOp>, Subterm<'t>),
    /// A term of the form `T op T`.
    Binary(Spanned<BinaryOp>, Subterm<'t>, Subterm<'t>),
    /// A term of the form `T'T`.
    Qual(Subterm<'t>, Subterm<'t>),
    /// A term of the form `new T`.
    New(Subterm<'t>),
}

#[allow(missing_docs)]
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum EitherUnit<'t> {
    Old(UnitRef),
    New(TypeVariantDef<'t>),
}

#[allow(missing_docs)]
impl<'t> EitherUnit<'t> {
    pub fn unwrap_old(self) -> UnitRef {
        match self {
            EitherUnit::Old(u) => u,
            _ => panic!("unit is not an old unit"),
        }
    }

    pub fn unwrap_new(self) -> TypeVariantDef<'t> {
        match self {
            EitherUnit::New(u) => u,
            _ => panic!("unit is not a new unit"),
        }
    }
}

/// A subterm.
pub type Subterm<'t> = Box<Spanned<Term<'t>>>;

/// A context within which termification can occur.
pub struct TermContext<C, S, D> {
    /// The underlying scoreboard context.
    pub ctx: C,
    /// The scope within which the terms will resolve their names.
    pub scope: S,
    marker: PhantomData<D>,
}

impl<C: DiagEmitter, S, D> DiagEmitter for TermContext<C, S, D> {
    fn emit(&self, diag: DiagBuilder2) {
        self.ctx.emit(diag)
    }
}

impl<C, S, D> TermContext<C, S, D> {
    /// Perform term folding.
    ///
    /// This is a post-processing step that should be applied to all terms once
    /// they are constructed. Folding applies transformations to the terms, e.g.
    /// changing `Ident(Type|Subtype)` to `TypeMark`, or gobbling up subtype
    /// constraints where appropriate.
    pub fn fold<'t>(&self, term: Spanned<Term<'t>>) -> Spanned<Term<'t>> {
        let new = match term.value {
            Term::Ident(Spanned {
                value: Def::Type(id),
                ..
            }) => Term::TypeMark(Spanned::new(id.into(), term.span)),
            Term::Ident(Spanned {
                value: Def::Subtype(id),
                ..
            }) => Term::TypeMark(Spanned::new(id.into(), term.span)),
            other => other,
        };
        Spanned::new(new, term.span)
    }
}

impl<'a, C, S, D> TermContext<C, S, D>
where
    Self: DefSpecificTermContext<'a, D>,
    Self: ScopeSpecificTermContext<'a, S, D>,
    C: DiagEmitter + Copy,
    S: Copy,
    D: Copy,
{
    /// Map an AST literal to a term.
    pub fn termify_literal(&self, ast: Spanned<&Literal>) -> Result<Spanned<Term<'a>>> {
        Ok(Spanned::new(
            match *ast.value {
                Literal::Abstract(base, int, frac, exp) => {
                    let base = match base {
                        Some(base) => match base.as_str().parse() {
                            Ok(base) => base,
                            Err(_) => {
                                self.emit(
                                    DiagBuilder2::error(format!(
                                        "`{}` is not a valid base for a number literal",
                                        base
                                    ))
                                    .span(ast.span),
                                );
                                return Err(());
                            }
                        },
                        None => 10,
                    };
                    let int = match BigInt::parse_bytes(int.as_str().as_bytes(), base) {
                        Some(v) => v,
                        None => {
                            self.emit(
                                DiagBuilder2::error(format!(
                                    "`{}` is not a valid base-{} integer",
                                    int, base
                                ))
                                .span(ast.span),
                            );
                            return Err(());
                        }
                    };
                    let exp: isize = match exp {
                        Some(Exponent(sign, exp)) => match exp.as_str().parse() {
                            Ok(v) => {
                                if sign == ExponentSign::Positive {
                                    v
                                } else {
                                    -v
                                }
                            }
                            Err(_) => {
                                self.emit(
                                    DiagBuilder2::error(format!(
                                        "`{}` is not a valid exponent for a number literal",
                                        exp
                                    ))
                                    .span(ast.span),
                                );
                                return Err(());
                            }
                        },
                        None => 0,
                    };

                    // Parse the rest of the number.
                    if let Some(frac) = frac {
                        use num::pow;
                        let shift = frac.as_str().len();
                        let frac = match BigInt::parse_bytes(frac.as_str().as_bytes(), base) {
                            Some(v) => v,
                            None => {
                                self.emit(
                                    DiagBuilder2::error(format!(
                                        "`{}` is not a valid base-{} integer",
                                        frac, base
                                    ))
                                    .span(ast.span),
                                );
                                return Err(());
                            }
                        };
                        let num = int * pow(BigInt::from(base), shift) + frac;
                        let adj = exp - shift as isize;
                        let q = if adj < 0 {
                            BigRational::new(num, pow(BigInt::from(base), (-adj) as usize))
                        } else if adj > 0 {
                            BigRational::from(num * pow(BigInt::from(base), adj as usize))
                        } else {
                            BigRational::from(num)
                        };
                        Term::FloatLit(q)
                    } else {
                        if exp < 0 {
                            self.emit(
                                DiagBuilder2::error(format!(
                                    "integer literal `{}` has negative exponent",
                                    ast.span.extract()
                                ))
                                .span(ast.span),
                            );
                            return Err(());
                        }
                        if exp > 0 {
                            use num::pow;
                            Term::IntLit(int * pow(BigInt::from(base), exp as usize))
                        } else {
                            Term::IntLit(int)
                        }
                    }
                }
                ref wrong => {
                    self.emit(
                        DiagBuilder2::bug(format!(
                            "termification of literal `{}` not implemented",
                            ast.span.extract()
                        ))
                        .span(ast.span)
                        .add_note(format!("{:?}", wrong)),
                    );
                    return Err(());
                }
            },
            ast.span,
        ))
    }

    /// Make sure the term is not an unresolved name.
    ///
    /// This function emits a diagnostic if the term is `Term::Unresolved(..)`.
    /// It is non-recursive.
    pub fn ensure_resolved<'t>(&self, term: Spanned<Term<'t>>) -> Result<Spanned<Term<'t>>> {
        match term.value {
            Term::Unresolved(name) => {
                self.emit(DiagBuilder2::error(format!("`{}` is unknown", name)).span(term.span));
                Err(())
            }
            _ => Ok(term),
        }
    }

    /// Map an AST compound name to a term.
    pub fn termify_compound_name(&self, ast: &ast::CompoundName) -> Result<Spanned<Term<'a>>> {
        // Map the primary name.
        let mut term = self.fold(match ast.primary.kind {
            ast::PrimaryNameKind::String(s) => Spanned::new(Term::StrLit(s), ast.primary.span),
            _ => self.termify_name(ResolvableName::from_primary_name(&ast.primary, self.ctx)?)?,
        });

        // For each name part, wrap the term in another layer. Like an onion.
        for part in &ast.parts {
            term = self.fold(match *part {
                ast::NamePart::Select(ref primary) => {
                    let n = ResolvableName::from_primary_name(primary, self.ctx)?;
                    let sp = Span::union(term.span, n.span);
                    let selectable_scope = self.maybe_selectable_scope(&term.value);
                    match selectable_scope {
                        Some(id) => {
                            let t = self.ensure_resolved(self.termify_name_in_scope(n, id)?)?;
                            Spanned::new(t.value, sp)
                        }
                        None => Spanned::new(Term::Select(Box::new(term), n), sp),
                    }
                }
                ast::NamePart::SelectAll(span) => {
                    let sp = Span::union(term.span, span);
                    Spanned::new(Term::SelectAll(Box::new(term)), sp)
                }
                ast::NamePart::Signature(ref sig) => {
                    self.emit(
                        DiagBuilder2::bug(format!(
                            "termification of signature suffix `{}` not implemented",
                            sig.span.extract()
                        ))
                        .span(sig.span),
                    );
                    return Err(());
                }
                ast::NamePart::Attribute(ident) => {
                    let attr = self.termify_name(Spanned::new(ident.name.into(), ident.span))?;
                    match attr.value {
                        // TODO: Enable this as soon as we handle attribute
                        // declarations.
                        // Term::Ident(Spanned { value: Def::Attr(id), span }) => {
                        //  let sp = Span::union(term.span, attr.span);
                        //  Spanned::new(Term::Attribute(Box::new(term), Spanned::new(id, span)), sp)
                        // }
                        Term::Ident(other) => {
                            self.emit(
                                DiagBuilder2::error(format!(
                                    "`{}` is not an attribute name",
                                    ident.name
                                ))
                                .span(ident.span)
                                .add_note("Declared here:")
                                .span(other.span),
                            );
                            return Err(());
                        }
                        _ => unreachable!(),
                    }
                }
                ast::NamePart::Call(ref paren_elems) => {
                    let subterm = self.termify_paren_elems(paren_elems)?;
                    let sp = Span::union(term.span, subterm.span);
                    Spanned::new(Term::SuffixParen(Box::new(term), Box::new(subterm)), sp)
                }
                ast::NamePart::Range(ref expr) => {
                    if expr.data == ast::BoxExpr {
                        let sp = Span::union(term.span, expr.span);
                        Spanned::new(Term::UnboundedRange(Box::new(term)), sp)
                    } else {
                        let expr = self.termify_expr(expr)?;
                        let sp = Span::union(term.span, expr.span);
                        Spanned::new(Term::RangeSuffix(Box::new(term), Box::new(expr)), sp)
                    }
                }
            });
        }

        Ok(term)
    }

    /// Map a resolvable name to a term.
    ///
    /// This function is the bottom of the pit. Names are resolved here and
    /// mapped to the corresponding term. Calling functions may then proceed to
    /// handle the term as they see fit, usually inspecting what exact kind the
    /// term is of.
    pub fn termify_name(&self, name: Spanned<ResolvableName>) -> Result<Spanned<Term<'a>>> {
        self.termify_name_in_scope(name, self.scope)
    }

    /// Map multiple parenthesis elements to a term.
    pub fn termify_paren_elems(&self, elems: &ast::ParenElems) -> Result<Spanned<Term<'a>>> {
        let is_aggregate = elems.value.iter().any(|e| !e.choices.value.is_empty());
        let term = if is_aggregate {
            Term::Aggregate(
                elems
                    .value
                    .iter()
                    .map(|e| {
                        Ok((
                            e.choices
                                .value
                                .iter()
                                .map(|c| self.termify_expr(c))
                                .collect::<Result<Vec<_>>>()?,
                            self.termify_expr(&e.expr)?,
                        ))
                    })
                    .collect::<Result<Vec<_>>>()?,
            )
        } else {
            Term::Paren(
                elems
                    .value
                    .iter()
                    .map(|e| self.termify_expr(&e.expr))
                    .collect::<Result<Vec<_>>>()?,
            )
        };
        Ok(self.fold(Spanned::new(term, elems.span)))
    }

    /// Map an AST subtype indication to a term.
    pub fn termify_subtype_ind(&self, subty: &ast::SubtypeInd) -> Result<Spanned<Term<'a>>> {
        let name = self.termify_compound_name(&subty.name)?;
        let res = match subty.res {
            Some(ast::ResolInd::Exprs(ref paren_elems)) => {
                Some(self.termify_paren_elems(paren_elems)?)
            }
            Some(ast::ResolInd::Name(ref name)) => Some(self.termify_compound_name(name)?),
            None => None,
        };
        if let Some(res) = res {
            let sp = Span::union(name.span, res.span);
            Ok(Spanned::new(
                Term::PrefixParen(Box::new(res), Box::new(name)),
                sp,
            ))
        } else {
            Ok(name)
        }
    }

    /// Map an AST expression to a term.
    pub fn termify_expr(&self, ast: &ast::Expr) -> Result<Spanned<Term<'a>>> {
        let term = match ast.data {
            // Literals with optional unit.
            ast::LitExpr(ref lit, unit) => {
                let lit = self.termify_literal(Spanned::new(lit, ast.span))?;
                let unit = match unit {
                    Some(unit_name) => Some(self.termify_name(unit_name.map_into())?),
                    None => None,
                };
                if let Some(unit) = unit {
                    let unit = self.ensure_resolved(unit)?;
                    let unit = match unit.value {
                        Term::Ident(Spanned {
                            value: Def::Unit(u),
                            span,
                        }) => Spanned::new(EitherUnit::Old(u), span),
                        Term::Ident2(Spanned {
                            value: Def2::Unit(u),
                            span,
                        }) => Spanned::new(EitherUnit::New(u), span),
                        _ => {
                            self.emit(
                                DiagBuilder2::error(format!(
                                    "`{}` is not a valid physical unit",
                                    unit.span.extract()
                                ))
                                .span(unit.span),
                            );
                            debugln!("It is a {:#?}", unit.value);
                            return Err(());
                        }
                    };
                    let lit = match lit.value {
                        Term::IntLit(v) => v,
                        _ => {
                            self.emit(
                                DiagBuilder2::error(format!(
                                    "`{}` is not a valid value for a physical literal",
                                    lit.span.extract()
                                ))
                                .span(lit.span),
                            );
                            debugln!("It is a {:#?}", lit.value);
                            return Err(());
                        }
                    };
                    Term::PhysLit(lit, unit)
                } else {
                    lit.value
                }
            }
            ast::NameExpr(ref name) => return self.termify_compound_name(name),
            // ast::ResolExpr(ref paren_elems, ref name) => {
            //  let name = self.termify_compound_name(name)?;
            // }
            ast::UnaryExpr(op, ref arg) => {
                Term::Unary(UnaryOp::from(op, self.ctx)?, self.termify_expr(arg)?.into())
            }
            ast::BinaryExpr(op, ref lhs, ref rhs) => {
                if let ast::BinaryOp::Dir(d) = op.value {
                    Term::Range(
                        Spanned::new(d, op.span),
                        self.termify_expr(lhs)?.into(),
                        self.termify_expr(rhs)?.into(),
                    )
                } else {
                    Term::Binary(
                        BinaryOp::from(op, self.ctx)?,
                        self.termify_expr(lhs)?.into(),
                        self.termify_expr(rhs)?.into(),
                    )
                }
            }
            ast::NullExpr => Term::Null,
            ast::OpenExpr => Term::Open,
            ast::OthersExpr => Term::Others,
            ast::DefaultExpr => Term::Default,
            ast::ParenExpr(ref elems) => self.termify_paren_elems(elems)?.value,
            ast::QualExpr(ref name, ref arg) => Term::Qual(
                self.termify_compound_name(name)?.into(),
                self.termify_paren_elems(arg)?.into(),
            ),
            ast::NewExpr(ref expr) => Term::New(self.termify_expr(expr)?.into()),
            ref wrong => {
                self.emit(
                    DiagBuilder2::bug(format!(
                        "termification of expression `{}` not implemented",
                        ast.span.extract()
                    ))
                    .span(ast.span)
                    .add_note(format!("{:?}", wrong)),
                );
                return Err(());
            }
        };
        Ok(self.fold(Spanned::new(term, ast.span)))
    }
}

#[allow(missing_docs)]
pub trait DefSpecificTermContext<'t, D> {
    /// Termify the result of a name resolution.
    ///
    /// This ensures that there is only one definition, or in case of something
    /// overloadable, that all definitions are of the same kind.
    fn termify_defs(
        &self,
        name: Spanned<ResolvableName>,
        defs: Vec<Spanned<D>>,
    ) -> Result<Spanned<Term<'t>>>;
}

impl<'t, C: DiagEmitter, S> DefSpecificTermContext<'t, Def> for TermContext<C, S, Def> {
    fn termify_defs(
        &self,
        name: Spanned<ResolvableName>,
        mut defs: Vec<Spanned<Def>>,
    ) -> Result<Spanned<Term<'t>>> {
        if defs.is_empty() {
            return Ok(name.map(Term::Unresolved));
        }

        fn is_enum(def: &Spanned<Def>) -> bool {
            match def.value {
                Def::Enum(..) => true,
                _ => false,
            }
        }
        let all_enum = defs.iter().all(is_enum);

        // Handle overloading. Basically if the definitions are all enum fields
        // or functions, that's fine. For everything else the name must be
        // unique.
        let first_def = defs.pop().unwrap();
        let term = match first_def.value {
            Def::Enum(id) if all_enum => {
                let mut ids = vec![Spanned::new(id, first_def.span)];
                for def in defs {
                    match def.value {
                        Def::Enum(id) => ids.push(Spanned::new(id, def.span)),
                        _ => unreachable!(),
                    }
                }
                Term::Enum(ids)
            }
            // TODO: Handle the function case.
            _ if !defs.is_empty() => {
                let mut d =
                    DiagBuilder2::error(format!("`{}` is ambiguous", name.value)).span(name.span);
                d = d.add_note("Found the following definitions:");
                if first_def.span() != INVALID_SPAN {
                    d = d.span(first_def.span());
                }
                for def in defs {
                    if def.span() != INVALID_SPAN {
                        d = d.span(def.span());
                    }
                }
                self.emit(d);
                return Err(());
            }
            _ => Term::Ident(first_def),
        };

        Ok(self.fold(Spanned::new(term, name.span)))
    }
}

impl<'t, C: DiagEmitter, S> DefSpecificTermContext<'t, Def2<'t>> for TermContext<C, S, Def2<'t>> {
    fn termify_defs(
        &self,
        name: Spanned<ResolvableName>,
        mut defs: Vec<Spanned<Def2<'t>>>,
    ) -> Result<Spanned<Term<'t>>> {
        if defs.is_empty() {
            return Ok(name.map(Term::Unresolved));
        }

        fn is_enum(def: &Spanned<Def2>) -> bool {
            match def.value {
                Def2::Enum(..) => true,
                _ => false,
            }
        }
        let all_enum = defs.iter().all(is_enum);

        // Handle overloading. Basically if the definitions are all enum fields
        // or functions, that's fine. For everything else the name must be
        // unique.
        let first_def = defs.pop().unwrap();
        let term = match first_def.value {
            Def2::Enum(..) if all_enum => {
                let mut ids = vec![first_def];
                for def in defs {
                    match def.value {
                        Def2::Enum(..) => ids.push(def),
                        _ => unreachable!(),
                    }
                }
                Term::Enum2(ids)
            }
            // TODO: Handle the function case.
            _ if !defs.is_empty() => {
                let mut d =
                    DiagBuilder2::error(format!("`{}` is ambiguous", name.value)).span(name.span);
                d = d.add_note("Found the following definitions:");
                if first_def.span() != INVALID_SPAN {
                    d = d.span(first_def.span());
                }
                for def in defs {
                    if def.span() != INVALID_SPAN {
                        d = d.span(def.span());
                    }
                }
                self.emit(d);
                return Err(());
            }
            _ => Term::Ident2(first_def),
        };

        Ok(self.fold(Spanned::new(term, name.span)))
    }
}

#[allow(missing_docs)]
pub trait ScopeSpecificTermContext<'t, S, D> {
    fn termify_name_in_scope(
        &self,
        name: Spanned<ResolvableName>,
        scope: S,
    ) -> Result<Spanned<Term<'t>>>;

    fn maybe_selectable_scope(&self, term: &Term<'t>) -> Option<S>;
}

impl<'t, 'sbc, 'lazy, 'sb, 'ast, 'ctx> ScopeSpecificTermContext<'t, ScopeRef, Def>
    for TermContext<&'sbc ScoreContext<'lazy, 'sb, 'ast, 'ctx>, ScopeRef, Def>
where
    'lazy: 'sbc,
    'sb: 'lazy,
    'ast: 'sb,
    'ctx: 'sb,
{
    /// Map a resolvable name to a term, resolving it within a scope.
    fn termify_name_in_scope(
        &self,
        name: Spanned<ResolvableName>,
        scope: ScopeRef,
    ) -> Result<Spanned<Term<'t>>> {
        let defs = self.ctx.resolve_name(name, scope, false, true)?;
        self.termify_defs(name, defs)
    }

    fn maybe_selectable_scope(&self, term: &Term<'t>) -> Option<ScopeRef> {
        if let Term::Ident(Spanned { value: def, .. }) = *term {
            match def {
                Def::Pkg(id) => Some(id.into()),
                Def::BuiltinPkg(id) => Some(id.into()),
                Def::Lib(id) => Some(id.into()),
                _ => None,
            }
        } else {
            None
        }
    }
}

impl<'t> ScopeSpecificTermContext<'t, &'t ScopeData<'t>, Def2<'t>>
    for TermContext<hir::AllocContext<'t>, &'t ScopeData<'t>, Def2<'t>>
{
    /// Map a resolvable name to a term, resolving it within a scope.
    fn termify_name_in_scope(
        &self,
        name: Spanned<ResolvableName>,
        scope: &'t ScopeData<'t>,
    ) -> Result<Spanned<Term<'t>>> {
        let defs = scope.resolve(name.value, false);
        self.termify_defs(name, defs)
    }

    fn maybe_selectable_scope(&self, term: &Term<'t>) -> Option<&'t ScopeData<'t>> {
        if let Term::Ident2(Spanned { value: def, .. }) = *term {
            match def {
                Def2::Lib(x) => Some(x.scope()),
                Def2::Pkg(x) => x.poll().ok().map(|x| x.scope()),
                _ => None,
            }
        } else {
            None
        }
    }
}

impl<'sbc, 'lazy, 'sb, 'ast, 'ctx>
    TermContext<&'sbc ScoreContext<'lazy, 'sb, 'ast, 'ctx>, ScopeRef, Def>
where
    'lazy: 'sbc,
    'sb: 'lazy,
    'ast: 'sb,
    'ctx: 'sb,
{
    /// Create a new termification context.
    pub fn new(ctx: &'sbc ScoreContext<'lazy, 'sb, 'ast, 'ctx>, scope: ScopeRef) -> Self {
        TermContext {
            ctx: ctx,
            scope: scope,
            marker: PhantomData,
        }
    }

    /// Map a latent name to a term.
    pub fn termify_latent_name(&self, name: LatentName<'ast>) -> Result<Spanned<Term>> {
        match name {
            LatentName::Simple(n) => self.termify_name(n.map_into()),
            LatentName::Primary(n) => {
                self.termify_name(ResolvableName::from_primary_name(n, self.ctx)?)
            }
            LatentName::Compound(n) => self.termify_compound_name(n),
        }
    }

    /// Map a term to an expression and schedule the necessary tasks.
    pub fn term_to_expr(&self, term: Spanned<Term>) -> Result<ExprRef> {
        let ctx = AddContext::new(self.ctx, self.scope);
        ctx.add_expr_hir(self.term_to_expr_raw(term)?)
    }

    /// Same as `term_to_expr`, but the result is spanned.
    pub fn term_to_expr_spanned(&self, term: Spanned<Term>) -> Result<Spanned<ExprRef>> {
        let sp = term.span;
        Ok(Spanned::new(self.term_to_expr(term)?, sp))
    }

    /// Map a term to an expression.
    pub fn term_to_expr_raw(&self, term: Spanned<Term>) -> Result<hir::Expr> {
        let term_span = term.span;
        let data = match term.value {
            Term::Unresolved(name) => {
                self.emit(DiagBuilder2::error(format!("`{}` is unknown", name)).span(term.span));
                return Err(());
            }
            Term::IntLit(value) => hir::ExprData::IntegerLiteral(ConstInt::new(None, value)),
            Term::StrLit(value) => {
                // Create a set of characters used in the literal. Then resolve
                // each as an individual bit literal. This yields multiple enums
                // per bit, the intersection of all of them being the possible
                // enums for this literal.
                let set: HashSet<_> = value.as_str().chars().collect();
                debugln!("string literal `{}` contains characters {:?}", value, set);
                let char_defs = set
                    .into_iter()
                    .map(|chr| {
                        let rn = Spanned::new(ResolvableName::Bit(chr), term_span);
                        let defs = self.ctx.resolve_name(rn, self.scope, false, true)?;
                        if defs.is_empty() {}
                        let term = self.termify_defs(rn, defs)?;
                        match term.value {
                            Term::Unresolved(name) => {
                                self.emit(
                                    DiagBuilder2::error(format!("`{}` is unknown", name))
                                        .span(term.span),
                                );
                                Err(())
                            }
                            Term::Enum(ids) => Ok((chr, ids.into_iter().collect())),
                            _ => {
                                self.emit(
                                    DiagBuilder2::error(format!("`{}` is not a bit literal", chr))
                                        .span(term_span)
                                        .add_note(format!("`{}` has been defined here:", rn.value))
                                        .span(term.span),
                                );
                                Err(())
                            }
                        }
                    })
                    .collect::<Vec<Result<(_, HashSet<_>)>>>()
                    .into_iter()
                    .collect::<Result<Vec<(_, HashSet<_>)>>>()?
                    .into_iter()
                    .collect::<HashMap<_, HashSet<_>>>();
                debugln!("string literal `{}` has def sets {:?}", value, char_defs);

                // Find the intersection of all available enums.
                let mut decl_sets = char_defs
                    .iter()
                    .map(|(_, defs)| defs.iter().map(|d| d.value.0).collect::<HashSet<_>>());
                let valid_decls: HashSet<_> = decl_sets
                    .next()
                    .map(|set| decl_sets.fold(set, |a, b| a.intersection(&b).map(|v| *v).collect()))
                    .unwrap_or_else(|| HashSet::new());
                debugln!(
                    "string literal `{}` has available decls {:?}",
                    value,
                    valid_decls
                );

                // Assemble the possible enum decls and bit strings.
                let maps: Vec<(_, Vec<_>)> = valid_decls
                    .into_iter()
                    .map(|decl| {
                        (
                            decl,
                            value
                                .as_str()
                                .chars()
                                .map(|chr| {
                                    char_defs[&chr]
                                        .iter()
                                        .filter_map(|enum_def| {
                                            if enum_def.value.0 == decl {
                                                Some(enum_def.value.1)
                                            } else {
                                                None
                                            }
                                        })
                                        .next()
                                        .unwrap()
                                })
                                .collect(),
                        )
                    })
                    .collect();
                debugln!("string literal `{}` has maps {:?}", value, maps);
                hir::ExprData::StringLiteral(maps)
            }
            Term::Unary(op, arg) => {
                // Look for implementations of this operator.
                let name: Spanned<ResolvableName> = op.map_into();
                let defs = self.ctx.resolve_name(name, self.scope, false, false)?;
                debugln!("resolved unary op `{}` to {:?}", name.value, defs);
                hir::ExprData::Unary(op, defs, self.term_to_expr(*arg)?)
            }
            Term::Binary(op, lhs, rhs) => {
                // Look for implementations of this operator.
                let name: Spanned<ResolvableName> = op.map_into();
                let defs = self.ctx.resolve_name(name, self.scope, false, false)?;
                debugln!("resolved binary op `{}` to {:?}", name.value, defs);
                hir::ExprData::Binary(op, defs, self.term_to_expr(*lhs)?, self.term_to_expr(*rhs)?)
            }
            Term::Ident(def) => match def.value {
                Def::Const(id) => hir::ExprData::ConstName(id),
                Def::Signal(id) => hir::ExprData::SignalName(id),
                Def::Var(id) => hir::ExprData::VarName(id),
                Def::File(id) => hir::ExprData::FileName(id),
                _ => {
                    self.emit(
                        DiagBuilder2::error(format!(
                            "`{}` cannot be used in an expression",
                            term_span.extract()
                        ))
                        .span(term_span)
                        .add_note(format!("`{}` was declared here:", term_span.extract()))
                        .span(def.span),
                    );
                    return Err(());
                }
            },
            Term::Enum(defs) => hir::ExprData::EnumName(defs),
            Term::Select(term, name) => hir::ExprData::Select(self.term_to_expr(*term)?, name),
            Term::Paren(subterm) => {
                // A parenthesis with only one element is just a parenthesized
                // expression. If there's more than one element, this is a
                // purely positional aggregate.
                if subterm.len() == 1 {
                    return self.term_to_expr_raw(subterm.into_iter().next().unwrap());
                } else {
                    hir::ExprData::Aggregate(
                        self.term_to_aggregate(Spanned::new(Term::Paren(subterm), term.span))?
                            .value,
                    )
                }
            }
            Term::Aggregate(..) => hir::ExprData::Aggregate(self.term_to_aggregate(term)?.value),
            Term::Qual(tm, term) => {
                let tm = self.term_to_type_mark(*tm)?;
                let expr = self.term_to_expr(*term)?;
                self.ctx.set_type_context(
                    expr,
                    self.ctx.intern_ty(Ty::Named(tm.span.into(), tm.value)),
                );
                hir::ExprData::Qualified(tm, expr)
            }

            // Allocators (`new` expressions) either have a type mark as their
            // argument, or a qualified expression which also provides the value
            // of the thing to allocate.
            Term::New(arg) => {
                let arg = *arg;
                match arg.value {
                    Term::Qual(tm, value) => {
                        let tm = self.term_to_type_mark(*tm)?;
                        let expr = self.term_to_expr(*value)?;
                        self.ctx.set_type_context(
                            expr,
                            self.ctx.intern_ty(Ty::Named(tm.span.into(), tm.value)),
                        );
                        hir::ExprData::Allocator(tm, Some(expr))
                    }
                    other => hir::ExprData::Allocator(
                        self.term_to_type_mark(Spanned::new(other, arg.span))?,
                        None,
                    ),
                }
            }

            // Function calls and cast expression look almost the same. The only
            // way to differentiate them is to look at the kind of the callee.
            Term::SuffixParen(callee, args) => {
                let callee = *callee;
                let args = self.term_to_assoc_list(*args)?;
                match callee.value {
                    Term::TypeMark(tm) => {
                        if args.value.len() != 1 {
                            self.emit(
                                DiagBuilder2::error(format!(
                                    "cast `{}` must have exactly one argument",
                                    term_span.extract()
                                ))
                                .span(args.span),
                            );
                            return Err(());
                        }
                        let arg = args.value.into_iter().next().unwrap();
                        if let Some(formal) = arg.formal {
                            self.emit(
                                DiagBuilder2::error(format!(
                                    "cast argument `{}` cannot have a formal part",
                                    arg.span.extract()
                                ))
                                .span(formal.span),
                            );
                        }
                        let arg = match arg.actual.value {
                            hir::AssocActual::Expr(id) => id,
                            _ => {
                                self.emit(
                                    DiagBuilder2::error(format!(
                                        "`{}` is not a valid cast argument",
                                        arg.actual.span.extract()
                                    ))
                                    .span(arg.actual.span),
                                );
                                return Err(());
                            }
                        };
                        self.ctx.set_type_context(
                            arg,
                            self.ctx.intern_ty(Ty::Named(tm.span.into(), tm.value)),
                        );
                        hir::ExprData::Cast(tm, arg)
                    }
                    other => hir::ExprData::Call(
                        self.term_to_expr(Spanned::new(other, callee.span))?,
                        args,
                    ),
                }
            }

            _ => {
                self.emit(
                    DiagBuilder2::error(format!(
                        "`{}` is not a valid expression",
                        term.span.extract()
                    ))
                    .span(term.span),
                );
                debugln!("It is a {:#?}", term);
                return Err(());
            }
        };
        Ok(hir::Expr {
            parent: self.scope,
            span: term_span,
            data: data,
        })
    }

    /// Map a term to a type mark.
    pub fn term_to_type_mark(&self, term: Spanned<Term>) -> Result<Spanned<TypeMarkRef>> {
        match term.value {
            Term::TypeMark(tm) => Ok(tm),
            _ => {
                self.emit(
                    DiagBuilder2::error(format!(
                        "`{}` is not a type or subtype",
                        term.span.extract()
                    ))
                    .span(term.span),
                );
                debugln!("It is a {:#?}", term);
                return Err(());
            }
        }
    }

    /// Perform term folding expecting to yield a type.
    ///
    /// This is a pre-processing step on terms. It is applied as soon as it is
    /// clear that a certain term should yield a type, e.g. when mapping to a
    /// subtype indication. This function performs certain precedence swaps and
    /// combines terms into higher level ones, e.g. `Term::SubtypeInd`.
    pub fn fold_term_as_type<'t>(&self, term: Spanned<Term<'t>>) -> Result<Spanned<Term<'t>>> {
        let (new, new_term) = match term.value {
            Term::Unresolved(name) => {
                self.emit(DiagBuilder2::error(format!("`{}` is unknown", name)).span(term.span));
                return Err(());
            }
            Term::RangeSuffix(subterm, range) => {
                let subterm = self.fold_term_as_type(*subterm)?;
                let range = self.fold_term_as_type(*range)?;
                match subterm.value {
                    // Fold `TypeMark range T` to `SubtypeInd`.
                    Term::TypeMark(tm) => (true, Term::SubtypeInd(tm, None, Some(Box::new(range)))),
                    // Fold `SubtypeInd range T` to `SubtypeInd`.
                    Term::SubtypeInd(tm, resol, Some(con)) => {
                        let sp = Span::union(con.span, range.span);
                        let new_con = Spanned::new(Term::RangeSuffix(con, Box::new(range)), sp);
                        (true, Term::SubtypeInd(tm, resol, Some(Box::new(new_con))))
                    }
                    _ => (false, Term::RangeSuffix(Box::new(subterm), Box::new(range))),
                }
            }
            Term::SuffixParen(subterm, suffix) => {
                let subterm = self.fold_term_as_type(*subterm)?;
                let suffix = self.fold_term_as_type(*suffix)?;
                match subterm.value {
                    // Fold `TypeMark (T)` to `SubtypeInd`.
                    Term::TypeMark(tm) => {
                        (true, Term::SubtypeInd(tm, None, Some(Box::new(suffix))))
                    }
                    // Fold `SubtypeInd (T)` to `SubtypeInd`.
                    Term::SubtypeInd(tm, resol, Some(con)) => {
                        let sp = Span::union(con.span, suffix.span);
                        let new_con = Spanned::new(Term::SuffixParen(con, Box::new(suffix)), sp);
                        (true, Term::SubtypeInd(tm, resol, Some(Box::new(new_con))))
                    }
                    // Fold `SubtypeInd (T)` to `SubtypeInd`.
                    Term::SubtypeInd(tm, resol, None) => {
                        (true, Term::SubtypeInd(tm, resol, Some(Box::new(suffix))))
                    }
                    _ => (
                        false,
                        Term::SuffixParen(Box::new(subterm), Box::new(suffix)),
                    ),
                }
            }
            others => (false, others),
        };
        let new_term = Spanned::new(new_term, term.span);
        if new {
            self.fold_term_as_type(new_term)
        } else {
            Ok(new_term)
        }
    }

    /// Map a term to a subtype indication.
    pub fn term_to_subtype_ind(&self, term: Spanned<Term>) -> Result<Spanned<hir::SubtypeInd>> {
        let term = self.fold_term_as_type(term)?;
        let (tm, resol, con) = match term.value {
            Term::SubtypeInd(tm, resol, con) => (tm, resol, con),
            Term::TypeMark(tm) => (tm, None, None),
            _ => {
                self.emit(
                    DiagBuilder2::error(format!(
                        "`{}` is not a subtype indication",
                        term.span.extract()
                    ))
                    .span(term.span),
                );
                debugln!("It is a {:#?}", term);
                return Err(());
            }
        };
        let _resol = match resol {
            Some(x) => Some(self.term_to_resolution_indication(*x)?),
            None => None,
        };
        let con = match con {
            Some(x) => Some(self.term_to_constraint(*x)?),
            None => None,
        };
        Ok(Spanned::new(
            hir::SubtypeInd {
                span: term.span,
                type_mark: tm,
                // TODO: Track resolution indication.
                constraint: con,
            },
            term.span,
        ))
        // let id = SubtypeIndRef::new(NodeId::alloc());
        // self.ctx.set_hir(id, self.ctx.sb.arenas.hir.subtype_ind.alloc(hir));
        // Ok(Spanned::new(id, term.span))
    }

    /// Map a term to a resolution indication.
    pub fn term_to_resolution_indication(&self, term: Spanned<Term>) -> Result<Spanned<()>> {
        self.emit(
            DiagBuilder2::bug(format!(
                "interpretation of `{}` as a resolution indication not implemented",
                term.span.extract()
            ))
            .span(term.span),
        );
        Err(())
    }

    /// Map a term to a constraint.
    pub fn term_to_constraint(&self, term: Spanned<Term>) -> Result<Spanned<hir::Constraint>> {
        // Handle range constraints.
        match term.value {
            Term::Range(..) => {
                return Ok(self.term_to_range(term)?.map(|r| hir::Constraint::Range(r)))
            }
            _ => (),
        };

        // Unpack the optional element constraint on array constraints.
        let (term, elem) = match term.value {
            Term::RangeSuffix(subterm, con) | Term::SuffixParen(subterm, con) => {
                (*subterm, Some(*con))
            }
            _ => (term, None),
        };

        // Otherwise handle the array and record constraint cases.
        match term.value {
            Term::Paren(terms) => {
                let any_records = terms.iter().any(|t| match t.value {
                    Term::SuffixParen(..) => true,
                    _ => false,
                });
                if any_records && elem.is_none() {
                    self.term_to_record_constraint(term.span, terms)
                        .map(|t| t.map_into())
                } else {
                    self.term_to_array_constraint(term.span, terms, elem)
                        .map(|t| t.map_into())
                }
            }
            _ => {
                self.emit(
                    DiagBuilder2::error(format!(
                        "`{}` is not a valid constraint",
                        term.span.extract()
                    ))
                    .span(term.span)
                    .add_note(
                        "Did you mean a range constraint (`range ...`) or an array or record \
                         constraint (`(...)`)? See IEEE 1076-2008 section 6.3.",
                    ),
                );
                debugln!("It is a {:#?}", term);
                return Err(());
            }
        }
    }

    /// Map a term to an array constraint.
    pub fn term_to_array_constraint(
        &self,
        span: Span,
        terms: Vec<Spanned<Term>>,
        elem: Option<Spanned<Term>>,
    ) -> Result<Spanned<hir::ArrayConstraint>> {
        if terms.is_empty() {
            self.emit(DiagBuilder2::error(format!("array constraint cannot be empty")).span(span));
            return Err(());
        }
        let indices = if terms.len() == 1 && terms[0].value == Term::Open {
            vec![]
        } else {
            terms
                .into_iter()
                .map(|e| self.term_to_discrete_range(e))
                .collect::<Result<Vec<_>>>()?
        };
        let elem = match elem {
            Some(e) => Some(self.term_to_element_constraint(e)?),
            None => None,
        };
        Ok(Spanned::new(
            hir::ArrayConstraint {
                span: span,
                index: indices,
                elem: elem.map(|e| Box::new(e)),
            },
            span,
        ))
    }

    /// Map a term to a record constraint.
    pub fn term_to_record_constraint(
        &self,
        span: Span,
        terms: Vec<Spanned<Term>>,
    ) -> Result<Spanned<hir::RecordConstraint>> {
        if terms.is_empty() {
            self.emit(DiagBuilder2::error(format!("record constraint cannot be empty")).span(span));
            return Err(());
        }
        let mut fields = Vec::new();
        let mut has_fails = false;
        let mut used_names = HashMap::new();
        for term in terms {
            // Make sure that the term is of the form `field (constraint)`.
            let (name, con) = match term.value {
                Term::SuffixParen(name, con) => (name, *con),
                _ => {
                    self.emit(
                        DiagBuilder2::error(format!(
                            "`{}` is not a valid constraint for a record element",
                            term.span.extract()
                        ))
                        .span(term.span)
                        .add_note(
                            "Element constraints must be of the form `name (constraint)`. See \
                             IEEE 1076-2008 section 5.3.3.",
                        ),
                    );
                    debugln!("It is a {:#?}", term.value);
                    has_fails = true;
                    continue;
                }
            };
            let name = match name.value {
                Term::Unresolved(ResolvableName::Ident(i)) => Spanned::new(i, name.span),
                _ => {
                    self.emit(
                        DiagBuilder2::error(format!(
                            "`{}` is not a valid record element name",
                            name.span.extract()
                        ))
                        .span(name.span),
                    );
                    debugln!("It is a {:#?}", name.value);
                    has_fails = true;
                    continue;
                }
            };

            // Make sure a field is not constrained twice.
            if let Some(&span) = used_names.get(&name.value) {
                self.emit(
                    DiagBuilder2::error(format!(
                        "element `{}` has already been constrained",
                        name.value
                    ))
                    .span(name.span)
                    .add_note("Previous constraint was here:")
                    .span(span),
                );
                has_fails = true;
                continue;
            } else {
                used_names.insert(name.value, name.span);
            }

            // Parse the constraint.
            fields.push((name, Box::new(self.term_to_element_constraint(con)?)));
        }
        if has_fails {
            return Err(());
        }
        Ok(Spanned::new(
            hir::RecordConstraint {
                span: span,
                elems: fields,
            },
            span,
        ))
    }

    /// Map a term to an element constraint.
    pub fn term_to_element_constraint(
        &self,
        term: Spanned<Term>,
    ) -> Result<Spanned<hir::ElementConstraint>> {
        let con = self.term_to_constraint(term)?;
        Ok(Spanned::new(
            match con.value {
                hir::Constraint::Array(c) => c.into(),
                hir::Constraint::Record(c) => c.into(),
                _ => {
                    self.emit(
                        DiagBuilder2::error(format!(
                            "`{}` is not a valid element constraint",
                            con.span.extract()
                        ))
                        .span(con.span)
                        .add_note(
                            "Did you mean an array or record constraint (`(...)`)? See IEEE \
                             1076-2008 section 6.3.",
                        ),
                    );
                    debugln!("It is a {:#?}", con);
                    return Err(());
                }
            },
            con.span,
        ))
    }

    /// Map a term to a discrete range.
    pub fn term_to_discrete_range(
        &self,
        term: Spanned<Term>,
    ) -> Result<Spanned<hir::DiscreteRange>> {
        let term = self.fold_term_as_type(term)?;
        Ok(match term.value {
            Term::SubtypeInd(..) | Term::TypeMark(..) => {
                let hir = self.term_to_subtype_ind(term)?;
                let add_ctx = AddContext::new(self.ctx, self.scope);
                Spanned::new(add_ctx.add_subtype_ind_hir(hir.value)?.into(), hir.span)
            }
            Term::Range(..) => self.term_to_range(term)?.map_into(),
            _ => {
                self.emit(
                    DiagBuilder2::error(format!(
                        "`{}` is not a valid discrete range",
                        term.span.extract()
                    ))
                    .span(term.span)
                    .add_note("A discrete range can be one of the following:")
                    .add_note(
                        "- a subtype indication of the form `[resolution] type_mark [constraint]`",
                    )
                    .add_note("- a range of the form `a to b` or `a downto b`")
                    .add_note("- a range attribute of the form `T'range`")
                    .add_note("See IEEE 1076-2008 section 5.3.2.1."),
                );
                debugln!("It is a {:#?}", term);
                return Err(());
            }
        })
    }

    /// Map a term to a range.
    pub fn term_to_range(&self, term: Spanned<Term>) -> Result<Spanned<hir::Range>> {
        Ok(Spanned::new(
            match term.value {
                // Term::Attr(..) => ...
                Term::Range(dir, lb, rb) => hir::Range::Immediate(
                    dir.value,
                    self.term_to_expr(*lb)?,
                    self.term_to_expr(*rb)?,
                ),
                _ => {
                    self.emit(
                        DiagBuilder2::error(format!(
                            "`{}` is not a valid range",
                            term.span.extract()
                        ))
                        .span(term.span)
                        .add_note("A range can be one of the following:")
                        .add_note("- an ascending range of the form `a to b`")
                        .add_note("- a descending range of the form `a downto b`")
                        .add_note("- a range attribute of the form `T'range`")
                        .add_note("See IEEE 1076-2008 section 5.2.1."),
                    );
                    debugln!("It is a {:#?}", term);
                    return Err(());
                }
            },
            term.span,
        ))
    }

    /// Map a term to a definition.
    ///
    /// This works for terms that are actually identifiers.
    pub fn term_to_ident(&self, term: Spanned<Term>) -> Result<Spanned<Def>> {
        Ok(match term.value {
            Term::Unresolved(name) => {
                self.emit(DiagBuilder2::error(format!("`{}` is unknown", name)).span(term.span));
                return Err(());
            }
            Term::Ident(def) => def,
            Term::TypeMark(tm) => tm.map_into(),
            Term::Enum(defs) => {
                if defs.len() == 1 {
                    let e = defs.into_iter().next().unwrap();
                    e.map(|e| e.0.into())
                } else {
                    self.emit(
                        DiagBuilder2::error(format!("`{}` is ambiguous", term.span.extract()))
                            .span(term.span),
                    );
                    debugln!("Its definitions are {:#?}", defs);
                    return Err(());
                }
            }
            _ => {
                self.emit(
                    DiagBuilder2::error(format!("`{}` is not an identifier", term.span.extract()))
                        .span(term.span),
                );
                debugln!("It is a {:#?}", term);
                return Err(());
            }
        })
    }

    /// Map a term to a label.
    ///
    /// Returns the statement the label refers to.
    pub fn term_to_label(&self, term: Spanned<Term>) -> Result<Spanned<StmtRef>> {
        let span = term.span;
        let def = self.term_to_ident(term)?;
        Ok(Spanned::new(
            match def.value {
                Def::Stmt(id) => id,
                _ => {
                    self.emit(
                        DiagBuilder2::error(format!(
                            "`{}` is not a statement label",
                            span.extract()
                        ))
                        .span(span)
                        .add_note(format!("`{}` was defined here:", span.extract()))
                        .span(def.span),
                    );
                    debugln!("The definition is a {:?}", def.value);
                    return Err(());
                }
            },
            span,
        ))
    }

    /// Map a term to a signal.
    pub fn term_to_signal(&self, term: Spanned<Term>) -> Result<Spanned<SignalRef>> {
        let span = term.span;
        let def = self.term_to_ident(term)?;
        Ok(Spanned::new(
            match def.value {
                Def::Signal(id) => id,
                _ => {
                    self.emit(
                        DiagBuilder2::error(format!("`{}` is not a signal", span.extract()))
                            .span(span)
                            .add_note(format!("`{}` was defined here:", span.extract()))
                            .span(def.span),
                    );
                    debugln!("The definition is a {:?}", def.value);
                    return Err(());
                }
            },
            span,
        ))
    }

    /// Map a term to a choice.
    ///
    /// See IEEE 1076-2008 section 9.3.3.1. A choice can be a simple expression,
    /// a discrete range, an identifier, or the keyword `others`.
    pub fn term_to_choice(&self, term: Spanned<Term>) -> Result<Spanned<hir::Choice>> {
        let term_span = term.span;
        Ok(Spanned::new(
            match term.value {
                Term::Unresolved(ResolvableName::Ident(name)) => hir::Choice::Element(name),
                Term::Others => hir::Choice::Others,
                Term::SubtypeInd(..) | Term::TypeMark(..) | Term::Range(..) => {
                    hir::Choice::DiscreteRange(self.term_to_discrete_range(term)?.value)
                }
                Term::IntLit(..) | Term::Unary(..) | Term::Binary(..) => {
                    hir::Choice::Expr(self.term_to_expr(term)?)
                }
                _ => {
                    self.emit(
                        DiagBuilder2::error(format!(
                            "`{}` is not a valid choice",
                            term.span.extract()
                        ))
                        .span(term.span)
                        .add_note("A choice can be one of the following:")
                        .add_note("- an expression")
                        .add_note("- a discrete range")
                        .add_note("- an element name")
                        .add_note("- the `others` keyword")
                        .add_note("See IEEE 1076-2008 section 9.3.3.1."),
                    );
                    debugln!("It is a {:#?}", term);
                    return Err(());
                }
            },
            term_span,
        ))
    }

    /// Map a term to an aggregate.
    ///
    /// See IEEE 1076-2008 section 9.3.3.1.
    pub fn term_to_aggregate(&self, term: Spanned<Term>) -> Result<Spanned<AggregateRef>> {
        // Determine the fields of the aggregate.
        let fields = match term.value {
            Term::Aggregate(fields) => fields
                .into_iter()
                .map(|(choices, expr)| {
                    let choices = choices
                        .into_iter()
                        .map(|choice| self.term_to_choice(choice))
                        .collect::<Vec<Result<_>>>()
                        .into_iter()
                        .collect::<Result<Vec<_>>>();
                    let expr = self.term_to_expr_spanned(expr);
                    let choices = choices?;
                    let expr = expr?;
                    let mut span = expr.span;
                    if !choices.is_empty() {
                        span.expand(choices[0].span);
                    }
                    Ok(Spanned::new((choices, expr), span))
                })
                .collect::<Vec<Result<_>>>()
                .into_iter()
                .collect::<Result<Vec<_>>>()?,
            Term::Paren(fields) => fields
                .into_iter()
                .map(|expr| {
                    let expr = self.term_to_expr_spanned(expr)?;
                    Ok(Spanned::new((vec![], expr), expr.span))
                })
                .collect::<Vec<Result<_>>>()
                .into_iter()
                .collect::<Result<Vec<_>>>()?,
            _ => {
                self.emit(
                    DiagBuilder2::error(format!(
                        "`{}` is not a valid aggregate",
                        term.span.extract()
                    ))
                    .span(term.span)
                    .add_note("See IEEE 1076-2008 section 9.3.3.1."),
                );
                debugln!("It is a {:#?}", term);
                return Err(());
            }
        };

        // Split the fields into positional, named, and `others`.
        let mut positional = Vec::new();
        let mut named = Vec::new();
        let mut others = None;
        #[derive(PartialOrd, Ord, PartialEq, Eq)]
        enum Mode {
            Positional = 0,
            Named = 1,
            Others = 2,
        }
        let mut mode = Mode::Positional;
        for field in fields {
            if mode == Mode::Others {
                self.emit(
                    DiagBuilder2::error(format!(
                        "aggregate element `{}` appears after `others`",
                        field.span.extract()
                    ))
                    .span(field.span)
                    .add_note(
                        "The `others` element must be the last in an aggregate. See IEEE \
                         1076-2008 section 9.3.3.1.",
                    ),
                );
                return Err(());
            }

            // Handle positional elements.
            if field.value.0.is_empty() {
                if mode > Mode::Positional {
                    self.emit(
                        DiagBuilder2::error(format!(
                            "positional element `{}` must appear before all named elements in \
                             aggregate",
                            field.value.1.span.extract()
                        ))
                        .span(field.value.1.span)
                        .add_note("See IEEE 1076-2008 section 9.3.3.1."),
                    );
                    return Err(());
                }
                positional.push(field.value.1);
            }
            // Handle `others`.
            else if field.value.0.iter().any(|i| i.value.is_others()) {
                if field.value.0.len() != 1 {
                    self.emit(
                        DiagBuilder2::error("`others` must be the only thing left of `=>`")
                            .span(field.value.1.span)
                            .add_note("See IEEE 1076-2008 section 9.3.3.1."),
                    );
                    return Err(());
                }
                others = Some(field.value.1);
                mode = Mode::Others;
            }
            // Handle named elements.
            else {
                named.push(field);
                mode = Mode::Named;
            }
        }

        // For the named elements, decide whether they describe a record or an
        // array aggregate. If no named elements exist, the aggregate can be
        // both and its type must be determined from context.
        let named = if named.is_empty() {
            hir::AggregateKind::Both
        } else if named
            .iter()
            .any(|n| n.value.0.iter().any(|c| c.value.is_element()))
        {
            hir::AggregateKind::Record(
                named
                    .into_iter()
                    .map(
                        |Spanned {
                             value: (choices, expr),
                             span,
                         }| {
                            let choices = choices
                                .into_iter()
                                .map(
                                    |Spanned {
                                         value: choice,
                                         span,
                                     }| {
                                        let choice = match choice {
                                            hir::Choice::Element(n) => n,
                                            _ => {
                                                self.emit(
                                                    DiagBuilder2::error(format!(
                                                        "choice `{}` is not a record element",
                                                        span.extract()
                                                    ))
                                                    .span(span)
                                                    .add_note(
                                                        "An aggregate must either be a record or \
                                                         array aggregate. It cannot contain both \
                                                         record elements and array elements. See \
                                                         IEEE 1076-2008 section 9.3.3.1.",
                                                    ),
                                                );
                                                return Err(());
                                            }
                                        };
                                        Ok(Spanned::new(choice, span))
                                    },
                                )
                                .collect::<Vec<Result<_>>>()
                                .into_iter()
                                .collect::<Result<Vec<_>>>()?;
                            Ok(Spanned::new((choices, expr), span))
                        },
                    )
                    .collect::<Vec<Result<_>>>()
                    .into_iter()
                    .collect::<Result<Vec<_>>>()?,
            )
        } else {
            hir::AggregateKind::Array(
                named
                    .into_iter()
                    .map(
                        |Spanned {
                             value: (choices, expr),
                             span,
                         }| {
                            let choices = choices
                                .into_iter()
                                .map(
                                    |Spanned {
                                         value: choice,
                                         span,
                                     }| {
                                        let choice = match choice {
                                            hir::Choice::Expr(e) => hir::ArrayChoice::Expr(e),
                                            hir::Choice::DiscreteRange(e) => {
                                                hir::ArrayChoice::DiscreteRange(e)
                                            }
                                            _ => {
                                                self.emit(
                                                    DiagBuilder2::error(format!(
                                                        "choice `{}` is not an array element",
                                                        span.extract()
                                                    ))
                                                    .span(span)
                                                    .add_note(
                                                        "An aggregate must either be a record or \
                                                         array aggregate. It cannot contain both \
                                                         record elements and array elements. See \
                                                         IEEE 1076-2008 section 9.3.3.1.",
                                                    ),
                                                );
                                                return Err(());
                                            }
                                        };
                                        Ok(Spanned::new(choice, span))
                                    },
                                )
                                .collect::<Vec<Result<_>>>()
                                .into_iter()
                                .collect::<Result<Vec<_>>>()?;
                            Ok(Spanned::new((choices, expr), span))
                        },
                    )
                    .collect::<Vec<Result<_>>>()
                    .into_iter()
                    .collect::<Result<Vec<_>>>()?,
            )
        };

        let hir = hir::Aggregate {
            parent: self.scope,
            span: term.span,
            positional: positional,
            named: named,
            others: others,
        };
        let ctx = AddContext::new(self.ctx, self.scope);
        Ok(Spanned::new(ctx.add_aggregate_hir(hir)?, term.span))
    }

    /// Map a term to an association list.
    ///
    /// See IEEE 1076-2008 section 6.5.7.
    pub fn term_to_assoc_list(&self, term: Spanned<Term>) -> Result<Spanned<hir::AssocList>> {
        let term_span = term.span;
        Ok(Spanned::new(
            match term.value {
                Term::Paren(fields) => fields
                    .into_iter()
                    .map(|term| {
                        let actual = self.term_to_assoc_actual(term)?;
                        Ok(hir::AssocElement {
                            span: actual.span,
                            formal: None,
                            actual: actual,
                        })
                    })
                    .collect::<Vec<Result<_>>>()
                    .into_iter()
                    .collect::<Result<Vec<_>>>()?,
                Term::Aggregate(fields) => fields
                    .into_iter()
                    .map(|(formal, actual)| {
                        if formal.len() != 1 {
                            self.emit(
                                DiagBuilder2::error(
                                    "formal part of association element must be exactly one \
                                     expression",
                                )
                                .span(term_span),
                            );
                            return Err(());
                        }
                        let formal = formal.into_iter().next().unwrap();
                        let formal_span = formal.span;
                        let span = Span::union(formal.span, actual.span);
                        let formal = self.term_to_expr(formal)?;
                        let actual = self.term_to_assoc_actual(actual)?;
                        Ok(hir::AssocElement {
                            span: span,
                            formal: Some(Spanned::new(formal, formal_span)),
                            actual: actual,
                        })
                    })
                    .collect::<Vec<Result<_>>>()
                    .into_iter()
                    .collect::<Result<Vec<_>>>()?,
                _ => {
                    self.emit(
                        DiagBuilder2::error(format!(
                            "`{}` is not a valid association list",
                            term.span.extract()
                        ))
                        .span(term.span)
                        .add_note("See IEEE 1076-2008 section 6.5.7."),
                    );
                    debugln!("It is a {:#?}", term);
                    return Err(());
                }
            },
            term_span,
        ))
    }

    /// Map a term to an association actual.
    pub fn term_to_assoc_actual(&self, term: Spanned<Term>) -> Result<Spanned<hir::AssocActual>> {
        let term_span = term.span;
        // TODO: Don't just treat this as an expression, but rather
        // differentiate properly between the different designators.
        let expr = self.term_to_expr(term)?;
        Ok(Spanned::new(hir::AssocActual::Expr(expr), term_span))
    }
}

impl<'t> TermContext<hir::AllocContext<'t>, &'t ScopeData<'t>, Def2<'t>> {
    /// Create a new termification context.
    pub fn new2(ctx: hir::AllocContext<'t>) -> Self {
        TermContext {
            ctx: ctx,
            scope: ctx.scope,
            marker: PhantomData,
        }
    }
}

/// Map a term to a range.
pub fn term_to_range<'t, C>(term: Spanned<Term<'t>>, ctx: C) -> Result<Spanned<hir::Range2<'t>>>
where
    C: SessionContext + Copy + for<'a> Alloc<'a, 't, hir::LitExpr>,
{
    let v = match term.value {
        // Term::Attr(..) => ...
        Term::Range(dir, lb, rb) => {
            let le = term_to_expr(*lb, ctx);
            let re = term_to_expr(*rb, ctx);
            hir::Range2::Immediate(term.span, dir, le?, re?)
        }
        _ => {
            ctx.emit(
                DiagBuilder2::error(format!("`{}` is not a valid range", term.span.extract()))
                    .span(term.span)
                    .add_note("A range can be one of the following:")
                    .add_note("- an ascending range of the form `a to b`")
                    .add_note("- a descending range of the form `a downto b`")
                    .add_note("- a range attribute of the form `T'range`")
                    .add_note("See IEEE 1076-2008 section 5.2.1."),
            );
            debugln!("It is a {:#?}", term);
            return Err(());
        }
    };
    Ok(Spanned::new(v, term.span))
}

/// Map a term to a range.
pub fn term_to_expr<'t, C>(term: Spanned<Term<'t>>, ctx: C) -> Result<&'t hir::Expr2<'t>>
where
    C: SessionContext + Copy + for<'a> Alloc<'a, 't, hir::LitExpr>,
{
    match term.value {
        Term::Unresolved(name) => {
            ctx.emit(DiagBuilder2::error(format!("`{}` is unknown", name)).span(term.span));
            Err(())
        }
        Term::IntLit(value) => Ok(ctx.alloc(hir::LitExpr::new_integer(term.span, value))),
        Term::FloatLit(value) => Ok(ctx.alloc(hir::LitExpr::new_float(term.span, value))),
        // Throw an error for everything that does not look like an expression.
        _ => {
            ctx.emit(
                DiagBuilder2::error(format!(
                    "`{}` is not a valid expression",
                    term.span.extract()
                ))
                .span(term.span),
            );
            debugln!("It is a {:#?}", term);
            Err(())
        }
    }
}