type-mapper

A Rust procedural macro crate for powerful type-level pattern matching and transformation. This crate is particularly useful for macro authors who need to perform complex type manipulations at compile time.

Features

• Type pattern matching with wildcards
• Lifetime handling and manipulation
• Recursive type transformations
• Support for generic type parameters
• Path-aware type matching

Limitations

The macro matches on token patterns rather than full types. It performs no validation of generic parameters or type identity.

For example, you cannot tell whether a type named Ordering was imported from std::cmd::Ordering or std::sync::atomic::Ordering.

Usage

Add this to your Cargo.toml:

[dependencies]
type-mapper = "0.x.y"

Examples

Type Declaration

The map_types! macro can be used in a type position:

use type_mapper::map_types;

/// This is an unlikely usage, but macros will generate code like this:
static X: map_types!(
    match Vec<Option<u8>> {
        Vec<_T> => recurse!(_T),
        Option<_T> => recurse!(_T),
        _T => _T,
    }
) = 1_u8;

It's mostly useful when writing macros that need to transform types:

use type_mapper::map_types;
use static_assertions::assert_type_eq_all;

macro_rules! with_static_lifetime {
    ($ty:ty) => {
        map_types!(
            match $ty {
                &'_ _T => &'static recurse!(_T),
                _T<'_, _U> => _T<'static, recurse!(_U)>,
                _T<'_, _U, _V> => _T<'static, recurse!(_U), recurse!(_V)>,
                _T => _T,
            }
        )
    }
}

trait MyTrait<T> {
    /// Our type
    type Type;
    /// Our type but with static lifetimes
    type Static;
}

macro_rules! define_trait_for {
    ($lt:lifetime $name:ident $ty:ty) => {
        impl <$lt> MyTrait<$ty> for $name where $name: $lt {
            type Type = $ty;
            type Static = with_static_lifetime!($ty);
        }
    }
}

struct MyStruct{}

define_trait_for!('a MyStruct (u8, u8));
define_trait_for!('a MyStruct (&'a u8, &'a u8));

assert_type_eq_all!(<MyStruct as MyTrait<(u8, u8)>>::Type, (u8, u8));
assert_type_eq_all!(<MyStruct as MyTrait<(u8, u8)>>::Static, (u8, u8));
// This doesn't compile because we can't test against that missing lifetime
// assert_type_eq_all!(<MyStruct as MyTrait<(&'a u8, &'a u8)>>::Type, (&'static u8, &'static u8));
assert_type_eq_all!(<MyStruct as MyTrait<(&'static u8, &'static u8)>>::Static, (&'static u8, &'static u8));

Basic Type Matching

The crate provides a assert_type_matches! and assert_type_not_matches! macros for testing type matches.

use type_mapper::{assert_type_matches, assert_type_not_matches, map_types};

// Simple type matching
assert_type_matches!(Vec<u8>, Vec<u8>);
assert_type_matches!(Vec<u8>, Vec);
assert_type_matches!(Vec<u8>, _T <u8>);
assert_type_matches!(Vec<u8>, Vec<_>);

// Wildcard matching
assert_type_matches!(u8, _);
assert_type_matches!(u8, _T);
assert_type_matches!(u8, _T<>);
assert_type_not_matches!(u8, _T<_U>);

// Type mismatch
assert_type_not_matches!(Vec<u8>, Vec<u16>);
assert_type_not_matches!(u8, u16);
assert_type_not_matches!(std::sync::MutexGuard<'a, u8>, std::sync::MutexGuard<'_>);

Lifetime Handling

use type_mapper::map_types;
use static_assertions::assert_type_eq_all;

/// Make a slice type static.
macro_rules! make_slice_static {
    ($ty:ty) => {
        map_types!(
            match $ty {
                &'_ [_T] => &'static [_T],
            }
        )
    }
}

assert_type_eq_all!(make_slice_static!(&'a [u8]), &'static [u8]);
assert_type_eq_all!(make_slice_static!(&'static [u8]), &'static [u8]);

Recursive Type Matching

Using the internal recurse! macro, you can match on types recursively.

use type_mapper::map_types;
use static_assertions::assert_type_eq_all;

/// Unwrap an Option type recursively.
macro_rules! unwrap_option {
    ($ty:ty) => {
        map_types!(
            match $ty {
                Option<_T> => recurse!(_T),
                _T => _T,
            }
        )
    }
}

assert_type_eq_all!(unwrap_option!(Option<u8>), u8);
assert_type_eq_all!(unwrap_option!(Option<Option<u8>>), u8);
assert_type_eq_all!(unwrap_option!(Option<Option<Option<u8>>>), u8);

Pattern Matching Syntax

The crate supports several pattern matching features:

• _ - Wildcard for any type
• _T - Named wildcard that can be referenced in the result
• _T<T> - Generic type matching
• _T<> - Generic type matching with no generics (WARNING: rustfmt may eat the <> if you don't use #[rustfmt::skip])
• '_ - Wildcard lifetime (will also match no lifetime in a reference)
• '_A - Named wildcard lifetime (will also match no lifetime in a reference)
• 'a - Named lifetime matching
• recurse!(_T) - Recursive type transformation

Use Cases

This crate is particularly useful for:

1. Macro authors who need to transform types
2. Creating type-level DSLs
3. Implementing complex type-level logic
4. Generic type manipulation
5. Lifetime manipulation in macros

License

MIT License