fieldx v0.1.2

Procedural macro for constructing structs with lazily initialized fields, builder pattern, and serde support with a focus on declarative syntax.

Let's start with an example:

use fieldx::fxstruct;

#[fxstruct( lazy )]
struct Foo {
    count: usize,
    foo:   String,
    #[fieldx( lazy(off), get )]
    order: RefCell<Vec<&'static str>>,
}

impl Foo {
    fn build_count(&self) -> usize {
        self.order.borrow_mut().push("Building count.");
        12
    }

    fn build_foo(&self) -> String {
        self.order.borrow_mut().push("Building foo.");
        format!("foo is using count: {}", self.count())
    }
}

let foo = Foo::new();
assert_eq!(foo.order().borrow().len(), 0);
assert_eq!(foo.foo(), "foo is using count: 12");
assert_eq!(foo.foo(), "foo is using count: 12");
assert_eq!(foo.order().borrow().len(), 2);
assert_eq!(foo.order().borrow()[0], "Building foo.");
assert_eq!(foo.order().borrow()[1], "Building count.");

What happens here is:

• a struct with all fields been lazy by default
• laziness is explicitly disabled for field order
• methods build_count and build_foo return initial values for corresponding fields

At run-time we first ensure that the order vector is empty meaning none of the build_ methods was called. Then we read from foo using its accessor method. Then we make sure that each build_ method was invoked only once.

As one can notice, a minimal amount of handcraft is needed here as most of boilerplate is handled by the macro, which provides even basic new associated function.

Also notice that we don't need to remember the order of initialization of fields. Builder of foo is using count without worrying if it's been initialized yet or not because it will always be.

Basics

The module provides two attributes: fxstruct, and fieldx. The first is responsible for configuring structs, the second for adjusting field parameters.

The macro can only be used with named structures, no union types, nor enums are supported. When applied, it rewrites the type it is applied to according to the parameters provided. Here is a list of most notable changes and additions:

• field types may be be wrapped into container types

  In the above example foo and count become [OnceCell<String>][OnceCell] and OnceCell<usize>, whereas order remains unchanged.

• a partial implementation of Foo is added with support methods and associated functions

  I.e. this is where accessor methods and new live.

• depending on parameters, an implicit implementation of the [Default] trait may be be added

• if requested, builder struct and builder() associated function will be implemented

• also, if requested, a shadow struct for correct serde support will be there too

Note that user is highly discouraged from directly accessing modified fields. The module does its best to provide all necessary API via corresponding methods.

Sync And Non-Sync Structs

If a thread-safe struct is needed then fxstruct must take the sync argument: #[fxstruct(sync, ...)]. When instructed so, the macro will do its best to provide concurrency safety at the field level. It means that:

• builder methods are guaranteed to be invoked once and only once per each lazy initialization, be it single- or multi-threaded application
• access to struct fields is lock-protected (unless otherwise requested by the user)

Sync and non-sync structures also are very different in ways they act and interact with user code.

Also, non-mutable accessors of non-sync struct normally return a reference to their field. Accessors of sync structs, unless directed to use [clone][Clone] or [copy][Copy], or used with a non-protected field, return a kind of lock-guard.

Wrapper types for sync struct fields are non-std and provided with the module.

Protected And Unprotected Fields Of Sync Structs

For a fieldx sync struct to be Sync+Sent all of its fields are expected to be lock-protected (or, sometimes we could just say "protected"). But "expected" doesn't mean "has to be". Unless defaults, specified with fxstruct attribute (i.e. with struct-level arguments) tell otherwise, fields not marked with fieldx attribute with corresponding arguments will remain unprotected. I.e.:

#[fxstruct(sync)]
struct Foo {
    #[fieldx(lazy)]
    foo: String, // protected
    #[fieldx(get_mut)]
    bar: String, // unprotected
}

Of course, whether the struct remains thread-safe would then depend on the safety of unprotected fields.

Optional Fields

Optional in this context has the same meaning, as in the [Option] type. Sure thing, one can simply declare a field using the core type (and, as a matter of fact, this is what fieldx is using internally anyway). What's the advantages of using fieldx then?

First of all, manual declaration may mean additional boilerplate code to implement an accessor, among other things. With fieldx most of it can be hidden under a single declaration:

#[fxstruct]
struct Foo {
    #[fieldx(predicate, clearer, get, set(into))]
    description: String,
}

let mut obj = Foo::new();
assert!( !obj.has_description() );
obj.set_description("foo");
assert!( obj.has_description() );
assert_eq!( obj.description(), &Some(String::from("foo")) );
obj.clear_description();
assert!( !obj.has_description() );

<digression_mode> Besides, aesthetically, to some has_description is more appealing than obj.description().is_some(). </digression_mode>

Next, optional fields of sync structs are lock-protected by default. This can be changed with explicit lock(off), but one has to be aware that then sync status of the struct will depend the safety of the field.

And the last note to be made is that if at some point it would prove to be useful to convert a field into a lazy then refactoring could be reduced to simply adding corresponding argument the fieldx attribute and implementing a new builder for it.

Laziness Protocol

Though being very simple concept, laziness has its own peculiarities. The basics, as shown above, are such that when we declare a field as lazy the macro wraps it into some kind of proxy container type ([OnceCell] for non-sync structs). The first read¹ from an uninitialized field will result in the builder method to be invoked and the value it returns to be stored in the field.

Here come the caveats:

1. A builder is expected to be infallible. This requirement comes from the fact that when we call field's accessor we expect a value of field's type to be returned. Since Rust requires errors to be handled semi-in-place (contrary to exceptions in many other languages) there is no way for us to overcome this limitation. The builder could panic, but this is rarely a good option.

   For cases when it is important to have controllable error handling, one could give the field a [Result] type. Then obj.field()? could be a way to take care of errors.

2. Field builder methods cannot mutate their objects. This limitation also comes from the fact that a typical accessor method doesn't need and must not use mutable &self. Of course, it is always possible to use internal mutability, as in the first example here.

Builder Pattern

IMPORTANT! First of all, it is necessary to mention unintended terminological ambiguity here. The terms build and builder are used for different, though identical in nature, processes. As mentioned in the previous section, the lazy builders are methods that return initial values for associated fields. The struct builder in this section is an object that collects initial values from user and then is able to create the final instance of the original struct. This ambiguity has some history spanning back to the times when Perl's Moo module was one of the author's primary tools. Then it was borrowed by Raku AttrX::Mooish and, finally, automatically made its way into fieldx which, initially, didn't implement the builder pattern.

The default new method generated by fxstruct macro accepts no arguments and simply creates a bare-bones object initialized from type defaults. Submitting custom values for struct fields is better be done by using the builder pattern:

#[fxstruct(builder)]
struct Foo {
    #[fieldx(lazy)]
    description: String,
    count: usize,
}

impl Foo {
    fn build_description(&self) -> String {
        format!("this is item #{}", self.count)
    }
}

let obj = Foo::builder()
            .count(42)
            .build()
            .expect("Foo builder failure");
assert_eq!( obj.description(), &String::from("this is item #42") );

let obj = Foo::builder()
            .count(13)
            .description(String::from("count is ignored"))
            .build()
            .expect("Foo builder failure");
// Since the `description` is given a value the `count` field is not used
assert_eq!( obj.description(), &String::from("count is ignored") );

Since the only fieldx-related failure that may happen when building a new object instance is a required field not given a value, the build() method would return FieldXError if this happens.

Usage

Most arguments of both fxstruct and fieldx can take either of the two forms: a keyword (arg), or a "function" (arg(subarg)).

Also, most of the arguments are shared by both fxstruct and fieldx. But their meaning and the way their arguments are interpreted could be slightly different for each attribute. For example, if an argument takes a literal string sub-argument it is likely to be a method name when associated with fieldx; but for fxstruct it would define common prefix for method names.

There is also a commonality between most of the arguments: they can be temporarily (say, for testing purposes) or permanently turned off by using off sub-argument with them. See lazy(off) in the above example.

Attribute Arguments

A few words on terminology:

• argument Type determines what sub-arguments can be received:

  • keyword – boolean-like, only accepts off: keyword(off)
  • flag – similar to the keyword above but takes no arguments; as a matter of fact, the off above is a flag
  • helper - introduce functionality that is bound to a helper method (see below)
  • list or function – can take multiple sub-arguments
  • meta - can take some syntax constructs

• helper method – implements certain functionality

  Almost all helpers are generated by the macro. The only exception are lazy builders which must be provided by the user.

• For specifies if argument is specific to an attribute

Sub-Arguments of Helper Arguments

Helper arguments share a bunch of common sub-arguments. We will describe them here, but if their meaning is unclear it'd be better to skip this section and get back to it later.

Sub-argument	In fxstruct	In fxfield
off	disable helper	disable helper
a non-empty string literal ("foo")	method name prefix	explicit method name (prefix not used)
attributes_fn	default attributes for corresponding kind of helper methods	attributes for field's helper method
public, public(crate), public(super), public(some::module), private	default visibility	visibility for field helper

For example:

#[fxstruct( get( "get_", public(crate) ) )]

will generate accessor methods with names prefixed with get_ and visibility pub(crate):

let foo = obj.get_foo();

With:

#[fieldx( get( "special_type", private ) )]
ty: String,

a method of the field owning struct can use the accessor as follows:

let foo = self.special_type();

attributes* Family of Sub-Arguments

Sometimes it might be necessary to specify attributes for various generated syntax elements like methods, or auxiliary structs. Where applicable, this functionality is supported by attributes* (sub)arguments. Their syntax is attributes(<attr1>, <attr2>, ...) where an <attr> is specified exactly, as it would be specified in the code, but with starting #[ and finishing ] being omitted.

For example, attributes_fn(allow(dead_code), cfg(feature = "myfeature")) will expand into something like:

#[allow(dead_code)]
#[cfg(feature = "myfeature")]

The following members of the family are currently supported: attributes, attributes_fn, and attributes_impl. Which ones are supported in a particular context is documented below.

Arguments of fxstruct

attributes

Type: list

Fallback attributes for structs produced by the builder and serde arguments.

attributes_impl

Type: list

Attributes to be applied to the struct implementation.

sync

Type: keyword

Declare a struct as thread-safe.

lazy

Type: helper

Enables lazy mode for all fields except those marked with lazy(off).

builder

Type: helper

Enables builder functionality by introducing a builder() associated function and builder type:

#[fxstruct(builder, get)]
struct Foo {
    description: String,
}
let obj = Foo::builder()
               .description(String::from("some description"))
               .build()?;
assert_eq!(obj.description(), "some description");

Literal string sub-argument of builder defines common prefix for methods-setters of the builder. For example, with builder("set_") one would then use .set_description(...) call.

Additional sub-arguments:

• attributes (see the section above) – builder struct attributes

• attributes_impl - attributes of the struct implementation

• into – force all builder setter methods to attempt automatic type conversion using .into() method

  With into the example above wouldn't need String::from and the call could look like this: .description("some description")

no_new

Type: keyword

Disable generation of method new. This is useful for cases when a user wants their own new method.

With this option the macro may avoid generating Default implementation for the struct. More details in a section below.

default

Type: keyword

Forces the Default implementation to be generated for the struct.

get

Type: helper

Enables or disables getter methods for all fields, unless a field is marked otherwise.

Additionally to the standard helper arguments accessors can also be configured as:

• clone - cloning, i.e. returning a clone of the field value (must implement [Clone])
• copy - copying, i.e. returning a copy of the field value (must implement [Copy])
• as_ref – only applicable if field value is optional; it makes the accessor to return an Option<&T> instead of &Option<T>

get_mut

Type: helper

Request for a mutable accessor. Since neither of additional options of get are applicable here² only basic helper sub-arguments are accepted.

Mutable accessors have the same name, as immutable ones, but with _mut suffix, unless given explicit name by the user:

#[fxstruct(get, get_mut)]
struct Foo {
    description: String,
}
let mut obj = Foo::new();
*obj.description_mut() = "some description".to_string();
assert_eq!(obj.description(), "some description");

set

Type: helper

Request for setter methods. If a literal string sub-argument is supplied it is used as setter method prefix instead of the default set_.

Takes an additional sub-argument:

• into: use the [Into] trait to automatically convert a value into the field type

#[fxstruct(set(into), get)]
struct Foo {
    description: String,
}
let mut obj = Foo::new();
obj.set_description("some description");
assert_eq!(obj.description(), &"some description".to_string());

reader, writer

Type: helper

Only meaningful for sync structs. Request for reader and writer methods that would return either read-only or read-write lock guards.

Akin to setters, method names are formed using read_ and write_ prefixes, correspondingly, prepended to the field name.

#[fxstruct(sync, reader, writer)]
struct Foo {
    description: String,
}
let obj = Foo::new();
{
    let mut wguard = obj.write_description();
    *wguard = String::from("let's use something different");
}
{
    let rguard = obj.read_description();
    assert_eq!(*rguard, "let's use something different".to_string());
}

See the section about differences between get/get_mut and reader/writer

lock

Type: flag

Forces lock-wrapping of all fields by default. Can be explicitly disabled with lock(off). Identical to the reader/writer arguments but without installing any methods.

clearer and predicate

Type: helper

These two are tightly coupled by their meaning, though can be used separately.

Predicate helper methods return [bool] and are the way to find out if a field is set. They're universal in the way that no matter wether a struct is sync or non-sync, or a field is lazy or just optional – you always use the same method.

Clearer helpers are the way to reset a field into uninitialized state. For optional fields it would simply mean it will contain [None]. A lazy field would be re-initialized the next time it is read from.

Clearers return the current field value. If field is already uninitialized (or never has been yet) None will be given back.

Using either of the two automatically make fields optional unless lazy.

Check out the example in the Optional Fields section.

optional

Type: keyword

Explicitly make all fields optional. Useful when neither predicate nor clearer helpers are needed.

public(...), private

Specify defaults for helpers. See the sub-arguments section above for more details.

clone, copy

Specify defaults for accessor helpers.

serde

Type: function

Enabled with serde feature, which is off by default.

Support for de/serialization will be discussed in more details in a section below. What is important to know at this point is that due to use of container types direct serialization of a struct is hardly possible. Therefore fieldx utilizes serde's into and from by creating a special shadow struct. The shadow, by default, is named after the original by prepending the name with double underscore and appending Shadow suffix: __FooShadow.

The following sub-arguments are supported:

• a string literal is used to give the shadow struct a user-specified name
• off disables de/serialization support altogether
• attributes(...) - custom attributes to be applied to the shadow struct
• public(...), private – specify visibility of the shadow struct
• serialize - enable or disable (serialize(off)) serialization support for the struct
• deserialize - enable or disable (deserialize(off)) deserialization support for the struct
• default - wether serde must use defaults for missing fields and, perhaps, where to take the defaults from\
• forward_attrs - a list of field attributes that are to be forwarded to the corresponding field of the shadow struct

Notes about default

Valid arguments for the sub-argument are:

• a string literal that has the same meaning as for the container-level serde attribute default
• a path to a symbol that is bound to an instance of our type: my_crate::FOO_DEFAULT
• a call-like path that'd be used literally: Self::serde_default()

The last option is preferable because fieldx will parse it and replace any found Self reference with the actual structure name making possible future renaming of it much easier.

There is a potentially useful "trick" in how default works. Internally, whatever type is returned by the sub-argument it gets converted into the shadow type with trait [Into]. This allows you to use the original struct as the trait implementation is automatically generated for it. See this example from a test:

#[cfg(feature = "serde")]
#[fxstruct(sync, get, serde("BazDup", default(Self::serde_default())))]
#[derive(Clone)]
pub(super) struct Baz {
    #[fieldx(reader)]
    f1: String,
    f2: String,
}

impl Baz {
    fn serde_default() -> Fubar {
        Fubar {
            postfix: "from fubar".into()
        }
    }
}

struct Fubar {
    postfix: String,
}

impl From<Fubar> for BazDup {
    fn from(value: Fubar) -> Self {
        Self {
            f1: format!("f1 {}", value.postfix),
            f2: format!("f2 {}", value.postfix),
        }
    }
}

let json_src = r#"{"f1": "f1 json"}"#;
let foo_de = serde_json::from_str::<Baz>(&json_src).expect("Bar deserialization failure");
assert_eq!(*foo_de.f1(), "f1 json".to_string());
assert_eq!(*foo_de.f2(), "f2 from fubar".to_string());

Arguments of fieldx

At this point, it's worth refreshing your memory about sub-arguments of helpers and how they differ in semantics between fxstruct and fieldx attributes.

skip

Type: flag

Leave this field alone. The only respected argument of fieldx when skipped is the default.

lazy

Type: helper

Mark field as lazy.

rename

Type: function

Specify alternative name for the field. The alternative will be used to form method names and, with serde feature enabled, serialization name³.

get, get_mut, set, reader, writer, clearer, predicate, optional

Type: helper

Have similar syntax and semantics to corresponding fxstruct arguments:

• get
• get_mut
• set
• reader and writer
• clearer
• predicate
• optional

optional

Type: keyword

Explicitly mark field as optional even if neither predicate nor clearer are requested.

public(...), private

Field-default visibility for helper methods. See the sub-arguments section above for more details.

serde

Type: function

At the field-level this option acts mostly the same way, as at the struct-level. With a couple of differences:

• string literal sub-argument is bypassed into serde field-level rename
• default is responsible for field default value; contrary to the struct-level, it doesn't use [Into] trait
• attributes will be applied to the field itself
• serialize/deserialize control field marshalling

into

Type: keyword

Sets default for set and builder arguments.

builder

Type: function

Mostly identical to the struct-level builder. Field specifics are:

• no attributes_impl (consumed, but ignored)
• string literal specifies setter method name if the builder type for this field
• attributes and attributes_fn are correspondingly applies to builder field and builder setter method

Do We Need The Default Trait?

Unless explicit default argument is used with the fxstruct attribute, fieldx tries to avoid implementing the Default trait unless really required. Here is the conditions which determine if the implementation is needed:

1. Method new is generated by the procedural macro.

   This is, actually, the default behavior which is disabled with no_new argument of the fxstruct attribute.

2. A field is given a default value.

3. The struct is sync and has a lazy field.

Why get/get_mut and reader/writer For Sync Structs?

It may be confusing at first as to why there are, basically, two different kinds of accessors for sync structs. But there are reasons for it.

First of all, let's take into account these important factors:

• fields, that are protected, cannot provide their values directly; lock-guards are required for this
• lazy fields are expected to always get some value when read from

Let's focus on a case of lazy fields. They have all properties of lock-protected and optional fields, so we loose nothing in the context of the get/get_mut and reader/writer differences.

get vs reader

A bare bones get accessor helper is the same thing, as the reader helper⁴. But, as soon as a user decides that they want copy or clone accessor behavior, reader becomes the only means of reaching out to field's lock-guard:

#[fxstruct(sync)]
struct Foo {
    #[fieldx(get(copy), reader, lazy)]
    bar: u32
}
impl Foo {
    fn build_bar(&self) -> u32 { 1234 }
    fn do_something(&self) -> u32 {
        // We need to protect the field value until we're done using it.
        let bar_guard = self.read_bar();
        let outcome = *bar_guard * 2;
        outcome
    }
}
let foo = Foo::new();
assert_eq!(foo.do_something(), 2468);

get_mut vs writer

This case if significantly different. Despite both helpers are responsible for mutating fields, the get_mut helper remains an accessor in first place, whereas the writer is not. In the context of lazy fields it means that get_mut guarantees the field to be initialized first. Then we can mutate its value.

writer, instead, provides direct and immediate access to the field's container. It allows to store a value into it without the builder method to be involved. Since building a lazy field can be expensive, it could be helpful to avoid it in cases when we don't actually need it⁵.

Basically, the guard returned by the writer helper can only do two things: store an entire value into the field, and clear the field.

#[fxstruct(sync)]
struct Foo {
    #[fieldx(get_mut, get(copy), writer, lazy)]
    bar: u32
}
impl Foo {
    fn build_bar(&self) -> u32 {
        eprintln!("Building bar");
        1234
    }
    fn do_something1(&self) {
        eprintln!("Using writer.");
        let mut bar_guard = self.write_bar();
        bar_guard.store(42);
    }
    fn do_something2(&self) {
        eprintln!("Using get_mut.");
        let mut bar_guard = self.bar_mut();
        *bar_guard = 12;
    }
}

let foo = Foo::new();
foo.do_something1();
assert_eq!(foo.bar(), 42);

let foo = Foo::new();
foo.do_something2();
assert_eq!(foo.bar(), 12);

This example is expected to output something like this:

Using writer.
Using get_mut.
Building bar

As you can see, use of the bar_mut accessor results in the build_bar method invoked.

The Inner Workings

As it was mentioned in the Basics section, fieldx rewrites structures with fxstruct applied. The following table reveals the final types of fields. T in the table represents the original field type, as specified by the user; O is the original struct type.

Field Parameters	Non-Sync Type	Sync Type
lazy	OnceCell<T>	[FXProxy<O, T>]
optional (also activated with clearer and proxy)	Option<T>	[FXRwLock<Option<T>>]
lock, reader and/or writer	N/A	[FXRwLock<T>]

Apparently, skipped fields retain their original type. Sure enough, if such a field is of non-Send or non-Sync type the entire struct would be missing these traits despite all the efforts from the fxstruct macro.

There is also a difference in how the initialization of lazy fields is implemented. Non-sync structs do it directly in their accessor methods. Sync structs delegate this functionality to the [FXProxy] type.

Traits

fieldx additionally implement traits FXStructNonSync and FXStructSync for corresponding kind of structs. Both traits are empty and only used to distinguish structs from non-fieldx ones and from each other. For both of them FXStruct is a super-trait.

Sync Primitives

The functionality of sync structs are backed by primitives provided by the parking_lot crate.

Support Of De-/Serialization With serde

Transparently de-/serializing container types is a non-trivial task. Luckily, serde allows us to use special parameters from and into to perform indirect marshalling via a shadow struct. The way this functionality implemented by serde (and it is for a good reason) requires our original struct to implement the [Clone] trait. fxstruct doesn't automatically add a #[derive(Clone)] because implementing the trait might require manual work from the user.

Normally one doesn't need to interfere with the marshalling process. But if such a need emerges then the following implementation details might be helpful to know about:

• shadow struct mirror-fields of lazy and optional originals are [Option]-wrapped
• the struct may be given a custom name using string literal sub-argument of the serde argument
• a shadow field may share its attributes with the original if they are listed in forward_attrs sub-argument of the serde argument
• forward_attrs is always applied to the fields, no matter if it is used with struct- or field-level serde argument
• if you need custom attributes applied to the shadow struct, use the attributes*-family of serde sub-arguments
• same is about non-shared field-level custom attributes: they are to be declared with field-level attributes* of serde

License

Licensed under the BSD 3-Clause License.

Footnotes

1. Apparently, the access has to be made by calling a corresponding method. Mostly it'd be field's accessor, but for sync structs it's more likely to be a reader. ↩

2. What sense is in having a mutable copy if you own it already? ↩

3. Unless a different alternative name is specified for serialization with serde argument. ↩

4. As a matter of fact, internally they even use the same method-generation code. ↩

5. Sometimes, if the value is known before a struct instance is created, it might make sense to use the builder instead of the writer. ↩