impl_tools/

lib.rs

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License in the LICENSE-APACHE file or at:
//     https://www.apache.org/licenses/LICENSE-2.0

#![allow(clippy::needless_doctest_main)]
// Lint advocates use of bool::then_some, stablizied in rustc 1.62.0
#![allow(clippy::unnecessary_lazy_evaluations)]

//! # Impl-tools
//!
//! [`#[autoimpl]`](macro@autoimpl) is an alternative to
//! [`#[derive]`](macro@derive) with more features (also usable on traits).
//!
//! [`#[impl_default]`](macro@impl_default) is shorthand for implementing
//! [`Default`] with an explicit default value.
//! It supports structs and enums.
//!
//! [`impl_scope!`] is a function-like macro used to define a type together with
//! its implementations. This allows:
//!
//! -   `impl Self` syntax (avoid repeated definitions of generics)
//! -   Evaluation of some more complex attribute macros
//!
//! [`impl_anon!`] is a function-like macro used to define and instantiate a
//! unique (single-use) type. It supports everything supported by [`impl_scope!`]
//! plus field initializers and (limited) automatic typing of fields.
//!
//! User-extensions to both [`#[autoimpl]`](macro@autoimpl) and [`impl_scope!`]
//! are possible with a custom proc-macro crate depending on
//! [impl-tools-lib](https://crates.io/crates/impl-tools-lib).

#[cfg(doctest)]
doc_comment::doctest!("../README.md");

extern crate proc_macro;

use lib::{anon, scope};
use proc_macro::TokenStream;
use proc_macro_error2::{emit_call_site_error, proc_macro_error};
use syn::parse_macro_input;

use impl_tools_lib::{self as lib, autoimpl};

/// Impl [`Default`] with given field or type initializers
///
/// This macro may be used in one of two ways.
///
/// ### Type-level initializer
///
/// ```
/// # use impl_tools::impl_default;
/// /// A simple enum; default value is Blue
/// #[impl_default(Colour::Blue)]
/// enum Colour {
///     Red,
///     Green,
///     Blue,
/// }
///
/// fn main() {
///     assert!(matches!(Colour::default(), Colour::Blue));
/// }
/// ```
///
/// A where clause is optional: `#[impl_default(EXPR where BOUNDS)]`.
///
/// ### Field-level initializer
///
/// This variant only supports structs. Fields specified as `name: type = expr`
/// will be initialized with `expr`, while other fields will be initialized with
/// `Default::default()`.
///
/// ```
/// # use impl_tools::{impl_default, impl_scope};
/// impl_scope! {
///     #[impl_default]
///     struct Person {
///         name: String = "Jane Doe".to_string(),
///         age: u32 = 72,
///         occupation: String,
///     }
/// }
///
/// fn main() {
///     let person = Person::default();
///     assert_eq!(person.name, "Jane Doe");
///     assert_eq!(person.age, 72);
///     assert_eq!(person.occupation, "");
/// }
/// ```
///
/// A where clause is optional: `#[impl_default(where BOUNDS)]`.
#[proc_macro_attribute]
#[proc_macro_error]
pub fn impl_default(args: TokenStream, item: TokenStream) -> TokenStream {
    let mut toks = item.clone();
    match syn::parse::<lib::ImplDefault>(args) {
        Ok(attr) => toks.extend(TokenStream::from(attr.expand(item.into()))),
        Err(err) => {
            emit_call_site_error!(err);
            // Since this form of invocation only adds implementations, we can
            // safely output the original item, thus reducing secondary errors.
        }
    }
    toks
}

/// An alternative to the standard [`macro@derive`] macro
///
/// This macro may be used:
///
/// -   [On a type definition](#on-type-definitions), to implement a specified trait
/// -   [On a trait definition](#on-trait-definitions), to implement the trait for specified types
///     supporting [`Deref`]
///
/// # On type definitions
///
/// `#[autoimpl]` on type definitions functions similarly to [`#[derive]`](macro@derive). The differences are as follows.
///
/// There is no implied bound on generic parameters. Instead, bounds must be specified explicitly, using syntax like `where T: Clone`. The special syntax `where T: trait` may be used where `trait` desugars to the target trait for each implementation. An example:
/// ```
/// # use impl_tools::autoimpl;
/// #[autoimpl(Clone, Debug where T: trait)]
/// struct Wrapper<T>(pub T);
/// ```
///
/// ### `ignore`
///
/// Traits like [`Debug`] may be implemented while `ignore`-ing some fields, for example:
/// ```
/// # use impl_tools::autoimpl;
/// #[autoimpl(Debug ignore self.f)]
/// struct PairWithFn<T> {
///     x: f32,
///     y: f32,
///     f: fn(&T),
/// }
/// ```
///
/// ### `using`
///
/// Traits like [`Deref`] may be implemented by `using` a named field, for example:
/// ```
/// # use impl_tools::autoimpl;
/// #[autoimpl(Deref, DerefMut using self.1)]
/// struct AnnotatedWrapper<T>(String, T);
/// ```
/// In the above example, [`Deref::Target`] will be implemented as `T` (the type
/// of the field `self.1`). The `Target` type may instead be specified explicitly:
/// ```
/// # use impl_tools::autoimpl;
/// #[autoimpl(Deref<Target = T> using self.0)]
/// struct MyBoxingWrapper<T: ?Sized>(Box<T>);
/// ```
///
/// ## Supported traits
///
/// | Path | *ignore* | *using* | *notes* |
/// |----- |--- |--- |--- |
/// | [`::core::borrow::Borrow<T>`] | - | borrow target | `T` is type of target field |
/// | [`::core::borrow::BorrowMut<T>`] | - | borrow target | `T` is type of target field |
/// | [`::core::clone::Clone`] | yes | - | ignored fields use `Default::default()` |
/// | [`::core::cmp::Eq`] | * | - | *allowed with `PartialEq` |
/// | [`::core::cmp::Ord`] | yes | - | |
/// | [`::core::cmp::PartialEq`] | yes | - | |
/// | [`::core::cmp::PartialOrd`] | yes | - | |
/// | [`::core::convert::AsRef<T>`] | - | ref target | `T` is type of target field |
/// | [`::core::convert::AsMut<T>`] | - | ref target | `T` is type of target field |
/// | [`::core::default::Default`] | - | - | [`macro@impl_default`] is a more flexible alternative |
/// | [`::core::fmt::Debug`] | yes | - | |
/// | [`::core::hash::Hash`] | yes | - | |
/// | [`::core::marker::Copy`] | * | - | *allowed with `Clone` |
/// | [`::core::ops::Deref`] | - | deref target | See [`Deref::Target` type](#dereftarget-type) below |
/// | [`::core::ops::DerefMut`] | - | deref target | |
///
/// Traits are matched using the path, as follows:
///
/// -   Only the last component, e.g. `#[autoimpl(Clone)]`
/// -   The full path with leading `::`, e.g. `#[autoimpl(::core::clone::Clone)]`
/// -   The full path without leading `::`, e.g. `#[autoimpl(core::clone::Clone)]`
/// -   The full path with/without leading `::`, using `std` instead of `core` or `alloc`,
///     e.g. `#[autoimpl(std::clone::Clone)]`
///
/// ## Parameter syntax
///
/// > _ParamsMulti_ :\
/// > &nbsp;&nbsp; ( _Trait_ ),+ _Using_? _Ignores_? _WhereClause_?
/// >
/// > _Using_ :\
/// > &nbsp;&nbsp; `using` `self` `.` _Member_
/// >
/// > _Ignores_ :\
/// > &nbsp;&nbsp; `ignore` ( `self` `.` _Member_ ),+
/// >
/// > _WhereClause_ :\
/// > &nbsp;&nbsp; `where` ( _WherePredicate_ ),*
///
/// **Targets:** each *Trait* listed is implemented for the annotated type.
///
///
/// # On trait definitions
///
/// `#[autoimpl]` on trait definitions generates an implementation of that trait
/// for the given targets. This functions using an implementation of [`Deref`]
/// (and, where required, [`DerefMut`]) to lower the target type to some other
/// type supporting the trait. We call this latter type the **definitive type**.
///
/// It is required that the target type(s) implemented are generic over some
/// type parameter(s). These generic parameters are introduced using `for<..>`.
/// It is further required that at least one generic parameter has a bound on
/// `trait`; the first such parameter is inferred to be the *definitive type*.
///
/// For example, the following usage implements `MyTrait` for targets `&T`,
/// `&mut T` and `Box<dyn MyTrait>` using definitive type `T`:
/// ```
/// # use impl_tools::autoimpl;
/// #[autoimpl(for<T: trait + ?Sized> &T, &mut T, Box<T>)]
/// trait MyTrait {
///     fn f(&self) -> String;
/// }
/// ```
/// The expansion for target `Box<T>` looks like:
/// ```
/// # trait MyTrait {
/// #     fn f(&self) -> String;
/// # }
/// #[automatically_derived]
/// impl<T: MyTrait + ?Sized> MyTrait for Box<T> {
///     fn f(&self) -> String {
///         <T as MyTrait>::f(self)
///     }
/// }
/// ```
///
/// ## Generics
///
/// Traits using generics and trait items using generics are, for the most part,
/// supported.
///
/// Items with a where clause with a type bound on `Self` are not supported
/// since the item is not guaranteed to exist on the definitive type.
/// Exception: methods with a default implementation (in this case the item is
/// skipped).
///
/// An example:
/// ```
/// # use impl_tools::autoimpl;
/// # use std::fmt::Debug;
/// #[autoimpl(for<'a, T> &'a T, &'a mut T, Box<T> where T: trait + ?Sized)]
/// trait G<V>
/// where
///     V: Debug,
/// {
///     fn g(&self) -> V;
///
///     fn s<X>(&self, f: impl Fn(V) -> X) -> X
///     where
///         Self: Sized,
///     {
///         f(self.g())
///     }
/// }
/// ```
///
/// ## Parameter syntax
///
/// > _ParamsTrait_ :\
/// > &nbsp;&nbsp; `for` _Generics_ ( _Type_ ),+ _WhereClause_?
///
/// [`Deref`]: std::ops::Deref
/// [`Deref::Target`]: std::ops::Deref::Target
/// [`DerefMut`]: std::ops::DerefMut
#[proc_macro_attribute]
#[proc_macro_error]
pub fn autoimpl(attr: TokenStream, item: TokenStream) -> TokenStream {
    let mut toks = item.clone();
    match syn::parse::<autoimpl::Attr>(attr) {
        Ok(autoimpl::Attr::ForDeref(ai)) => toks.extend(TokenStream::from(ai.expand(item.into()))),
        Ok(autoimpl::Attr::ImplTraits(ai)) => {
            // We could use lazy_static to construct a HashMap for fast lookups,
            // but given the small number of impls a "linear map" is fine.
            let find_impl = |path: &syn::Path| {
                autoimpl::STD_IMPLS
                    .iter()
                    .cloned()
                    .find(|impl_| impl_.path().matches_ident_or_path(path))
            };
            toks.extend(TokenStream::from(ai.expand(item.into(), find_impl)))
        }
        Err(err) => {
            emit_call_site_error!(err);
            // Since autoimpl only adds implementations, we can safely output
            // the original item, thus reducing secondary errors.
        }
    }
    toks
}

/// Implement a type with `impl Self` syntax
///
/// This macro facilitates definition of a type (struct, enum or union) plus
/// implementations via `impl Self { .. }` syntax: `Self` is expanded to the
/// type's name, including generics and bounds (as defined on the type).
///
/// Caveat: `rustfmt` can not yet format contents (see
/// [rustfmt#5254](https://github.com/rust-lang/rustfmt/issues/5254),
/// [rustfmt#5538](https://github.com/rust-lang/rustfmt/pull/5538)).
///
/// ## Special attribute macros
///
/// Additionally, `impl_scope!` supports special attribute macros evaluated
/// within its scope:
///
/// -   [`#[impl_default]`](macro@impl_default): implement [`Default`] using
///     field initializers (which are not legal syntax outside of `impl_scope!`)
///
/// Note: matching these macros within `impl_scope!` does not use path
/// resolution. Using `#[impl_tools::impl_default]` would resolve the variant
/// of this macro which *doesn't support* field initializers.
///
/// ## Syntax
///
/// > _ImplScope_ :\
/// > &nbsp;&nbsp; `impl_scope!` `{` _ScopeItem_ _ItemImpl_ * `}`
/// >
/// > _ScopeItem_ :\
/// > &nbsp;&nbsp; _ItemEnum_ | _ItemStruct_ | _ItemType_ | _ItemUnion_
///
/// That is, one type definition followed by a set of implementations.
/// Impls must take one of two forms:
///
/// -   `impl Self { ... }` — generic parameters and bounds of the type are used
/// -   `impl MyType { ... }` where `MyType` matches the name of the defined type
///
/// Generic parameters from the type are included implicitly with the first form.
/// Additional generic parameters and where clauses are supported (parameters
/// and bounds are merged).
///
/// ## Example
///
/// ```
/// impl_tools::impl_scope! {
///     struct Pair<T>(T, T);
///
///     impl Self {
///         pub fn new(a: T, b: T) -> Self {
///             Pair(a, b)
///         }
///     }
///
///     impl Self where T: Clone {
///         pub fn splat(a: T) -> Self {
///             let b = a.clone();
///             Pair(a, b)
///         }
///     }
/// }
/// ```
#[proc_macro_error]
#[proc_macro]
pub fn impl_scope(input: TokenStream) -> TokenStream {
    let mut scope = parse_macro_input!(input as scope::Scope);
    scope.apply_attrs(scope::find_impl_default);
    scope.expand().into()
}

/// Construct an anonymous struct
///
/// Rust doesn't currently support [`impl Trait { ... }` expressions](https://github.com/canndrew/rfcs/blob/impl-trait-expressions/text/0000-impl-trait-expressions.md)
/// or implicit typing of struct fields. This macro is a **hack** allowing that.
///
/// Example:
/// ```
/// use std::fmt;
/// fn main() {
///     let world = "world";
///     let says_hello_world = impl_tools::impl_anon! {
///         struct(&'static str = world);
///         impl fmt::Display for Self {
///             fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
///                 write!(f, "hello {}", self.0)
///             }
///         }
///     };
///     assert_eq!(format!("{}", says_hello_world), "hello world");
/// }
/// ```
///
/// That is, this macro creates an anonymous struct type (must be a struct),
/// which may have trait implementations, then creates an instance of that
/// struct.
///
/// Struct fields may have a fixed type or may be generic. Syntax is as follows:
///
/// -   **regular struct:** `ident: ty = value`
/// -   **regular struct:** `ident: ty` (uses `Default` to construct value)
/// -   **regular struct:** `ident = value` (type is generic without bounds)
/// -   **tuple struct:** `ty = value`
/// -   **tuple struct:** `ty` (uses `Default` to construct value)
///
/// The field name, `ident`, may be `_` (anonymous field).
///
/// The field type, `ty`, may be or may contain inferred types (`_`) and/or
/// `impl Trait` type expressions. These are substituted with generics on the
/// type.
///
/// Refer to [examples](https://github.com/search?q=impl_anon+repo%3Akas-gui%2Fkas+path%3Aexamples&type=Code) for usage.
#[proc_macro_error]
#[proc_macro]
pub fn impl_anon(input: TokenStream) -> TokenStream {
    let mut scope = parse_macro_input!(input as anon::Anon).into_scope();
    scope.apply_attrs(scope::find_impl_default);
    scope.expand().into()
}