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//! Macro to specialize on the type of an expression. //! //! This crate implements *auto(de)ref specialization*: //! A trick to do specialization in non-generic contexts on stable Rust. //! //! For the details of this technique, see: //! - [*Autoref-based stable specialization* by David Tolnay][autoref] //! - [*Generalized Autoref-Based Specialization* by Lukas Kalbertodt][autoderef] //! //! [autoref]: https://github.com/dtolnay/case-studies/blob/master/autoref-specialization/README.md //! [autoderef]: http://lukaskalbertodt.github.io/2019/12/05/generalized-autoref-based-specialization.html //! //! # What it can and cannot do //! //! The auto(de)ref technique—and therefore this macro—is useless in generic //! functions, as Rust resolves the specialization based on the bounds defined //! on the generic context, not based on the actual type when instantiated. //! (See [the example below](#in-a-generic-function) for a demonstration of //! this.) //! //! In non-generic contexts, it's also mostly useless, as you probably already //! know the exact type of all variables. //! //! The only place where using this can make sense is in the implementation of //! macros that need to have different behaviour depending on the type of a //! value passed to it. For example, a macro that prints the `Debug` output of //! a value, but falls back to a default when it doesn't implement `Debug`. //! (See [the example below](#in-a-macro) for a demonstration of //! that.) //! //! # How to use it //! //! The basic syntax of the macro is: //! //! ```text //! spez! { //! for <expression>; //! match <type> { <body> } //! [match <type> { <body> }] //! [...] //! } //! ``` //! //! The examples below show more details. //! //! ## Simple specialization //! //! In the most simple case, you use this macro to match specific types: //! //! ``` //! # use spez::spez; //! let x = 0; //! spez! { //! for x; //! match i32 { //! println!("x is a 32-bit integer!"); //! } //! match &str { //! println!("x is a string slice!"); //! assert!(false); //! } //! } //! ``` //! //! ## Return types //! //! Values can be returned from the matches, but have to be explicitly //! specified for each `match`. They do not have to be the same for every //! `match`. //! //! ``` //! # use spez::spez; //! let x = 0; //! let result = spez! { //! for x; //! match i32 -> &'static str { //! "x is a 32-bit integer!" //! } //! match &str -> i32 { //! 123 //! } //! }; //! assert_eq!(result, "x is a 32-bit integer!"); //! ``` //! //! ## Generic matches //! //! Generic matches are also possible. Generic variables can be defined //! on the `match`, and a `where` clause can be added after the type. //! //! The matches are tried in order. The first matches get priority over later //! ones, even if later ones are perfect matches. //! //! ``` //! # use spez::spez; //! let x = 123i32; //! let result = spez! { //! for x; //! match<T> T where i8: From<T> -> i32 { //! 0 //! } //! match<T: std::fmt::Debug> T -> i32 { //! 1 //! } //! match i32 -> i32 { //! 2 //! } //! }; //! assert_eq!(result, 1); //! ``` //! //! # Consuming the input //! //! The input (after the `for`) is consumed and made available to the `match` //! bodies. //! //! (If you don't want to consume the input, take a reference and also prepend //! a `&` to the types you're matching.) //! //! ``` //! # use spez::spez; //! # use core::ops::Deref; //! let x = Box::new(123); //! let result = spez! { //! for x; //! match<T: Deref<Target = i32>> T -> i32 { //! *x //! } //! match i32 -> i32 { //! x //! } //! }; //! assert_eq!(result, 123); //! ``` //! //! # Expressions as input //! //! Not just variable names, but full expressions can be given as input. //! However, if you want to refer to them from the match bodies, you need to //! prepend `name =` to give the input a name. //! //! ``` //! # use spez::spez; //! let result = spez! { //! for 1 + 1; //! match i32 -> i32 { 0 } //! match i64 -> i32 { 1 } //! }; //! assert_eq!(result, 0); //! ``` //! //! ``` //! # use spez::spez; //! let result = spez! { //! for x = 1 + 1; //! match i32 -> i32 { x } //! match i64 -> i32 { 1 } //! }; //! assert_eq!(result, 2); //! ``` //! //! # Capturing variables //! //! Unfortunately, you can't refer to variables of the scope around the `spec! {}` macro: //! //! ```compile_fail //! let a = 1; //! let result = spez! { //! for x = 1; //! match i32 { //! println!("{}", a); // ERROR //! } //! }; //! ``` //! //! # In a generic function //! //! As mentioned above, the macro is of not much use in generic context, as the //! specialization is resolved based on the bounds rather than on the actual //! type in the instantiation of the generic function: //! //! ``` //! # use spez::spez; //! # use std::fmt::Debug; //! fn f<T: Debug>(v: T) -> &'static str { //! spez! { //! for v; //! match i32 -> &'static str { //! ":)" //! } //! match<T: Debug> T -> &'static str { //! ":(" //! } //! match<T> T -> &'static str { //! ":((" //! } //! } //! } //! assert_eq!(f(0i32), ":("); //! ``` //! //! # In a macro //! //! This is a demonstration of a macro that prints the `Debug` output of a //! value, but falls back to `"<object of type ...>"` if it doesn't implement //! `Debug`. //! //! ``` //! # use spez::spez; //! # use std::fmt::Debug; //! macro_rules! debug { //! ($e:expr) => { //! spez! { //! for x = $e; //! match<T: Debug> T { //! println!("{:?}", x); //! } //! match<T> T { //! println!("<object of type {}>", std::any::type_name::<T>()); //! } //! } //! } //! } //! debug!(123); //! # struct NoDebugType; //! debug!(NoDebugType); //! ``` #![no_std] use proc_macro_hack::proc_macro_hack; /// Specialize based on the type of an expression. /// /// See the [crate level documentation](index.html). #[proc_macro_hack] pub use spez_macros::spez;