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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 `spez! {}` 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);
//! ```
extern crate proc_macro;
mod parse;
use parse::Args;
use proc_macro::TokenStream;
use proc_macro2::Span;
use proc_macro2::TokenStream as TokenStream2;
use quote::quote;
/// Specialize based on the type of an expression.
///
/// See the [crate level documentation](index.html).
#[proc_macro]
pub fn spez(tokens: TokenStream) -> TokenStream {
spez_impl(syn::parse_macro_input!(tokens)).into()
}
fn refs(n: usize) -> TokenStream2 {
let mut refs = TokenStream2::new();
for _ in 0..n {
refs.extend(quote![&]);
}
refs
}
fn spez_impl(args: Args) -> TokenStream2 {
let mut traits = TokenStream2::new();
let param_def = match args.param {
Some(param) => quote! {
let #param = self.0.take().unwrap();
let _ = #param; // Suppress unused variable warning.
},
None => quote! {},
};
let n_arms = args.arms.len();
for (i, arm) in args.arms.into_iter().enumerate() {
let name = syn::Ident::new(&format!("Match{}", i + 1), Span::call_site());
let body = arm.body;
let ty = arm.ty;
let generics = &arm.generics;
let where_clause = &arm.generics.where_clause;
let refs = refs(n_arms - i - 1);
let return_type = match arm.return_type {
Some(return_type) => quote! { #return_type },
None => quote! { () },
};
traits.extend(quote! {
trait #name {
type Return;
fn spez(&self) -> Self::Return;
}
impl #generics #name for #refs Match<#ty> #where_clause {
type Return = #return_type;
fn spez(&self) -> Self::Return {
#param_def
#body
}
}
});
}
let expr = args.expr;
let refs = refs(n_arms);
quote! {
{
struct Match<T>(core::cell::Cell<Option<T>>);
#traits
(#refs Match(core::cell::Cell::new(Some(#expr)))).spez()
}
}
}