match_wrap/
lib.rs

1use proc_macro::TokenStream;
2use quote::quote;
3use syn::{
4    Attribute, Expr, Pat, Result as SynResult, Token, Type,
5    parse::{Parse, ParseStream},
6    parse_macro_input,
7};
8
9struct MatchWrapInput {
10    trait_type: Type,
11    expr: Expr,
12    arms: Vec<(Vec<Attribute>, Pat, Expr)>,
13}
14
15impl Parse for MatchWrapInput {
16    fn parse(input: ParseStream) -> SynResult<Self> {
17        let trait_type = input.parse()?;
18        input.parse::<Token![;]>()?;
19
20        let expr = input.parse()?;
21        input.parse::<Token![;]>()?;
22
23        let mut arms = Vec::new();
24        while !input.is_empty() {
25            let attrs = input.call(syn::Attribute::parse_outer)?;
26            let pat = Pat::parse_single(input)?;
27            input.parse::<Token![=>]>()?;
28            let arm_expr = input.parse()?;
29            arms.push((attrs, pat, arm_expr));
30
31            if input.peek(Token![,]) {
32                input.parse::<Token![,]>()?;
33            }
34        }
35
36        Ok(MatchWrapInput {
37            trait_type,
38            expr,
39            arms,
40        })
41    }
42}
43
44#[proc_macro]
45pub fn match_box(input: TokenStream) -> TokenStream {
46    const DIVERGE_ATTR: &str = "diverges";
47
48    let input = parse_macro_input!(input as MatchWrapInput);
49    let trait_type = &input.trait_type;
50    let expr = &input.expr;
51
52    let arms = input.arms.iter().map(|(attrs, pat, arm_expr)| {
53        let is_diverging = attrs.iter().any(|attr| attr.path().is_ident(DIVERGE_ATTR));
54
55        if is_diverging {
56            quote! {#pat => #arm_expr}
57        } else {
58            quote! { #pat => ::std::boxed::Box::new(#arm_expr) as ::std::boxed::Box<#trait_type> }
59        }
60    });
61
62    quote! { match #expr { #(#arms,)* } }.into()
63}
match_wrap/lib.rs

match_wrap/
lib.rs
