tryvial_proc/
lib.rs

1use proc_macro::TokenStream;
2use proc_macro2::TokenStream as TokenStream2;
3use quote::quote;
4use venial::{Declaration, Error, Function};
5
6/// An attribute macro that performs "Ok-wrapping" on the return value of a `fn` item.
7/// This is compatible with [`Result`], [`Option`], [`ControlFlow`], and any type that
8/// implements the unstable [`std::ops::Try`] trait.
9///
10/// Using this macro is equivalent to wrapping the body of a fn in a try block.
11///
12/// Nightly:
13/// ```ignore
14/// fn fallible_fn(x: T) -> Result<U, E> {
15///     try {
16///         let a = do_one(x)?;
17///         let b = do_two(a)?;
18///         b
19///     }
20/// }
21/// ```
22///
23/// With `try_fn`:
24/// ```ignore
25/// #[try_fn]
26/// fn fallible_fn(x: T) -> Result<U, E> {
27///     let a = do_one(x)?;
28///     let b = do_two(a)?;
29///     b
30/// }
31/// ```
32///
33/// [`ControlFlow`]: core::ops::ControlFlow
34#[proc_macro_attribute]
35pub fn try_fn(_attr: TokenStream, item: TokenStream) -> TokenStream {
36    impl_try_fn(item.into())
37        .unwrap_or_else(|e| e.to_compile_error())
38        .into()
39}
40
41#[proc_macro_attribute]
42#[deprecated(note = "renamed to `try_fn`")]
43pub fn tryvial(_attr: TokenStream, item: TokenStream) -> TokenStream {
44    impl_try_fn(item.into())
45        .unwrap_or_else(|e| e.to_compile_error())
46        .into()
47}
48
49fn impl_try_fn(input: TokenStream2) -> Result<TokenStream2, Error> {
50    let decl = venial::parse_declaration(input)?;
51    let Function {
52        attributes,
53        vis_marker,
54        qualifiers,
55        tk_fn_keyword,
56        name,
57        generic_params,
58        tk_params_parens: _,
59        params,
60        where_clause,
61        tk_return_arrow: _,
62        return_ty,
63        tk_semicolon: _,
64        body,
65    } = match decl {
66        Declaration::Function(item) => item,
67        _ => Err(Error::new("`#[try_fn]` is supported only on `fn` items"))?,
68    };
69
70    let body = body.ok_or(Error::new(
71        "`#[try_fn]` can only be used on functions with a body",
72    ))?;
73
74    let return_ty = return_ty.map_or_else(|| quote! { () }, |ty| quote! { #ty });
75
76    Ok(quote! {
77        #(#attributes)*
78        #vis_marker #qualifiers #tk_fn_keyword #name #generic_params ( #params ) -> #return_ty
79        #where_clause
80        {
81            ::core::iter::empty().try_fold(#body, |_, __x: ::core::convert::Infallible| match __x {})
82        }
83    })
84}
tryvial_proc/lib.rs

tryvial_proc/
lib.rs
