rustc_macros/
try_from.rs

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1use proc_macro2::TokenStream;
2use quote::{quote, quote_spanned};
3use syn::Data;
4use syn::spanned::Spanned;
5use synstructure::Structure;
6
7pub(crate) fn try_from_u32(s: Structure<'_>) -> TokenStream {
8    let span_error = |span, message: &str| {
9        quote_spanned! { span => const _: () = ::core::compile_error!(#message); }
10    };
11
12    // Must be applied to an enum type.
13    if let Some(span) = match &s.ast().data {
14        Data::Enum(_) => None,
15        Data::Struct(s) => Some(s.struct_token.span()),
16        Data::Union(u) => Some(u.union_token.span()),
17    } {
18        return span_error(span, "type is not an enum (TryFromU32)");
19    }
20
21    // The enum's variants must not have fields.
22    let variant_field_errors = s
23        .variants()
24        .iter()
25        .filter_map(|v| v.ast().fields.iter().map(|f| f.span()).next())
26        .map(|span| span_error(span, "enum variant cannot have fields (TryFromU32)"))
27        .collect::<TokenStream>();
28    if !variant_field_errors.is_empty() {
29        return variant_field_errors;
30    }
31
32    let ctor = s
33        .variants()
34        .iter()
35        .map(|v| v.construct(|_, _| -> TokenStream { unreachable!() }))
36        .collect::<Vec<_>>();
37    // FIXME(edition_2024): Fix the `keyword_idents_2024` lint to not trigger here?
38    #[allow(keyword_idents_2024)]
39    s.gen_impl(quote! {
40        // The surrounding code might have shadowed these identifiers.
41        use ::core::convert::TryFrom;
42        use ::core::primitive::u32;
43        use ::core::result::Result::{self, Ok, Err};
44
45        gen impl TryFrom<u32> for @Self {
46            type Error = u32;
47
48            #[allow(deprecated)] // Don't warn about deprecated variants.
49            fn try_from(value: u32) -> Result<Self, Self::Error> {
50                #( if value == const { #ctor as u32 } { return Ok(#ctor) } )*
51                Err(value)
52            }
53        }
54    })
55}
rustc_macros/try_from.rs

rustc_macros/
try_from.rs
