rustc_macros/
type_foldable.rs

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1use quote::{ToTokens, quote};
2use syn::parse_quote;
3
4pub(super) fn type_foldable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::TokenStream {
5    if let syn::Data::Union(_) = s.ast().data {
6        panic!("cannot derive on union")
7    }
8
9    s.underscore_const(true);
10
11    if !s.ast().generics.lifetimes().any(|lt| lt.lifetime.ident == "tcx") {
12        s.add_impl_generic(parse_quote! { 'tcx });
13    }
14
15    s.add_bounds(synstructure::AddBounds::Generics);
16    s.bind_with(|_| synstructure::BindStyle::Move);
17    let body_fold = s.each_variant(|vi| {
18        let bindings = vi.bindings();
19        vi.construct(|_, index| {
20            let bind = &bindings[index];
21
22            let mut fixed = false;
23
24            // retain value of fields with #[type_foldable(identity)]
25            bind.ast().attrs.iter().for_each(|x| {
26                if !x.path().is_ident("type_foldable") {
27                    return;
28                }
29                let _ = x.parse_nested_meta(|nested| {
30                    if nested.path.is_ident("identity") {
31                        fixed = true;
32                    }
33                    Ok(())
34                });
35            });
36
37            if fixed {
38                bind.to_token_stream()
39            } else {
40                quote! {
41                    ::rustc_middle::ty::fold::TypeFoldable::try_fold_with(#bind, __folder)?
42                }
43            }
44        })
45    });
46
47    s.bound_impl(
48        quote!(::rustc_middle::ty::fold::TypeFoldable<::rustc_middle::ty::TyCtxt<'tcx>>),
49        quote! {
50            fn try_fold_with<__F: ::rustc_middle::ty::fold::FallibleTypeFolder<::rustc_middle::ty::TyCtxt<'tcx>>>(
51                self,
52                __folder: &mut __F
53            ) -> Result<Self, __F::Error> {
54                Ok(match self { #body_fold })
55            }
56        },
57    )
58}
rustc_macros/type_foldable.rs

rustc_macros/
type_foldable.rs
