rustc_macros/
type_foldable.rs

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    if !s.ast().generics.lifetimes().any(|lt| lt.lifetime.ident == "tcx") {
10        s.add_impl_generic(parse_quote! { 'tcx });
11    }
12
13    s.add_bounds(synstructure::AddBounds::Generics);
14    s.bind_with(|_| synstructure::BindStyle::Move);
15    let try_body_fold = s.each_variant(|vi| {
16        let bindings = vi.bindings();
17        vi.construct(|_, index| {
18            let bind = &bindings[index];
19
20            // retain value of fields with #[type_foldable(identity)]
21            if has_ignore_attr(&bind.ast().attrs, "type_foldable", "identity") {
22                bind.to_token_stream()
23            } else {
24                quote! {
25                    ::rustc_middle::ty::TypeFoldable::try_fold_with(#bind, __folder)?
26                }
27            }
28        })
29    });
30
31    let body_fold = s.each_variant(|vi| {
32        let bindings = vi.bindings();
33        vi.construct(|_, index| {
34            let bind = &bindings[index];
35
36            // retain value of fields with #[type_foldable(identity)]
37            if has_ignore_attr(&bind.ast().attrs, "type_foldable", "identity") {
38                bind.to_token_stream()
39            } else {
40                quote! {
41                    ::rustc_middle::ty::TypeFoldable::fold_with(#bind, __folder)
42                }
43            }
44        })
45    });
46
47    s.bound_impl(
48        quote!(::rustc_middle::ty::TypeFoldable<::rustc_middle::ty::TyCtxt<'tcx>>),
49        quote! {
50            fn try_fold_with<__F: ::rustc_middle::ty::FallibleTypeFolder<::rustc_middle::ty::TyCtxt<'tcx>>>(
51                self,
52                __folder: &mut __F
53            ) -> Result<Self, __F::Error> {
54                Ok(match self { #try_body_fold })
55            }
56
57            fn fold_with<__F: ::rustc_middle::ty::TypeFolder<::rustc_middle::ty::TyCtxt<'tcx>>>(
58                self,
59                __folder: &mut __F
60            ) -> Self {
61                match self { #body_fold }
62            }
63        },
64    )
65}
66
67fn has_ignore_attr(attrs: &[syn::Attribute], name: &'static str, meta: &'static str) -> bool {
68    let mut ignored = false;
69    attrs.iter().for_each(|attr| {
70        if !attr.path().is_ident(name) {
71            return;
72        }
73        let _ = attr.parse_nested_meta(|nested| {
74            if nested.path.is_ident(meta) {
75                ignored = true;
76            }
77            Ok(())
78        });
79    });
80
81    ignored
82}
rustc_macros/type_foldable.rs

rustc_macros/
type_foldable.rs
