Derive traversables over generic interners

Author: eggyal

compiler/rustc_macros/src/lib.rs

@@ -141,13 +141,12 @@ decl_derive!(
     /// such derived implementations are only applicable if the annotated type does not contain
     /// anything that may be of interest to a folder.
     ///
-    /// If the annotated type has a `'tcx` lifetime parameter, then that will be used as the
-    /// lifetime for the type context/interner; otherwise the lifetime of the type context/interner
-    /// will be unrelated to the annotated type. It therefore matters how any lifetime parameters of
-    /// the annotated type are named. For example, deriving `TypeFoldable` for both `Foo<'a>` and
-    /// `Bar<'tcx>` will respectively produce:
+    /// The derived implementation will use `TyCtxt<'tcx>` as the interner iff the annotated type
+    /// has a `'tcx` lifetime parameter; otherwise it will be generic over all interners. It
+    /// therefore matters how any lifetime parameters of the annotated type are named. For example,
+    /// deriving `TypeFoldable` for both `Foo<'a>` and `Bar<'tcx>` will respectively produce:
     ///
-    /// `impl<'a, 'tcx> TypeFoldable<TyCtxt<'tcx>> for Foo<'a>`
+    /// `impl<'a, I: Interner> TypeFoldable<I> for Foo<'a>`
     ///
     /// and
     ///
@@ -176,13 +175,12 @@ decl_derive!(
     /// above, such derived implementations are only applicable if the annotated type does not
     /// contain anything that may be of interest to visitors.
     ///
-    /// If the annotated type has a `'tcx` lifetime parameter, then that will be used as the
-    /// lifetime for the type context/interner; otherwise the lifetime of the type context/interner
-    /// will be unrelated to the annotated type. It therefore matters how any lifetime parameters of
-    /// the annotated type are named. For example, deriving `TypeVisitable` for both `Foo<'a>` and
-    /// `Bar<'tcx>` will respectively produce:
+    /// The derived implementation will use `TyCtxt<'tcx>` as the interner iff the annotated type
+    /// has a `'tcx` lifetime parameter; otherwise it will be generic over all interners. It
+    /// therefore matters how any lifetime parameters of the annotated type are named. For example,
+    /// deriving `TypeVisitable` for both `Foo<'a>` and `Bar<'tcx>` will respectively produce:
     ///
-    /// `impl<'a, 'tcx> TypeVisitable<TyCtxt<'tcx>> for Foo<'a>`
+    /// `impl<'a, I: Interner> TypeVisitable<I> for Foo<'a>`
     ///
     /// and
     ///

compiler/rustc_macros/src/traversable.rs

@@ -1,7 +1,7 @@
 use proc_macro2::{Span, TokenStream};
 use quote::{quote, quote_spanned, ToTokens};
 use std::collections::HashMap;
-use syn::{parse_quote, spanned::Spanned, Attribute, Generics, Ident, Lifetime, LifetimeDef};
+use syn::{parse_quote, spanned::Spanned, Attribute, Generics, Ident};
 
 /// Generate a type parameter with the given `suffix` that does not conflict with
 /// any of the `existing` generics.
@@ -13,25 +13,22 @@ fn gen_param(suffix: impl ToString, existing: &Generics) -> Ident {
     Ident::new(&suffix, Span::call_site())
 }
 
-/// Return the `TyCtxt` interner for the given `structure`.
+/// Return the interner for the given `structure`.
 ///
-/// If the input represented by `structure` has a `'tcx` lifetime parameter, then that will be used
-/// used as the lifetime of the `TyCtxt`. Otherwise a `'tcx` lifetime parameter that is unrelated
-/// to the input will be used.
+/// If the input represented by `structure` has a `'tcx` lifetime parameter, then we `TyCtxt<'tcx>`
+/// will be returned; otherwise our derived implementation will be generic over the interner.
 fn gen_interner(structure: &mut synstructure::Structure<'_>) -> TokenStream {
-    let lt = structure
+    structure
         .ast()
         .generics
         .lifetimes()
         .find_map(|def| (def.lifetime.ident == "tcx").then_some(&def.lifetime))
-        .cloned()
+        .map(|lt| quote! { ::rustc_middle::ty::TyCtxt<#lt> })
         .unwrap_or_else(|| {
-            let tcx: Lifetime = parse_quote! { 'tcx };
-            structure.add_impl_generic(LifetimeDef::new(tcx.clone()).into());
-            tcx
-        });
-
-    quote! { ::rustc_middle::ty::TyCtxt<#lt> }
+            let ident = gen_param("I", &structure.ast().generics);
+            structure.add_impl_generic(parse_quote! { #ident: ::rustc_type_ir::Interner });
+            ident.into_token_stream()
+        })
 }
 
 /// Returns the `Span` of the first `#[skip_traversal]` attribute in `attrs`.