Auto merge of #138464 - compiler-errors:less-type-ir, r=lcnr

Use `rustc_type_ir` directly less in the codebase

cc https://github.com/rust-lang/rust/issues/138449

This is a somewhat opinionated bundle of changes that will make working on https://github.com/rust-lang/rust/issues/138449 more easy, since it cuts out the bulk of the changes that would be necessitated by the lint. Namely:

1. Fold `rustc_middle::ty::fold` and `rustc_middle::ty::visit` into `rustc_middle::ty`. This is because we already reexport some parts of these modules into `rustc_middle::ty`, and there's really no benefit from namespacing away the rest of these modules's functionality given how important folding and visiting is to the type layer.
2. Rename `{Decodable,Encodable}_Generic` to `{Decodable,Encodable}_NoContext`[^why], change it to be "perfect derive" (`synstructure::AddBounds::Fields`), use it throughout `rustc_type_ir` instead of `TyEncodable`/`TyDecodable`.
3. Make `TyEncodable` and `TyDecodable` derives use `::rustc_middle::ty::codec::TyEncoder` (etc) for its generated paths, and move the `rustc_type_ir::codec` module back to `rustc_middle::ty::codec` :tada:.
4. Stop using `rustc_type_ir` in crates that aren't "fundamental" to the type system, namely middle/infer/trait-selection. This amounted mostly to changing imports from `use rustc_type_ir::...` to `use rustc_middle::ty::...`, but also this means that we can't glob import `TyKind::*` since the reexport into `rustc_middle::ty::TyKind` is a type alias. Instead, use the prefixed variants like `ty::Str` everywhere -- IMO this is a good change, since it makes it more regularized with most of the rest of the compiler.

[^why]: `_NoContext` is the name for derive macros with no additional generic bounds and which do "perfect derive" by generating bounds based on field types. See `HashStable_NoContext`.

I'm happy to cut out some of these changes into separate PRs to make landing it a bit easier, though I don't expect to have much trouble with bitrot.

r? lcnr

1 files changed, 12 insertions, 22 deletions

diff --git a/compiler/rustc_macros/src/serialize.rs b/compiler/rustc_macros/src/serialize.rs
index 2c33a0ac0aa..673e6cd618f 100644
--- a/compiler/rustc_macros/src/serialize.rs
+++ b/compiler/rustc_macros/src/serialize.rs
@@ -6,16 +6,11 @@ use syn::spanned::Spanned;
 pub(super) fn type_decodable_derive(
     mut s: synstructure::Structure<'_>,
 ) -> proc_macro2::TokenStream {
+    if !s.ast().generics.lifetimes().any(|lt| lt.lifetime.ident == "tcx") {
+        s.add_impl_generic(parse_quote! { 'tcx });
+    }
     let decoder_ty = quote! { __D };
-    let bound = if s.ast().generics.lifetimes().any(|lt| lt.lifetime.ident == "tcx") {
-        quote! { <I = ::rustc_middle::ty::TyCtxt<'tcx>> }
-    } else if s.ast().generics.type_params().any(|ty| ty.ident == "I") {
-        quote! { <I = I> }
-    } else {
-        quote! {}
-    };
-
-    s.add_impl_generic(parse_quote! { #decoder_ty: ::rustc_type_ir::codec::TyDecoder #bound });
+    s.add_impl_generic(parse_quote! { #decoder_ty: ::rustc_middle::ty::codec::TyDecoder<'tcx> });
     s.add_bounds(synstructure::AddBounds::Fields);
     s.underscore_const(true);
 
@@ -45,12 +40,12 @@ pub(super) fn decodable_derive(mut s: synstructure::Structure<'_>) -> proc_macro
     decodable_body(s, decoder_ty)
 }
 
-pub(super) fn decodable_generic_derive(
+pub(super) fn decodable_nocontext_derive(
     mut s: synstructure::Structure<'_>,
 ) -> proc_macro2::TokenStream {
     let decoder_ty = quote! { __D };
     s.add_impl_generic(parse_quote! { #decoder_ty: ::rustc_serialize::Decoder });
-    s.add_bounds(synstructure::AddBounds::Generics);
+    s.add_bounds(synstructure::AddBounds::Fields);
     s.underscore_const(true);
 
     decodable_body(s, decoder_ty)
@@ -132,16 +127,11 @@ fn decode_field(field: &syn::Field) -> proc_macro2::TokenStream {
 pub(super) fn type_encodable_derive(
     mut s: synstructure::Structure<'_>,
 ) -> proc_macro2::TokenStream {
-    let bound = if s.ast().generics.lifetimes().any(|lt| lt.lifetime.ident == "tcx") {
-        quote! { <I = ::rustc_middle::ty::TyCtxt<'tcx>> }
-    } else if s.ast().generics.type_params().any(|ty| ty.ident == "I") {
-        quote! { <I = I> }
-    } else {
-        quote! {}
-    };
-
     let encoder_ty = quote! { __E };
-    s.add_impl_generic(parse_quote! { #encoder_ty: ::rustc_type_ir::codec::TyEncoder #bound });
+    if !s.ast().generics.lifetimes().any(|lt| lt.lifetime.ident == "tcx") {
+        s.add_impl_generic(parse_quote! { 'tcx });
+    }
+    s.add_impl_generic(parse_quote! { #encoder_ty: ::rustc_middle::ty::codec::TyEncoder<'tcx> });
     s.add_bounds(synstructure::AddBounds::Fields);
     s.underscore_const(true);
 
@@ -171,12 +161,12 @@ pub(super) fn encodable_derive(mut s: synstructure::Structure<'_>) -> proc_macro
     encodable_body(s, encoder_ty, false)
 }
 
-pub(super) fn encodable_generic_derive(
+pub(super) fn encodable_nocontext_derive(
     mut s: synstructure::Structure<'_>,
 ) -> proc_macro2::TokenStream {
     let encoder_ty = quote! { __E };
     s.add_impl_generic(parse_quote! { #encoder_ty: ::rustc_serialize::Encoder });
-    s.add_bounds(synstructure::AddBounds::Generics);
+    s.add_bounds(synstructure::AddBounds::Fields);
     s.underscore_const(true);
 
     encodable_body(s, encoder_ty, false)