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, 2 insertions, 2 deletions

diff --git a/compiler/rustc_data_structures/src/svh.rs b/compiler/rustc_data_structures/src/svh.rs
index 391a7c9f30d..f51fcb8ed22 100644
--- a/compiler/rustc_data_structures/src/svh.rs
+++ b/compiler/rustc_data_structures/src/svh.rs
@@ -7,12 +7,12 @@
 
 use std::fmt;
 
-use rustc_macros::{Decodable_Generic, Encodable_Generic};
+use rustc_macros::{Decodable_NoContext, Encodable_NoContext};
 
 use crate::fingerprint::Fingerprint;
 use crate::stable_hasher;
 
-#[derive(Copy, Clone, PartialEq, Eq, Debug, Encodable_Generic, Decodable_Generic, Hash)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Encodable_NoContext, Decodable_NoContext, Hash)]
 pub struct Svh {
     hash: Fingerprint,
 }