Rollup merge of #121059 - compiler-errors:extension, r=davidtwco,Nilstrieb

Add and use a simple extension trait derive macro in the compiler

Adds `#[extension]` to `rustc_macros` for implementing an extension trait. This expands an impl (with an optional visibility) into two parallel trait + impl definitions.

before:
```rust
pub trait Extension {
  fn a();
}
impl Extension for () {
  fn a() {}
}
```

to:
```rust
#[extension]
pub impl Extension for () {
  fn a() {}
}
```

Opted to just implement it by hand because I couldn't figure if there was a "canonical" choice of extension trait macro in the ecosystem. It's really lightweight anyways, and can always be changed.

I'm interested in adding this because I'd like to later split up the large `TypeErrCtxtExt` traits into several different files. This should make it one step easier.

compiler/rustc_middle/src/ty/layout.rs

@@ -23,20 +23,8 @@ use std::fmt;
 use std::num::NonZero;
 use std::ops::Bound;
 
-pub trait IntegerExt {
-    fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>, signed: bool) -> Ty<'tcx>;
-    fn from_int_ty<C: HasDataLayout>(cx: &C, ity: ty::IntTy) -> Integer;
-    fn from_uint_ty<C: HasDataLayout>(cx: &C, uty: ty::UintTy) -> Integer;
-    fn repr_discr<'tcx>(
-        tcx: TyCtxt<'tcx>,
-        ty: Ty<'tcx>,
-        repr: &ReprOptions,
-        min: i128,
-        max: i128,
-    ) -> (Integer, bool);
-}
-
-impl IntegerExt for Integer {
+#[extension(pub trait IntegerExt)]
+impl Integer {
     #[inline]
     fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>, signed: bool) -> Ty<'tcx> {
         match (*self, signed) {
@@ -123,12 +111,8 @@ impl IntegerExt for Integer {
     }
 }
 
-pub trait PrimitiveExt {
-    fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx>;
-    fn to_int_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx>;
-}
-
-impl PrimitiveExt for Primitive {
+#[extension(pub trait PrimitiveExt)]
+impl Primitive {
     #[inline]
     fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
         match *self {

compiler/rustc_middle/src/ty/util.rs

@@ -96,13 +96,8 @@ impl<'tcx> Discr<'tcx> {
     }
 }
 
-pub trait IntTypeExt {
-    fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx>;
-    fn disr_incr<'tcx>(&self, tcx: TyCtxt<'tcx>, val: Option<Discr<'tcx>>) -> Option<Discr<'tcx>>;
-    fn initial_discriminant<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Discr<'tcx>;
-}
-
-impl IntTypeExt for IntegerType {
+#[extension(pub trait IntTypeExt)]
+impl IntegerType {
     fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
         match self {
             IntegerType::Pointer(true) => tcx.types.isize,