Auto merge of #112988 - spastorino:new-rpitit-24, r=compiler-errors

 Replace RPITIT current impl with new strategy that lowers as a GAT

This PR replaces the current implementation of RPITITs with the new implementation that we had under -Zlower-impl-trait-in-trait-to-assoc-ty flag that lowers the RPIT as a GAT on the trait and on the impls that implement that trait.

Opening this PR as a draft because this goes after #112682, ~#112981~ and ~#112983~.
As soon as those are merged, I can rebase and we should run perf, crater and test a lot.

r? `@compiler-errors`

Author: bors

compiler/rustc_hir/src/def.rs

@@ -109,8 +109,6 @@ pub enum DefKind {
     InlineConst,
     /// Opaque type, aka `impl Trait`.
     OpaqueTy,
-    /// A return-position `impl Trait` in a trait definition
-    ImplTraitPlaceholder,
     Field,
     /// Lifetime parameter: the `'a` in `struct Foo<'a> { ... }`
     LifetimeParam,
@@ -143,7 +141,6 @@ impl DefKind {
             DefKind::Ctor(CtorOf::Struct, CtorKind::Fn) => "tuple struct",
             DefKind::Ctor(CtorOf::Struct, CtorKind::Const) => "unit struct",
             DefKind::OpaqueTy => "opaque type",
-            DefKind::ImplTraitPlaceholder => "opaque type in trait",
             DefKind::TyAlias => "type alias",
             DefKind::TraitAlias => "trait alias",
             DefKind::AssocTy => "associated type",
@@ -227,8 +224,7 @@ impl DefKind {
             | DefKind::Use
             | DefKind::ForeignMod
             | DefKind::GlobalAsm
-            | DefKind::Impl { .. }
-            | DefKind::ImplTraitPlaceholder => None,
+            | DefKind::Impl { .. } => None,
         }
     }
 
@@ -262,7 +258,6 @@ impl DefKind {
             | DefKind::Use
             | DefKind::ForeignMod
             | DefKind::OpaqueTy
-            | DefKind::ImplTraitPlaceholder
             | DefKind::Impl { .. }
             | DefKind::Field
             | DefKind::TyParam

compiler/rustc_hir/src/target.rs

@@ -36,7 +36,6 @@ pub enum Target {
     GlobalAsm,
     TyAlias,
     OpaqueTy,
-    ImplTraitPlaceholder,
     Enum,
     Variant,
     Struct,
@@ -80,13 +79,7 @@ impl Target {
             ItemKind::ForeignMod { .. } => Target::ForeignMod,
             ItemKind::GlobalAsm(..) => Target::GlobalAsm,
             ItemKind::TyAlias(..) => Target::TyAlias,
-            ItemKind::OpaqueTy(ref opaque) => {
-                if opaque.in_trait {
-                    Target::ImplTraitPlaceholder
-                } else {
-                    Target::OpaqueTy
-                }
-            }
+            ItemKind::OpaqueTy(..) => Target::OpaqueTy,
             ItemKind::Enum(..) => Target::Enum,
             ItemKind::Struct(..) => Target::Struct,
             ItemKind::Union(..) => Target::Union,
@@ -110,7 +103,6 @@ impl Target {
             DefKind::GlobalAsm => Target::GlobalAsm,
             DefKind::TyAlias => Target::TyAlias,
             DefKind::OpaqueTy => Target::OpaqueTy,
-            DefKind::ImplTraitPlaceholder => Target::ImplTraitPlaceholder,
             DefKind::Enum => Target::Enum,
             DefKind::Struct => Target::Struct,
             DefKind::Union => Target::Union,
@@ -165,7 +157,6 @@ impl Target {
             Target::GlobalAsm => "global asm",
             Target::TyAlias => "type alias",
             Target::OpaqueTy => "opaque type",
-            Target::ImplTraitPlaceholder => "opaque type in trait",
             Target::Enum => "enum",
             Target::Variant => "enum variant",
             Target::Struct => "struct",