Auto merge of #126049 - compiler-errors:rework-use, r=oli-obk

Rework `feature(precise_capturing)` to represent `use<...>` as a syntactical bound

Reworks `precise_capturing` for a recent lang-team consensus.

Specifically:

> The conclusion of the team is that we'll make use<..> a bound. That is, we'll support impl use<..> + Trait, impl Trait + use<..>, etc.

> For now, we will support at most one such bound in a list of bounds, and semantically we'll only support these bounds in the item bounds of RPIT-like impl Trait opaque types (i.e., in the places discussed in the RFC).

Lang decision in favor of this approach:

- #125836 (comment)

Tracking:

- #123432

Author: bors

compiler/rustc_ast/src/ast.rs

@@ -307,13 +307,16 @@ impl TraitBoundModifiers {
 pub enum GenericBound {
     Trait(PolyTraitRef, TraitBoundModifiers),
     Outlives(Lifetime),
+    /// Precise capturing syntax: `impl Sized + use<'a>`
+    Use(ThinVec<PreciseCapturingArg>, Span),
 }
 
 impl GenericBound {
     pub fn span(&self) -> Span {
         match self {
             GenericBound::Trait(t, ..) => t.span,
             GenericBound::Outlives(l) => l.ident.span,
+            GenericBound::Use(_, span) => *span,
         }
     }
 }
@@ -2162,7 +2165,7 @@ pub enum TyKind {
     /// The `NodeId` exists to prevent lowering from having to
     /// generate `NodeId`s on the fly, which would complicate
     /// the generation of opaque `type Foo = impl Trait` items significantly.
-    ImplTrait(NodeId, GenericBounds, Option<P<(ThinVec<PreciseCapturingArg>, Span)>>),
+    ImplTrait(NodeId, GenericBounds),
     /// No-op; kept solely so that we can pretty-print faithfully.
     Paren(P<Ty>),
     /// Unused for now.

compiler/rustc_ast/src/mut_visit.rs

@@ -523,14 +523,9 @@ pub fn noop_visit_ty<T: MutVisitor>(ty: &mut P<Ty>, vis: &mut T) {
         TyKind::TraitObject(bounds, _syntax) => {
             visit_vec(bounds, |bound| vis.visit_param_bound(bound))
         }
-        TyKind::ImplTrait(id, bounds, precise_capturing) => {
+        TyKind::ImplTrait(id, bounds) => {
             vis.visit_id(id);
             visit_vec(bounds, |bound| vis.visit_param_bound(bound));
-            if let Some((precise_capturing, _span)) = precise_capturing.as_deref_mut() {
-                for arg in precise_capturing {
-                    vis.visit_precise_capturing_arg(arg);
-                }
-            }
         }
         TyKind::MacCall(mac) => vis.visit_mac_call(mac),
         TyKind::AnonStruct(id, fields) | TyKind::AnonUnion(id, fields) => {
@@ -923,6 +918,11 @@ fn noop_visit_param_bound<T: MutVisitor>(pb: &mut GenericBound, vis: &mut T) {
     match pb {
         GenericBound::Trait(ty, _modifier) => vis.visit_poly_trait_ref(ty),
         GenericBound::Outlives(lifetime) => noop_visit_lifetime(lifetime, vis),
+        GenericBound::Use(args, _) => {
+            for arg in args {
+                vis.visit_precise_capturing_arg(arg);
+            }
+        }
     }
 }