Auto merge of #123468 - compiler-errors:precise-capturing, r=oli-obk

Implement syntax for `impl Trait` to specify its captures explicitly (`feature(precise_capturing)`)

Implements `impl use<'a, 'b, T, U> Sized` syntax that allows users to explicitly list the captured parameters for an opaque, rather than inferring it from the opaque's bounds (or capturing *all* lifetimes under 2024-edition capture rules). This allows us to exclude some implicit captures, so this syntax may be used as a migration strategy for changes due to #117587.

We represent this list of captured params as `PreciseCapturingArg` in AST and HIR, resolving them between `rustc_resolve` and `resolve_bound_vars`. Later on, we validate that the opaques only capture the parameters in this list.

We artificially limit the feature to *require* mentioning all type and const parameters, since we don't currently have support for non-lifetime bivariant generics. This can be relaxed in the future.

We also may need to limit this to require naming *all* lifetime parameters for RPITIT, since GATs have no variance. I have to investigate this. This can also be relaxed in the future.

r? `@oli-obk`

Tracking issue:

- https://github.com/rust-lang/rust/issues/123432

2 files changed, 17 insertions, 3 deletions

diff --git a/src/tools/clippy/clippy_utils/src/ast_utils.rs b/src/tools/clippy/clippy_utils/src/ast_utils.rs
index f594a40ff59..9f0bd4ea7e2 100644
--- a/src/tools/clippy/clippy_utils/src/ast_utils.rs
+++ b/src/tools/clippy/clippy_utils/src/ast_utils.rs
@@ -709,7 +709,8 @@ pub fn eq_ty(l: &Ty, r: &Ty) -> bool {
         (Tup(l), Tup(r)) => over(l, r, |l, r| eq_ty(l, r)),
         (Path(lq, lp), Path(rq, rp)) => both(lq, rq, eq_qself) && eq_path(lp, rp),
         (TraitObject(lg, ls), TraitObject(rg, rs)) => ls == rs && over(lg, rg, eq_generic_bound),
-        (ImplTrait(_, lg), ImplTrait(_, rg)) => over(lg, rg, eq_generic_bound),
+        (ImplTrait(_, lg, lc), ImplTrait(_, rg, rc)) =>
+            over(lg, rg, eq_generic_bound) && both(lc, rc, |lc, rc| over(lc.0.as_slice(), rc.0.as_slice(), eq_precise_capture)),
         (Typeof(l), Typeof(r)) => eq_expr(&l.value, &r.value),
         (MacCall(l), MacCall(r)) => eq_mac_call(l, r),
         _ => false,
@@ -770,6 +771,14 @@ pub fn eq_generic_bound(l: &GenericBound, r: &GenericBound) -> bool {
     }
 }
 
+pub fn eq_precise_capture(l: &PreciseCapturingArg, r: &PreciseCapturingArg) -> bool {
+    match (l, r) {
+        (PreciseCapturingArg::Lifetime(l), PreciseCapturingArg::Lifetime(r)) => l.ident == r.ident,
+        (PreciseCapturingArg::Arg(l, _), PreciseCapturingArg::Arg(r, _)) => l.segments[0].ident == r.segments[0].ident,
+        _ => false,
+    }
+}
+
 fn eq_term(l: &Term, r: &Term) -> bool {
     match (l, r) {
         (Term::Ty(l), Term::Ty(r)) => eq_ty(l, r),
diff --git a/src/tools/rustfmt/src/types.rs b/src/tools/rustfmt/src/types.rs
index 10a87f6e698..fe2d28ae1b9 100644
--- a/src/tools/rustfmt/src/types.rs
+++ b/src/tools/rustfmt/src/types.rs
@@ -843,7 +843,11 @@ impl Rewrite for ast::Ty {
                 rewrite_macro(mac, None, context, shape, MacroPosition::Expression)
             }
             ast::TyKind::ImplicitSelf => Some(String::from("")),
-            ast::TyKind::ImplTrait(_, ref it) => {
+            ast::TyKind::ImplTrait(_, ref it, ref captures) => {
+                // FIXME(precise_capturing): Implement formatting.
+                if captures.is_some() {
+                    return None;
+                }
                 // Empty trait is not a parser error.
                 if it.is_empty() {
                     return Some("impl".to_owned());
@@ -1106,7 +1110,8 @@ fn join_bounds_inner(
 
 pub(crate) fn opaque_ty(ty: &Option<ptr::P<ast::Ty>>) -> Option<&ast::GenericBounds> {
     ty.as_ref().and_then(|t| match &t.kind {
-        ast::TyKind::ImplTrait(_, bounds) => Some(bounds),
+        // FIXME(precise_capturing): Implement support here
+        ast::TyKind::ImplTrait(_, bounds, _) => Some(bounds),
         _ => None,
     })
 }