Auto merge of #93505 - lcnr:substsref-vs-ty-list, r=michaelwoerister

safely `transmute<&List<Ty<'tcx>>, &List<GenericArg<'tcx>>>`

This PR has 3 relevant steps which are is split in distinct commits.

The first commit now interns `List<Ty<'tcx>>` and `List<GenericArg<'tcx>>` together, potentially reusing memory while allowing free conversions between these two using `List<Ty<'tcx>>::as_substs()` and `SubstsRef<'tcx>::try_as_type_list()`.

Using this, we then use `&'tcx List<Ty<'tcx>>` instead of a `SubstsRef<'tcx>` for tuple fields, simplifying a bunch of code.

Finally, as tuple fields and other generic arguments now use a different `TypeFoldable<'tcx>` impl, we optimize the impl for `List<Ty<'tcx>>` improving perf by slightly less than 1% in tuple heavy benchmarks.

Author: bors

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -2298,7 +2298,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 // Closure arguments are wrapped in a tuple, so we need to get the first
                 // from that.
                 if let ty::Tuple(elems) = argument_ty.kind() {
-                    let argument_ty = elems.first()?.expect_ty();
+                    let &argument_ty = elems.first()?;
                     if let ty::Ref(_, _, _) = argument_ty.kind() {
                         return Some(AnnotatedBorrowFnSignature::Closure {
                             argument_ty,

compiler/rustc_borrowck/src/diagnostics/region_name.rs

@@ -480,7 +480,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
         let search_stack: &mut Vec<(Ty<'tcx>, &hir::Ty<'_>)> = &mut vec![(ty, hir_ty)];
 
         while let Some((ty, hir_ty)) = search_stack.pop() {
-            match (&ty.kind(), &hir_ty.kind) {
+            match (ty.kind(), &hir_ty.kind) {
                 // Check if the `ty` is `&'X ..` where `'X`
                 // is the region we are looking for -- if so, and we have a `&T`
                 // on the RHS, then we want to highlight the `&` like so:
@@ -532,9 +532,8 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
                 // The following cases don't have lifetimes, so we
                 // just worry about trying to match up the rustc type
                 // with the HIR types:
-                (ty::Tuple(elem_tys), hir::TyKind::Tup(elem_hir_tys)) => {
-                    search_stack
-                        .extend(iter::zip(elem_tys.iter().map(|k| k.expect_ty()), *elem_hir_tys));
+                (&ty::Tuple(elem_tys), hir::TyKind::Tup(elem_hir_tys)) => {
+                    search_stack.extend(iter::zip(elem_tys, *elem_hir_tys));
                 }
 
                 (ty::Slice(elem_ty), hir::TyKind::Slice(elem_hir_ty))