Auto merge of rust-lang#135709 - lqd:bring-back-len, r=compiler-errors

Temporarily bring back `Rvalue::Len`

r? `@compiler-errors` as requested in rust-lang#135671 (comment)

> However, in the mean time, I'd rather we not crunch trying to find and more importantly validate the soundness of a solution 🤔

Agreed. To fix the IMO P-critical rust-lang#135671 for which we somehow didn't have test coverage, this PR temporarily reverts:
- rust-lang#133734
- its bugfix rust-lang#134371
- rust-lang#134330

cc `@scottmcm`

I added the few samples from that issue as a test, but we can add more in the future, in particular it seems `@steffahn` [will work on that](rust-lang#135671 (comment)).

Fixes rust-lang#135671. And if we want to land this, it should also be nominated for beta backport.

Author: bors

compiler/rustc_borrowck/src/lib.rs

@@ -829,6 +829,7 @@ use self::ReadOrWrite::{Activation, Read, Reservation, Write};
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 enum ArtificialField {
+    ArrayLength,
     FakeBorrow,
 }
 
@@ -1338,11 +1339,16 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
                 );
             }
 
-            &Rvalue::Discriminant(place) => {
+            &(Rvalue::Len(place) | Rvalue::Discriminant(place)) => {
+                let af = match *rvalue {
+                    Rvalue::Len(..) => Some(ArtificialField::ArrayLength),
+                    Rvalue::Discriminant(..) => None,
+                    _ => unreachable!(),
+                };
                 self.access_place(
                     location,
                     (place, span),
-                    (Shallow(None), Read(ReadKind::Copy)),
+                    (Shallow(af), Read(ReadKind::Copy)),
                     LocalMutationIsAllowed::No,
                     state,
                 );

compiler/rustc_borrowck/src/places_conflict.rs

@@ -203,7 +203,8 @@ fn place_components_conflict<'tcx>(
             let base_ty = base.ty(body, tcx).ty;
 
             match (elem, base_ty.kind(), access) {
-                (_, _, Shallow(Some(ArtificialField::FakeBorrow))) => {
+                (_, _, Shallow(Some(ArtificialField::ArrayLength)))
+                | (_, _, Shallow(Some(ArtificialField::FakeBorrow))) => {
                     // The array length is like additional fields on the
                     // type; it does not overlap any existing data there.
                     // Furthermore, if cannot actually be a prefix of any