Auto merge of #133734 - scottmcm:lower-indexing-to-ptrmetadata, r=davidtwco,RalfJung

Bounds-check with PtrMetadata instead of Len in MIR

Rather than emitting `Len(*_n)` in array index bounds checks, emit `PtrMetadata(copy _n)` instead -- with some asterisks for arrays and `&mut` that need it to be done slightly differently.

We're getting pretty close to removing `Len` entirely, actually.  I think just one more PR after this (for slice drop shims).

r? mir

compiler/rustc_span/src/hygiene.rs

@@ -1163,6 +1163,9 @@ pub enum DesugaringKind {
     WhileLoop,
     /// `async Fn()` bound modifier
     BoundModifier,
+    /// Marks a `&raw const *_1` needed as part of getting the length of a mutable
+    /// slice for the bounds check, so that MIRI's retag handling can recognize it.
+    IndexBoundsCheckReborrow,
 }
 
 impl DesugaringKind {
@@ -1179,6 +1182,7 @@ impl DesugaringKind {
             DesugaringKind::ForLoop => "`for` loop",
             DesugaringKind::WhileLoop => "`while` loop",
             DesugaringKind::BoundModifier => "trait bound modifier",
+            DesugaringKind::IndexBoundsCheckReborrow => "slice indexing",
         }
     }
 }