Rollup merge of #124761 - Urgau:ref-casting_bigger_slice_index, r=jieyouxu

Fix insufficient logic when searching for the underlying allocation

This PR fixes the logic inside the `invalid_reference_casting` lint, when trying to lint on bigger memory layout casts.

More specifically when looking for the "underlying allocation" we were wrongly assuming that when we got `&mut slice[index]` that `slice[index]` was the allocation, but it's not.

Fixes https://github.com/rust-lang/rust/issues/124685

compiler/rustc_lint/src/reference_casting.rs

@@ -199,6 +199,13 @@ fn is_cast_to_bigger_memory_layout<'tcx>(
     let e_alloc = cx.expr_or_init(e);
     let e_alloc =
         if let ExprKind::AddrOf(_, _, inner_expr) = e_alloc.kind { inner_expr } else { e_alloc };
+
+    // if the current expr looks like this `&mut expr[index]` then just looking
+    // at `expr[index]` won't give us the underlying allocation, so we just skip it
+    if let ExprKind::Index(..) = e_alloc.kind {
+        return None;
+    }
+
     let alloc_ty = cx.typeck_results().node_type(e_alloc.hir_id);
 
     // if we do not find it we bail out, as this may not be UB