Lint on reference casting to bigger underlying allocation

compiler/rustc_lint/src/lints.rs

@@ -716,7 +716,7 @@ pub enum InvalidFromUtf8Diag {
 
 // reference_casting.rs
 #[derive(LintDiagnostic)]
-pub enum InvalidReferenceCastingDiag {
+pub enum InvalidReferenceCastingDiag<'tcx> {
     #[diag(lint_invalid_reference_casting_borrow_as_mut)]
     #[note(lint_invalid_reference_casting_note_book)]
     BorrowAsMut {
@@ -733,6 +733,18 @@ pub enum InvalidReferenceCastingDiag {
         #[note(lint_invalid_reference_casting_note_ty_has_interior_mutability)]
         ty_has_interior_mutability: Option<()>,
     },
+    #[diag(lint_invalid_reference_casting_bigger_layout)]
+    #[note(lint_layout)]
+    BiggerLayout {
+        #[label]
+        orig_cast: Option<Span>,
+        #[label(lint_alloc)]
+        alloc: Span,
+        from_ty: Ty<'tcx>,
+        from_size: u64,
+        to_ty: Ty<'tcx>,
+        to_size: u64,
+    },
 }
 
 // hidden_unicode_codepoints.rs