safe transmute: forbid reference lifetime extension

Modifies `BikeshedIntrinsicFrom` to forbid lifetime extensions on
references. This static check can be opted out of with the
`Assume::lifetimes` flag.

Fixes #129097

compiler/rustc_transmute/src/layout/mod.rs

@@ -63,7 +63,9 @@ pub mod rustc {
     use std::fmt::{self, Write};
 
     use rustc_middle::mir::Mutability;
-    use rustc_middle::ty::{self, Ty};
+    use rustc_middle::ty::layout::{LayoutCx, LayoutError};
+    use rustc_middle::ty::{self, Ty, TyCtxt};
+    use rustc_target::abi::Layout;
 
     /// A reference in the layout.
     #[derive(Debug, Hash, Eq, PartialEq, Clone, Copy)]
@@ -120,4 +122,13 @@ pub mod rustc {
             self != &Self::Primitive
         }
     }
+
+    pub(crate) fn layout_of<'tcx>(
+        cx: LayoutCx<'tcx, TyCtxt<'tcx>>,
+        ty: Ty<'tcx>,
+    ) -> Result<Layout<'tcx>, &'tcx LayoutError<'tcx>> {
+        use rustc_middle::ty::layout::LayoutOf;
+        let ty = cx.tcx.erase_regions(ty);
+        cx.layout_of(ty).map(|tl| tl.layout)
+    }
 }