safe transmute: add rustc_on_unimplemented to BikeshedIntrinsicFrom

ref: rust-lang/rust#92268 (comment)

Author: jswrenn

core/src/mem/transmutability.rs

@@ -5,6 +5,10 @@
 /// notwithstanding whatever safety checks you have asked the compiler to [`Assume`] are satisfied.
 #[unstable(feature = "transmutability", issue = "none")]
 #[cfg_attr(not(bootstrap), lang = "transmute_trait")]
+#[rustc_on_unimplemented(
+    message = "`{Src}` cannot be safely transmuted into `{Self}` in the defining scope of `{Context}`.",
+    label = "`{Src}` cannot be safely transmuted into `{Self}` in the defining scope of `{Context}`."
+)]
 pub unsafe trait BikeshedIntrinsicFrom<
     Src,
     Context,