safe transmute: test to ensure that trait is correctly feature-gated

ref: #92268 (comment)

Author: jswrenn

src/test/ui/transmutability/malformed-program-gracefulness/feature-missing.rs

@@ -0,0 +1,6 @@
+// The trait must not be available if its feature flag is absent.
+
+#![crate_type = "lib"]
+
+use std::mem::BikeshedIntrinsicFrom;
+//~^ ERROR use of unstable library feature 'transmutability' [E0658]

src/test/ui/transmutability/malformed-program-gracefulness/feature-missing.stderr

@@ -0,0 +1,11 @@
+error[E0658]: use of unstable library feature 'transmutability'
+  --> $DIR/feature-missing.rs:5:5
+   |
+LL | use std::mem::BikeshedIntrinsicFrom;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(transmutability)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.