Add CastKind::Transmute to MIR

Updates `interpret`, `codegen_ssa`, and `codegen_cranelift` to consume the new cast instead of the intrinsic.

Includes `CastTransmute` for custom MIR building, to be able to test the extra UB.

compiler/rustc_const_eval/src/interpret/cast.rs

@@ -133,6 +133,22 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     bug!()
                 }
             }
+
+            Transmute => {
+                assert!(src.layout.is_sized());
+                assert!(dest.layout.is_sized());
+                if src.layout.size != dest.layout.size {
+                    throw_ub_format!(
+                        "transmuting from {}-byte type to {}-byte type: `{}` -> `{}`",
+                        src.layout.size.bytes(),
+                        dest.layout.size.bytes(),
+                        src.layout.ty,
+                        dest.layout.ty,
+                    );
+                }
+
+                self.copy_op(src, dest, /*allow_transmute*/ true)?;
+            }
         }
         Ok(())
     }

compiler/rustc_const_eval/src/interpret/intrinsics.rs

@@ -127,7 +127,6 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // First handle intrinsics without return place.
         let ret = match ret {
             None => match intrinsic_name {
-                sym::transmute => throw_ub_format!("transmuting to uninhabited type"),
                 sym::abort => M::abort(self, "the program aborted execution".to_owned())?,
                 // Unsupported diverging intrinsic.
                 _ => return Ok(false),
@@ -411,9 +410,6 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 self.exact_div(&val, &size, dest)?;
             }
 
-            sym::transmute => {
-                self.copy_op(&args[0], dest, /*allow_transmute*/ true)?;
-            }
             sym::assert_inhabited
             | sym::assert_zero_valid
             | sym::assert_mem_uninitialized_valid => {