Remove workaround for CastKind::Transmute from const prop

Since constants are no longer validated before propagation the
workaround is obsolete. Remove it.

compiler/rustc_mir_transform/src/const_prop.rs

@@ -501,16 +501,6 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 
                 return None;
             }
-            // Do not try creating references, nor any types with potentially-complex
-            // invariants. This avoids an issue where checking validity would do a
-            // bunch of work generating a nice message about the invariant violation,
-            // only to not show it to anyone (since this isn't the lint).
-            Rvalue::Cast(CastKind::Transmute, op, dst_ty) if !dst_ty.is_primitive() => {
-                trace!("skipping Transmute of {:?} to {:?}", op, dst_ty);
-
-                return None;
-            }
-
             // There's no other checking to do at this time.
             Rvalue::Aggregate(..)
             | Rvalue::Use(..)

tests/mir-opt/const_prop/transmute.invalid_bool.ConstProp.diff

@@ -7,7 +7,8 @@
       }
   
       bb0: {
-          _0 = const -1_i8 as bool (Transmute); // scope 1 at $DIR/transmute.rs:+1:14: +1:30
+-         _0 = const -1_i8 as bool (Transmute); // scope 1 at $DIR/transmute.rs:+1:14: +1:30
++         _0 = const {transmute(0xff): bool}; // scope 1 at $DIR/transmute.rs:+1:14: +1:30
           return;                          // scope 0 at $DIR/transmute.rs:+2:2: +2:2
       }
   }

tests/mir-opt/const_prop/transmute.invalid_char.ConstProp.diff

@@ -7,7 +7,8 @@
       }
   
       bb0: {
-          _0 = const _ as char (Transmute); // scope 1 at $DIR/transmute.rs:+1:14: +1:33
+-         _0 = const _ as char (Transmute); // scope 1 at $DIR/transmute.rs:+1:14: +1:33
++         _0 = const {transmute(0x7fffffff): char}; // scope 1 at $DIR/transmute.rs:+1:14: +1:33
           return;                          // scope 0 at $DIR/transmute.rs:+2:2: +2:2
       }
   }

tests/mir-opt/const_prop/transmute.unreachable_box.ConstProp.diff

@@ -15,7 +15,11 @@
       bb0: {
           StorageLive(_1);                 // scope 0 at $DIR/transmute.rs:+0:38: +3:2
           StorageLive(_2);                 // scope 0 at $DIR/transmute.rs:+1:9: +1:10
-          _2 = const 1_usize as std::boxed::Box<Never> (Transmute); // scope 2 at $DIR/transmute.rs:+1:34: +1:52
+-         _2 = const 1_usize as std::boxed::Box<Never> (Transmute); // scope 2 at $DIR/transmute.rs:+1:34: +1:52
++         _2 = const Box::<Never>(Unique::<Never> {{ pointer: NonNull::<Never> {{ pointer: {0x1 as *const Never} }}, _marker: PhantomData::<Never> }}, std::alloc::Global); // scope 2 at $DIR/transmute.rs:+1:34: +1:52
++                                          // mir::Constant
++                                          // + span: no-location
++                                          // + literal: Const { ty: Box<Never>, val: Value(Scalar(0x0000000000000001)) }
           StorageLive(_3);                 // scope 1 at $DIR/transmute.rs:+2:5: +2:16
           unreachable;                     // scope 1 at $DIR/transmute.rs:+2:11: +2:13
       }

tests/mir-opt/const_prop/transmute.unreachable_mut.ConstProp.diff

@@ -17,7 +17,11 @@
           StorageLive(_1);                 // scope 0 at $DIR/transmute.rs:+0:38: +3:2
           StorageLive(_2);                 // scope 0 at $DIR/transmute.rs:+1:9: +1:10
           StorageLive(_3);                 // scope 0 at $DIR/transmute.rs:+1:34: +1:52
-          _3 = const 1_usize as &mut Never (Transmute); // scope 2 at $DIR/transmute.rs:+1:34: +1:52
+-         _3 = const 1_usize as &mut Never (Transmute); // scope 2 at $DIR/transmute.rs:+1:34: +1:52
++         _3 = const {0x1 as &mut Never};  // scope 2 at $DIR/transmute.rs:+1:34: +1:52
++                                          // mir::Constant
++                                          // + span: no-location
++                                          // + literal: Const { ty: &mut Never, val: Value(Scalar(0x0000000000000001)) }
           _2 = &mut (*_3);                 // scope 0 at $DIR/transmute.rs:+1:34: +1:52
           StorageDead(_3);                 // scope 0 at $DIR/transmute.rs:+1:54: +1:55
           StorageLive(_4);                 // scope 1 at $DIR/transmute.rs:+2:5: +2:16

tests/mir-opt/const_prop/transmute.unreachable_ref.ConstProp.diff

@@ -15,7 +15,11 @@
       bb0: {
           StorageLive(_1);                 // scope 0 at $DIR/transmute.rs:+0:38: +3:2
           StorageLive(_2);                 // scope 0 at $DIR/transmute.rs:+1:9: +1:10
-          _2 = const 1_usize as &Never (Transmute); // scope 2 at $DIR/transmute.rs:+1:30: +1:48
+-         _2 = const 1_usize as &Never (Transmute); // scope 2 at $DIR/transmute.rs:+1:30: +1:48
++         _2 = const {0x1 as &Never};      // scope 2 at $DIR/transmute.rs:+1:30: +1:48
++                                          // mir::Constant
++                                          // + span: no-location
++                                          // + literal: Const { ty: &Never, val: Value(Scalar(0x0000000000000001)) }
           StorageLive(_3);                 // scope 1 at $DIR/transmute.rs:+2:5: +2:16
           unreachable;                     // scope 1 at $DIR/transmute.rs:+2:11: +2:13
       }