safe transmute: add `rustc_on_unimplemented` to `BikeshedIntrinsicFrom`

ref: https://github.com/rust-lang/rust/pull/92268#discussion_r925266583

2 files changed, 48 insertions, 36 deletions

diff --git a/src/test/ui/transmutability/structs/repr/should_require_well_defined_layout.rs b/src/test/ui/transmutability/structs/repr/should_require_well_defined_layout.rs
index 63419aceb6c..556be989dbc 100644
--- a/src/test/ui/transmutability/structs/repr/should_require_well_defined_layout.rs
+++ b/src/test/ui/transmutability/structs/repr/should_require_well_defined_layout.rs
@@ -18,39 +18,39 @@ fn should_reject_repr_rust()
 {
     fn unit() {
         struct repr_rust;
-        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR not satisfied
-        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR not satisfied
+        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR cannot be safely transmuted
+        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR cannot be safely transmuted
     }
 
     fn tuple() {
         struct repr_rust();
-        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR not satisfied
-        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR not satisfied
+        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR cannot be safely transmuted
+        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR cannot be safely transmuted
     }
 
     fn braces() {
         struct repr_rust{}
-        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR not satisfied
-        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR not satisfied
+        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR cannot be safely transmuted
+        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR cannot be safely transmuted
     }
 
     fn aligned() {
         #[repr(align(1))] struct repr_rust{}
-        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR not satisfied
-        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR not satisfied
+        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR cannot be safely transmuted
+        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR cannot be safely transmuted
     }
 
     fn packed() {
         #[repr(packed)] struct repr_rust{}
-        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR not satisfied
-        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR not satisfied
+        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR cannot be safely transmuted
+        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR cannot be safely transmuted
     }
 
     fn nested() {
         struct repr_rust;
         #[repr(C)] struct repr_c(repr_rust);
-        assert::is_maybe_transmutable::<repr_c, ()>(); //~ ERROR not satisfied
-        assert::is_maybe_transmutable::<u128, repr_c>(); //~ ERROR not satisfied
+        assert::is_maybe_transmutable::<repr_c, ()>(); //~ ERROR cannot be safely transmuted
+        assert::is_maybe_transmutable::<u128, repr_c>(); //~ ERROR cannot be safely transmuted
     }
 }
 
diff --git a/src/test/ui/transmutability/structs/repr/should_require_well_defined_layout.stderr b/src/test/ui/transmutability/structs/repr/should_require_well_defined_layout.stderr
index ab582dd6688..07355f7c2ad 100644
--- a/src/test/ui/transmutability/structs/repr/should_require_well_defined_layout.stderr
+++ b/src/test/ui/transmutability/structs/repr/should_require_well_defined_layout.stderr
@@ -1,9 +1,10 @@
-error[E0277]: the trait bound `(): BikeshedIntrinsicFrom<should_reject_repr_rust::unit::repr_rust, assert::Context, true, true, true, true>` is not satisfied
+error[E0277]: `should_reject_repr_rust::unit::repr_rust` cannot be safely transmuted into `()` in the defining scope of `assert::Context`.
   --> $DIR/should_require_well_defined_layout.rs:21:52
    |
 LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ the trait `BikeshedIntrinsicFrom<should_reject_repr_rust::unit::repr_rust, assert::Context, true, true, true, true>` is not implemented for `()`
+   |                                                    ^^ `should_reject_repr_rust::unit::repr_rust` cannot be safely transmuted into `()` in the defining scope of `assert::Context`.
    |
+   = help: the trait `BikeshedIntrinsicFrom<should_reject_repr_rust::unit::repr_rust, assert::Context, true, true, true, true>` is not implemented for `()`
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
    |
@@ -13,12 +14,13 @@ LL |     where
 LL |         Dst: BikeshedIntrinsicFrom<Src, Context, true, true, true, true>
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_maybe_transmutable`
 
-error[E0277]: the trait bound `should_reject_repr_rust::unit::repr_rust: BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not satisfied
+error[E0277]: `u128` cannot be safely transmuted into `should_reject_repr_rust::unit::repr_rust` in the defining scope of `assert::Context`.
   --> $DIR/should_require_well_defined_layout.rs:22:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ the trait `BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not implemented for `should_reject_repr_rust::unit::repr_rust`
+   |                                               ^^^^^^^^^ `u128` cannot be safely transmuted into `should_reject_repr_rust::unit::repr_rust` in the defining scope of `assert::Context`.
    |
+   = help: the trait `BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not implemented for `should_reject_repr_rust::unit::repr_rust`
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
    |
@@ -28,12 +30,13 @@ LL |     where
 LL |         Dst: BikeshedIntrinsicFrom<Src, Context, true, true, true, true>
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_maybe_transmutable`
 
-error[E0277]: the trait bound `(): BikeshedIntrinsicFrom<should_reject_repr_rust::tuple::repr_rust, assert::Context, true, true, true, true>` is not satisfied
+error[E0277]: `should_reject_repr_rust::tuple::repr_rust` cannot be safely transmuted into `()` in the defining scope of `assert::Context`.
   --> $DIR/should_require_well_defined_layout.rs:27:52
    |
 LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ the trait `BikeshedIntrinsicFrom<should_reject_repr_rust::tuple::repr_rust, assert::Context, true, true, true, true>` is not implemented for `()`
+   |                                                    ^^ `should_reject_repr_rust::tuple::repr_rust` cannot be safely transmuted into `()` in the defining scope of `assert::Context`.
    |
+   = help: the trait `BikeshedIntrinsicFrom<should_reject_repr_rust::tuple::repr_rust, assert::Context, true, true, true, true>` is not implemented for `()`
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
    |
@@ -43,12 +46,13 @@ LL |     where
 LL |         Dst: BikeshedIntrinsicFrom<Src, Context, true, true, true, true>
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_maybe_transmutable`
 
-error[E0277]: the trait bound `should_reject_repr_rust::tuple::repr_rust: BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not satisfied
+error[E0277]: `u128` cannot be safely transmuted into `should_reject_repr_rust::tuple::repr_rust` in the defining scope of `assert::Context`.
   --> $DIR/should_require_well_defined_layout.rs:28:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ the trait `BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not implemented for `should_reject_repr_rust::tuple::repr_rust`
+   |                                               ^^^^^^^^^ `u128` cannot be safely transmuted into `should_reject_repr_rust::tuple::repr_rust` in the defining scope of `assert::Context`.
    |
+   = help: the trait `BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not implemented for `should_reject_repr_rust::tuple::repr_rust`
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
    |
@@ -58,12 +62,13 @@ LL |     where
 LL |         Dst: BikeshedIntrinsicFrom<Src, Context, true, true, true, true>
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_maybe_transmutable`
 
-error[E0277]: the trait bound `(): BikeshedIntrinsicFrom<should_reject_repr_rust::braces::repr_rust, assert::Context, true, true, true, true>` is not satisfied
+error[E0277]: `should_reject_repr_rust::braces::repr_rust` cannot be safely transmuted into `()` in the defining scope of `assert::Context`.
   --> $DIR/should_require_well_defined_layout.rs:33:52
    |
 LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ the trait `BikeshedIntrinsicFrom<should_reject_repr_rust::braces::repr_rust, assert::Context, true, true, true, true>` is not implemented for `()`
+   |                                                    ^^ `should_reject_repr_rust::braces::repr_rust` cannot be safely transmuted into `()` in the defining scope of `assert::Context`.
    |
+   = help: the trait `BikeshedIntrinsicFrom<should_reject_repr_rust::braces::repr_rust, assert::Context, true, true, true, true>` is not implemented for `()`
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
    |
@@ -73,12 +78,13 @@ LL |     where
 LL |         Dst: BikeshedIntrinsicFrom<Src, Context, true, true, true, true>
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_maybe_transmutable`
 
-error[E0277]: the trait bound `should_reject_repr_rust::braces::repr_rust: BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not satisfied
+error[E0277]: `u128` cannot be safely transmuted into `should_reject_repr_rust::braces::repr_rust` in the defining scope of `assert::Context`.
   --> $DIR/should_require_well_defined_layout.rs:34:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ the trait `BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not implemented for `should_reject_repr_rust::braces::repr_rust`
+   |                                               ^^^^^^^^^ `u128` cannot be safely transmuted into `should_reject_repr_rust::braces::repr_rust` in the defining scope of `assert::Context`.
    |
+   = help: the trait `BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not implemented for `should_reject_repr_rust::braces::repr_rust`
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
    |
@@ -88,12 +94,13 @@ LL |     where
 LL |         Dst: BikeshedIntrinsicFrom<Src, Context, true, true, true, true>
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_maybe_transmutable`
 
-error[E0277]: the trait bound `(): BikeshedIntrinsicFrom<aligned::repr_rust, assert::Context, true, true, true, true>` is not satisfied
+error[E0277]: `aligned::repr_rust` cannot be safely transmuted into `()` in the defining scope of `assert::Context`.
   --> $DIR/should_require_well_defined_layout.rs:39:52
    |
 LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ the trait `BikeshedIntrinsicFrom<aligned::repr_rust, assert::Context, true, true, true, true>` is not implemented for `()`
+   |                                                    ^^ `aligned::repr_rust` cannot be safely transmuted into `()` in the defining scope of `assert::Context`.
    |
+   = help: the trait `BikeshedIntrinsicFrom<aligned::repr_rust, assert::Context, true, true, true, true>` is not implemented for `()`
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
    |
@@ -103,12 +110,13 @@ LL |     where
 LL |         Dst: BikeshedIntrinsicFrom<Src, Context, true, true, true, true>
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_maybe_transmutable`
 
-error[E0277]: the trait bound `aligned::repr_rust: BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not satisfied
+error[E0277]: `u128` cannot be safely transmuted into `aligned::repr_rust` in the defining scope of `assert::Context`.
   --> $DIR/should_require_well_defined_layout.rs:40:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ the trait `BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not implemented for `aligned::repr_rust`
+   |                                               ^^^^^^^^^ `u128` cannot be safely transmuted into `aligned::repr_rust` in the defining scope of `assert::Context`.
    |
+   = help: the trait `BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not implemented for `aligned::repr_rust`
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
    |
@@ -118,12 +126,13 @@ LL |     where
 LL |         Dst: BikeshedIntrinsicFrom<Src, Context, true, true, true, true>
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_maybe_transmutable`
 
-error[E0277]: the trait bound `(): BikeshedIntrinsicFrom<packed::repr_rust, assert::Context, true, true, true, true>` is not satisfied
+error[E0277]: `packed::repr_rust` cannot be safely transmuted into `()` in the defining scope of `assert::Context`.
   --> $DIR/should_require_well_defined_layout.rs:45:52
    |
 LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ the trait `BikeshedIntrinsicFrom<packed::repr_rust, assert::Context, true, true, true, true>` is not implemented for `()`
+   |                                                    ^^ `packed::repr_rust` cannot be safely transmuted into `()` in the defining scope of `assert::Context`.
    |
+   = help: the trait `BikeshedIntrinsicFrom<packed::repr_rust, assert::Context, true, true, true, true>` is not implemented for `()`
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
    |
@@ -133,12 +142,13 @@ LL |     where
 LL |         Dst: BikeshedIntrinsicFrom<Src, Context, true, true, true, true>
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_maybe_transmutable`
 
-error[E0277]: the trait bound `packed::repr_rust: BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not satisfied
+error[E0277]: `u128` cannot be safely transmuted into `packed::repr_rust` in the defining scope of `assert::Context`.
   --> $DIR/should_require_well_defined_layout.rs:46:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ the trait `BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not implemented for `packed::repr_rust`
+   |                                               ^^^^^^^^^ `u128` cannot be safely transmuted into `packed::repr_rust` in the defining scope of `assert::Context`.
    |
+   = help: the trait `BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not implemented for `packed::repr_rust`
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
    |
@@ -148,12 +158,13 @@ LL |     where
 LL |         Dst: BikeshedIntrinsicFrom<Src, Context, true, true, true, true>
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_maybe_transmutable`
 
-error[E0277]: the trait bound `(): BikeshedIntrinsicFrom<nested::repr_c, assert::Context, true, true, true, true>` is not satisfied
+error[E0277]: `nested::repr_c` cannot be safely transmuted into `()` in the defining scope of `assert::Context`.
   --> $DIR/should_require_well_defined_layout.rs:52:49
    |
 LL |         assert::is_maybe_transmutable::<repr_c, ()>();
-   |                                                 ^^ the trait `BikeshedIntrinsicFrom<nested::repr_c, assert::Context, true, true, true, true>` is not implemented for `()`
+   |                                                 ^^ `nested::repr_c` cannot be safely transmuted into `()` in the defining scope of `assert::Context`.
    |
+   = help: the trait `BikeshedIntrinsicFrom<nested::repr_c, assert::Context, true, true, true, true>` is not implemented for `()`
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
    |
@@ -163,12 +174,13 @@ LL |     where
 LL |         Dst: BikeshedIntrinsicFrom<Src, Context, true, true, true, true>
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_maybe_transmutable`
 
-error[E0277]: the trait bound `nested::repr_c: BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not satisfied
+error[E0277]: `u128` cannot be safely transmuted into `nested::repr_c` in the defining scope of `assert::Context`.
   --> $DIR/should_require_well_defined_layout.rs:53:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_c>();
-   |                                               ^^^^^^ the trait `BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not implemented for `nested::repr_c`
+   |                                               ^^^^^^ `u128` cannot be safely transmuted into `nested::repr_c` in the defining scope of `assert::Context`.
    |
+   = help: the trait `BikeshedIntrinsicFrom<u128, assert::Context, true, true, true, true>` is not implemented for `nested::repr_c`
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
    |