Safe Transmute: Enable handling references, including recursive types

This patch enables support for references in Safe Transmute, by generating
nested obligations during trait selection. Specifically, when we call
`confirm_transmutability_candidate(...)`, we now recursively traverse the
`rustc_transmute::Answer` tree and create obligations for all the `Answer`
variants, some of which include multiple nested `Answer`s.

Also, to handle recursive types, enable support for coinduction for the Safe
Transmute trait (`BikeshedIntrinsicFrom`) by adding the `#[rustc_coinduction]`
annotation.

Also fix some small logic issues when reducing the `or` and `and` combinations
in `rustc_transmute`, so that we don't end up with additional redundant
`Answer`s in the tree.

Co-authored-by: Jack Wrenn <jack@wrenn.fyi>

19 files changed, 216 insertions, 46 deletions

diff --git a/tests/ui/transmutability/arrays/should_require_well_defined_layout.stderr b/tests/ui/transmutability/arrays/should_require_well_defined_layout.stderr
index 1a0a5d3ae94..4a0cd176408 100644
--- a/tests/ui/transmutability/arrays/should_require_well_defined_layout.stderr
+++ b/tests/ui/transmutability/arrays/should_require_well_defined_layout.stderr
@@ -23,7 +23,7 @@ error[E0277]: `u128` cannot be safely transmuted into `[String; 0]` in the defin
   --> $DIR/should_require_well_defined_layout.rs:27:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ `[String; 0]` does not have a well-specified layout
+   |                                               ^^^^^^^^^ `u128` does not have a well-specified layout
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
@@ -65,7 +65,7 @@ error[E0277]: `u128` cannot be safely transmuted into `[String; 1]` in the defin
   --> $DIR/should_require_well_defined_layout.rs:33:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ `[String; 1]` does not have a well-specified layout
+   |                                               ^^^^^^^^^ `u128` does not have a well-specified layout
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
@@ -107,7 +107,7 @@ error[E0277]: `u128` cannot be safely transmuted into `[String; 2]` in the defin
   --> $DIR/should_require_well_defined_layout.rs:39:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ `[String; 2]` does not have a well-specified layout
+   |                                               ^^^^^^^^^ `u128` does not have a well-specified layout
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
diff --git a/tests/ui/transmutability/enums/repr/primitive_reprs_should_have_correct_length.stderr b/tests/ui/transmutability/enums/repr/primitive_reprs_should_have_correct_length.stderr
index 9877a6606a9..77d2dc4f50c 100644
--- a/tests/ui/transmutability/enums/repr/primitive_reprs_should_have_correct_length.stderr
+++ b/tests/ui/transmutability/enums/repr/primitive_reprs_should_have_correct_length.stderr
@@ -24,7 +24,7 @@ error[E0277]: `V0i8` cannot be safely transmuted into `u16` in the defining scop
   --> $DIR/primitive_reprs_should_have_correct_length.rs:50:44
    |
 LL |         assert::is_transmutable::<Current, Larger, Context>();
-   |                                            ^^^^^^ The size of `V0i8` is smaller than the size of `u16`
+   |                                            ^^^^^^ At least one value of `V0i8` isn't a bit-valid value of `u16`
    |
 note: required by a bound in `is_transmutable`
   --> $DIR/primitive_reprs_should_have_correct_length.rs:12:14
@@ -68,7 +68,7 @@ error[E0277]: `V0u8` cannot be safely transmuted into `u16` in the defining scop
   --> $DIR/primitive_reprs_should_have_correct_length.rs:58:44
    |
 LL |         assert::is_transmutable::<Current, Larger, Context>();
-   |                                            ^^^^^^ The size of `V0u8` is smaller than the size of `u16`
+   |                                            ^^^^^^ At least one value of `V0u8` isn't a bit-valid value of `u16`
    |
 note: required by a bound in `is_transmutable`
   --> $DIR/primitive_reprs_should_have_correct_length.rs:12:14
@@ -112,7 +112,7 @@ error[E0277]: `V0i16` cannot be safely transmuted into `u32` in the defining sco
   --> $DIR/primitive_reprs_should_have_correct_length.rs:74:44
    |
 LL |         assert::is_transmutable::<Current, Larger, Context>();
-   |                                            ^^^^^^ The size of `V0i16` is smaller than the size of `u32`
+   |                                            ^^^^^^ At least one value of `V0i16` isn't a bit-valid value of `u32`
    |
 note: required by a bound in `is_transmutable`
   --> $DIR/primitive_reprs_should_have_correct_length.rs:12:14
@@ -156,7 +156,7 @@ error[E0277]: `V0u16` cannot be safely transmuted into `u32` in the defining sco
   --> $DIR/primitive_reprs_should_have_correct_length.rs:82:44
    |
 LL |         assert::is_transmutable::<Current, Larger, Context>();
-   |                                            ^^^^^^ The size of `V0u16` is smaller than the size of `u32`
+   |                                            ^^^^^^ At least one value of `V0u16` isn't a bit-valid value of `u32`
    |
 note: required by a bound in `is_transmutable`
   --> $DIR/primitive_reprs_should_have_correct_length.rs:12:14
@@ -200,7 +200,7 @@ error[E0277]: `V0i32` cannot be safely transmuted into `u64` in the defining sco
   --> $DIR/primitive_reprs_should_have_correct_length.rs:98:44
    |
 LL |         assert::is_transmutable::<Current, Larger, Context>();
-   |                                            ^^^^^^ The size of `V0i32` is smaller than the size of `u64`
+   |                                            ^^^^^^ At least one value of `V0i32` isn't a bit-valid value of `u64`
    |
 note: required by a bound in `is_transmutable`
   --> $DIR/primitive_reprs_should_have_correct_length.rs:12:14
@@ -244,7 +244,7 @@ error[E0277]: `V0u32` cannot be safely transmuted into `u64` in the defining sco
   --> $DIR/primitive_reprs_should_have_correct_length.rs:106:44
    |
 LL |         assert::is_transmutable::<Current, Larger, Context>();
-   |                                            ^^^^^^ The size of `V0u32` is smaller than the size of `u64`
+   |                                            ^^^^^^ At least one value of `V0u32` isn't a bit-valid value of `u64`
    |
 note: required by a bound in `is_transmutable`
   --> $DIR/primitive_reprs_should_have_correct_length.rs:12:14
@@ -288,7 +288,7 @@ error[E0277]: `V0i64` cannot be safely transmuted into `u128` in the defining sc
   --> $DIR/primitive_reprs_should_have_correct_length.rs:122:44
    |
 LL |         assert::is_transmutable::<Current, Larger, Context>();
-   |                                            ^^^^^^ The size of `V0i64` is smaller than the size of `u128`
+   |                                            ^^^^^^ At least one value of `V0i64` isn't a bit-valid value of `u128`
    |
 note: required by a bound in `is_transmutable`
   --> $DIR/primitive_reprs_should_have_correct_length.rs:12:14
@@ -332,7 +332,7 @@ error[E0277]: `V0u64` cannot be safely transmuted into `u128` in the defining sc
   --> $DIR/primitive_reprs_should_have_correct_length.rs:130:44
    |
 LL |         assert::is_transmutable::<Current, Larger, Context>();
-   |                                            ^^^^^^ The size of `V0u64` is smaller than the size of `u128`
+   |                                            ^^^^^^ At least one value of `V0u64` isn't a bit-valid value of `u128`
    |
 note: required by a bound in `is_transmutable`
   --> $DIR/primitive_reprs_should_have_correct_length.rs:12:14
@@ -376,7 +376,7 @@ error[E0277]: `V0isize` cannot be safely transmuted into `[usize; 2]` in the def
   --> $DIR/primitive_reprs_should_have_correct_length.rs:146:44
    |
 LL |         assert::is_transmutable::<Current, Larger, Context>();
-   |                                            ^^^^^^ The size of `V0isize` is smaller than the size of `[usize; 2]`
+   |                                            ^^^^^^ At least one value of `V0isize` isn't a bit-valid value of `[usize; 2]`
    |
 note: required by a bound in `is_transmutable`
   --> $DIR/primitive_reprs_should_have_correct_length.rs:12:14
@@ -420,7 +420,7 @@ error[E0277]: `V0usize` cannot be safely transmuted into `[usize; 2]` in the def
   --> $DIR/primitive_reprs_should_have_correct_length.rs:154:44
    |
 LL |         assert::is_transmutable::<Current, Larger, Context>();
-   |                                            ^^^^^^ The size of `V0usize` is smaller than the size of `[usize; 2]`
+   |                                            ^^^^^^ At least one value of `V0usize` isn't a bit-valid value of `[usize; 2]`
    |
 note: required by a bound in `is_transmutable`
   --> $DIR/primitive_reprs_should_have_correct_length.rs:12:14
diff --git a/tests/ui/transmutability/enums/repr/should_require_well_defined_layout.stderr b/tests/ui/transmutability/enums/repr/should_require_well_defined_layout.stderr
index 1612b6b3661..6508d535b05 100644
--- a/tests/ui/transmutability/enums/repr/should_require_well_defined_layout.stderr
+++ b/tests/ui/transmutability/enums/repr/should_require_well_defined_layout.stderr
@@ -24,7 +24,7 @@ error[E0277]: `u128` cannot be safely transmuted into `void::repr_rust` in the d
   --> $DIR/should_require_well_defined_layout.rs:29:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ `void::repr_rust` does not have a well-specified layout
+   |                                               ^^^^^^^^^ `u128` does not have a well-specified layout
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:14:14
@@ -68,7 +68,7 @@ error[E0277]: `u128` cannot be safely transmuted into `singleton::repr_rust` in
   --> $DIR/should_require_well_defined_layout.rs:35:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ `singleton::repr_rust` does not have a well-specified layout
+   |                                               ^^^^^^^^^ `u128` does not have a well-specified layout
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:14:14
@@ -112,7 +112,7 @@ error[E0277]: `u128` cannot be safely transmuted into `duplex::repr_rust` in the
   --> $DIR/should_require_well_defined_layout.rs:41:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ `duplex::repr_rust` does not have a well-specified layout
+   |                                               ^^^^^^^^^ `u128` does not have a well-specified layout
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:14:14
diff --git a/tests/ui/transmutability/enums/should_pad_variants.stderr b/tests/ui/transmutability/enums/should_pad_variants.stderr
index bfbef8b25fc..c82a7968022 100644
--- a/tests/ui/transmutability/enums/should_pad_variants.stderr
+++ b/tests/ui/transmutability/enums/should_pad_variants.stderr
@@ -2,7 +2,7 @@ error[E0277]: `Src` cannot be safely transmuted into `Dst` in the defining scope
   --> $DIR/should_pad_variants.rs:44:36
    |
 LL |     assert::is_transmutable::<Src, Dst, Context>();
-   |                                    ^^^ The size of `Src` is smaller than the size of `Dst`
+   |                                    ^^^ At least one value of `Src` isn't a bit-valid value of `Dst`
    |
 note: required by a bound in `is_transmutable`
   --> $DIR/should_pad_variants.rs:13:14
diff --git a/tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.rs b/tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.rs
new file mode 100644
index 00000000000..918147a0862
--- /dev/null
+++ b/tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.rs
@@ -0,0 +1,27 @@
+// check-fail
+#![feature(transmutability)]
+
+mod assert {
+    use std::mem::{Assume, BikeshedIntrinsicFrom};
+    pub struct Context;
+
+    pub fn is_maybe_transmutable<Src, Dst>()
+    where
+        Dst: BikeshedIntrinsicFrom<Src, Context, {
+            Assume {
+                alignment: true,
+                lifetimes: false,
+                safety: true,
+                validity: false,
+            }
+        }>
+    {}
+}
+
+fn main() {
+    #[repr(C)] struct A(bool, &'static A);
+    #[repr(C)] struct B(u8, &'static B);
+    // FIXME(bryangarza): Make 2 variants of this test, depending on mutability.
+    // Right now, we are being strict by default and checking A->B and B->A both.
+    assert::is_maybe_transmutable::<&'static A, &'static B>(); //~ ERROR `B` cannot be safely transmuted into `A`
+}
diff --git a/tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.stderr b/tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.stderr
new file mode 100644
index 00000000000..fac0e4f032e
--- /dev/null
+++ b/tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.stderr
@@ -0,0 +1,25 @@
+error[E0277]: `B` cannot be safely transmuted into `A` in the defining scope of `assert::Context`
+  --> $DIR/recursive-wrapper-types-bit-compatible.rs:26:49
+   |
+LL |     assert::is_maybe_transmutable::<&'static A, &'static B>();
+   |                                                 ^^^^^^^^^^ At least one value of `B` isn't a bit-valid value of `A`
+   |
+note: required by a bound in `is_maybe_transmutable`
+  --> $DIR/recursive-wrapper-types-bit-compatible.rs:10:14
+   |
+LL |       pub fn is_maybe_transmutable<Src, Dst>()
+   |              --------------------- required by a bound in this function
+LL |       where
+LL |           Dst: BikeshedIntrinsicFrom<Src, Context, {
+   |  ______________^
+LL | |             Assume {
+LL | |                 alignment: true,
+LL | |                 lifetimes: false,
+...  |
+LL | |             }
+LL | |         }>
+   | |__________^ required by this bound in `is_maybe_transmutable`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/transmutability/references/recursive-wrapper-types-bit-incompatible.rs b/tests/ui/transmutability/references/recursive-wrapper-types-bit-incompatible.rs
new file mode 100644
index 00000000000..6dcb7df9feb
--- /dev/null
+++ b/tests/ui/transmutability/references/recursive-wrapper-types-bit-incompatible.rs
@@ -0,0 +1,25 @@
+// check-fail
+#![feature(transmutability)]
+
+mod assert {
+    use std::mem::{Assume, BikeshedIntrinsicFrom};
+    pub struct Context;
+
+    pub fn is_maybe_transmutable<Src, Dst>()
+    where
+        Dst: BikeshedIntrinsicFrom<Src, Context, {
+            Assume {
+                alignment: true,
+                lifetimes: false,
+                safety: true,
+                validity: false,
+            }
+        }>
+    {}
+}
+
+fn main() {
+    #[repr(C)] struct A(bool, &'static A);
+    #[repr(C)] struct B(u8, &'static B);
+    assert::is_maybe_transmutable::<&'static B, &'static A>(); //~ ERROR `B` cannot be safely transmuted into `A`
+}
diff --git a/tests/ui/transmutability/references/recursive-wrapper-types-bit-incompatible.stderr b/tests/ui/transmutability/references/recursive-wrapper-types-bit-incompatible.stderr
new file mode 100644
index 00000000000..ecfe4865962
--- /dev/null
+++ b/tests/ui/transmutability/references/recursive-wrapper-types-bit-incompatible.stderr
@@ -0,0 +1,25 @@
+error[E0277]: `B` cannot be safely transmuted into `A` in the defining scope of `assert::Context`
+  --> $DIR/recursive-wrapper-types-bit-incompatible.rs:24:49
+   |
+LL |     assert::is_maybe_transmutable::<&'static B, &'static A>();
+   |                                                 ^^^^^^^^^^ At least one value of `B` isn't a bit-valid value of `A`
+   |
+note: required by a bound in `is_maybe_transmutable`
+  --> $DIR/recursive-wrapper-types-bit-incompatible.rs:10:14
+   |
+LL |       pub fn is_maybe_transmutable<Src, Dst>()
+   |              --------------------- required by a bound in this function
+LL |       where
+LL |           Dst: BikeshedIntrinsicFrom<Src, Context, {
+   |  ______________^
+LL | |             Assume {
+LL | |                 alignment: true,
+LL | |                 lifetimes: false,
+...  |
+LL | |             }
+LL | |         }>
+   | |__________^ required by this bound in `is_maybe_transmutable`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/transmutability/references/recursive-wrapper-types.rs b/tests/ui/transmutability/references/recursive-wrapper-types.rs
new file mode 100644
index 00000000000..090c1fea6db
--- /dev/null
+++ b/tests/ui/transmutability/references/recursive-wrapper-types.rs
@@ -0,0 +1,26 @@
+// check-pass
+#![feature(transmutability)]
+
+mod assert {
+    use std::mem::{Assume, BikeshedIntrinsicFrom};
+    pub struct Context;
+
+    pub fn is_maybe_transmutable<Src, Dst>()
+    where
+        Dst: BikeshedIntrinsicFrom<Src, Context, {
+            Assume {
+                alignment: true,
+                lifetimes: false,
+                safety: true,
+                validity: false,
+            }
+        }>
+    {}
+}
+
+fn main() {
+    #[repr(C)] struct A(&'static B);
+    #[repr(C)] struct B(&'static A);
+    assert::is_maybe_transmutable::<&'static A, &'static B>();
+    assert::is_maybe_transmutable::<&'static B, &'static A>();
+}
diff --git a/tests/ui/transmutability/references.rs b/tests/ui/transmutability/references/u8-to-unit.rs
index 8c2b25ebba1..b7dd638b952 100644
--- a/tests/ui/transmutability/references.rs
+++ b/tests/ui/transmutability/references/u8-to-unit.rs
@@ -1,11 +1,5 @@
-// revisions: current next
-//[next] compile-flags: -Ztrait-solver=next
-
-//! Transmutations involving references are not yet supported.
-
-#![crate_type = "lib"]
+// check-fail
 #![feature(transmutability)]
-#![allow(dead_code, incomplete_features, non_camel_case_types)]
 
 mod assert {
     use std::mem::{Assume, BikeshedIntrinsicFrom};
@@ -24,7 +18,7 @@ mod assert {
     {}
 }
 
-fn not_yet_implemented() {
+fn main() {
     #[repr(C)] struct Unit;
-    assert::is_maybe_transmutable::<&'static Unit, &'static Unit>(); //~ ERROR cannot be safely transmuted
+    assert::is_maybe_transmutable::<&'static u8, &'static Unit>(); //~ ERROR `Unit` cannot be safely transmuted into `u8`
 }
diff --git a/tests/ui/transmutability/references.next.stderr b/tests/ui/transmutability/references/u8-to-unit.stderr
index 819c9b92bc8..81b0b57f0cf 100644
--- a/tests/ui/transmutability/references.next.stderr
+++ b/tests/ui/transmutability/references/u8-to-unit.stderr
@@ -1,11 +1,11 @@
-error[E0277]: `&Unit` cannot be safely transmuted into `&Unit` in the defining scope of `assert::Context`
-  --> $DIR/references.rs:29:52
+error[E0277]: `Unit` cannot be safely transmuted into `u8` in the defining scope of `assert::Context`
+  --> $DIR/u8-to-unit.rs:23:50
    |
-LL |     assert::is_maybe_transmutable::<&'static Unit, &'static Unit>();
-   |                                                    ^^^^^^^^^^^^^ `&Unit` does not have a well-specified layout
+LL |     assert::is_maybe_transmutable::<&'static u8, &'static Unit>();
+   |                                                  ^^^^^^^^^^^^^ The size of `Unit` is smaller than the size of `u8`
    |
 note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/references.rs:16:14
+  --> $DIR/u8-to-unit.rs:10:14
    |
 LL |       pub fn is_maybe_transmutable<Src, Dst>()
    |              --------------------- required by a bound in this function
diff --git a/tests/ui/transmutability/references/unit-to-itself.rs b/tests/ui/transmutability/references/unit-to-itself.rs
new file mode 100644
index 00000000000..04a7e16d7cc
--- /dev/null
+++ b/tests/ui/transmutability/references/unit-to-itself.rs
@@ -0,0 +1,24 @@
+// check-pass
+#![feature(transmutability)]
+
+mod assert {
+    use std::mem::{Assume, BikeshedIntrinsicFrom};
+    pub struct Context;
+
+    pub fn is_maybe_transmutable<Src, Dst>()
+    where
+        Dst: BikeshedIntrinsicFrom<Src, Context, {
+            Assume {
+                alignment: true,
+                lifetimes: false,
+                safety: true,
+                validity: false,
+            }
+        }>
+    {}
+}
+
+fn main() {
+    #[repr(C)] struct Unit;
+    assert::is_maybe_transmutable::<&'static Unit, &'static Unit>();
+}
diff --git a/tests/ui/transmutability/references/unit-to-u8.rs b/tests/ui/transmutability/references/unit-to-u8.rs
new file mode 100644
index 00000000000..73e1db3d2d5
--- /dev/null
+++ b/tests/ui/transmutability/references/unit-to-u8.rs
@@ -0,0 +1,24 @@
+// check-fail
+#![feature(transmutability)]
+
+mod assert {
+    use std::mem::{Assume, BikeshedIntrinsicFrom};
+    pub struct Context;
+
+    pub fn is_maybe_transmutable<Src, Dst>()
+    where
+        Dst: BikeshedIntrinsicFrom<Src, Context, {
+            Assume {
+                alignment: true,
+                lifetimes: true,
+                safety: true,
+                validity: true,
+            }
+        }>
+    {}
+}
+
+fn main() {
+    #[repr(C)] struct Unit;
+    assert::is_maybe_transmutable::<&'static Unit, &'static u8>(); //~ ERROR `Unit` cannot be safely transmuted into `u8`
+}
diff --git a/tests/ui/transmutability/references.current.stderr b/tests/ui/transmutability/references/unit-to-u8.stderr
index 819c9b92bc8..f2b72357f79 100644
--- a/tests/ui/transmutability/references.current.stderr
+++ b/tests/ui/transmutability/references/unit-to-u8.stderr
@@ -1,11 +1,11 @@
-error[E0277]: `&Unit` cannot be safely transmuted into `&Unit` in the defining scope of `assert::Context`
-  --> $DIR/references.rs:29:52
+error[E0277]: `Unit` cannot be safely transmuted into `u8` in the defining scope of `assert::Context`
+  --> $DIR/unit-to-u8.rs:23:52
    |
-LL |     assert::is_maybe_transmutable::<&'static Unit, &'static Unit>();
-   |                                                    ^^^^^^^^^^^^^ `&Unit` does not have a well-specified layout
+LL |     assert::is_maybe_transmutable::<&'static Unit, &'static u8>();
+   |                                                    ^^^^^^^^^^^ The size of `Unit` is smaller than the size of `u8`
    |
 note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/references.rs:16:14
+  --> $DIR/unit-to-u8.rs:10:14
    |
 LL |       pub fn is_maybe_transmutable<Src, Dst>()
    |              --------------------- required by a bound in this function
diff --git a/tests/ui/transmutability/region-infer.stderr b/tests/ui/transmutability/region-infer.stderr
index d6b65e9e4a0..307d0dfe50d 100644
--- a/tests/ui/transmutability/region-infer.stderr
+++ b/tests/ui/transmutability/region-infer.stderr
@@ -2,7 +2,7 @@ error[E0277]: `()` cannot be safely transmuted into `W<'_>` in the defining scop
   --> $DIR/region-infer.rs:20:5
    |
 LL |     test();
-   |     ^^^^ `W<'_>` does not have a well-specified layout
+   |     ^^^^ The size of `()` is smaller than the size of `W<'_>`
    |
 note: required by a bound in `test`
   --> $DIR/region-infer.rs:11:12
diff --git a/tests/ui/transmutability/structs/repr/should_require_well_defined_layout.stderr b/tests/ui/transmutability/structs/repr/should_require_well_defined_layout.stderr
index 4c5062cd3b3..7f26bc49817 100644
--- a/tests/ui/transmutability/structs/repr/should_require_well_defined_layout.stderr
+++ b/tests/ui/transmutability/structs/repr/should_require_well_defined_layout.stderr
@@ -24,7 +24,7 @@ error[E0277]: `u128` cannot be safely transmuted into `should_reject_repr_rust::
   --> $DIR/should_require_well_defined_layout.rs:29:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ `should_reject_repr_rust::unit::repr_rust` does not have a well-specified layout
+   |                                               ^^^^^^^^^ `u128` does not have a well-specified layout
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
@@ -68,7 +68,7 @@ error[E0277]: `u128` cannot be safely transmuted into `should_reject_repr_rust::
   --> $DIR/should_require_well_defined_layout.rs:35:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ `should_reject_repr_rust::tuple::repr_rust` does not have a well-specified layout
+   |                                               ^^^^^^^^^ `u128` does not have a well-specified layout
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
@@ -112,7 +112,7 @@ error[E0277]: `u128` cannot be safely transmuted into `should_reject_repr_rust::
   --> $DIR/should_require_well_defined_layout.rs:41:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ `should_reject_repr_rust::braces::repr_rust` does not have a well-specified layout
+   |                                               ^^^^^^^^^ `u128` does not have a well-specified layout
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
@@ -156,7 +156,7 @@ error[E0277]: `u128` cannot be safely transmuted into `aligned::repr_rust` in th
   --> $DIR/should_require_well_defined_layout.rs:47:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ `aligned::repr_rust` does not have a well-specified layout
+   |                                               ^^^^^^^^^ `u128` does not have a well-specified layout
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
@@ -200,7 +200,7 @@ error[E0277]: `u128` cannot be safely transmuted into `packed::repr_rust` in the
   --> $DIR/should_require_well_defined_layout.rs:53:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ `packed::repr_rust` does not have a well-specified layout
+   |                                               ^^^^^^^^^ `u128` does not have a well-specified layout
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
@@ -244,7 +244,7 @@ error[E0277]: `u128` cannot be safely transmuted into `nested::repr_c` in the de
   --> $DIR/should_require_well_defined_layout.rs:60:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_c>();
-   |                                               ^^^^^^ `nested::repr_c` does not have a well-specified layout
+   |                                               ^^^^^^ `u128` does not have a well-specified layout
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
diff --git a/tests/ui/transmutability/unions/repr/should_require_well_defined_layout.stderr b/tests/ui/transmutability/unions/repr/should_require_well_defined_layout.stderr
index 4293d34f47b..305ecc71733 100644
--- a/tests/ui/transmutability/unions/repr/should_require_well_defined_layout.stderr
+++ b/tests/ui/transmutability/unions/repr/should_require_well_defined_layout.stderr
@@ -24,7 +24,7 @@ error[E0277]: `u128` cannot be safely transmuted into `should_reject_repr_rust::
   --> $DIR/should_require_well_defined_layout.rs:31:43
    |
 LL |     assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                           ^^^^^^^^^ `should_reject_repr_rust::repr_rust` does not have a well-specified layout
+   |                                           ^^^^^^^^^ `u128` does not have a well-specified layout
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:13:14
diff --git a/tests/ui/transmutability/unions/should_pad_variants.stderr b/tests/ui/transmutability/unions/should_pad_variants.stderr
index bfbef8b25fc..c82a7968022 100644
--- a/tests/ui/transmutability/unions/should_pad_variants.stderr
+++ b/tests/ui/transmutability/unions/should_pad_variants.stderr
@@ -2,7 +2,7 @@ error[E0277]: `Src` cannot be safely transmuted into `Dst` in the defining scope
   --> $DIR/should_pad_variants.rs:44:36
    |
 LL |     assert::is_transmutable::<Src, Dst, Context>();
-   |                                    ^^^ The size of `Src` is smaller than the size of `Dst`
+   |                                    ^^^ At least one value of `Src` isn't a bit-valid value of `Dst`
    |
 note: required by a bound in `is_transmutable`
   --> $DIR/should_pad_variants.rs:13:14
diff --git a/tests/ui/transmute/transmute-padding-ice.stderr b/tests/ui/transmute/transmute-padding-ice.stderr
index f5480e0b9fb..2739fd16645 100644
--- a/tests/ui/transmute/transmute-padding-ice.stderr
+++ b/tests/ui/transmute/transmute-padding-ice.stderr
@@ -2,7 +2,7 @@ error[E0277]: `B` cannot be safely transmuted into `A` in the defining scope of
   --> $DIR/transmute-padding-ice.rs:27:40
    |
 LL |     assert::is_maybe_transmutable::<B, A>();
-   |                                        ^ The size of `B` is smaller than the size of `A`
+   |                                        ^ At least one value of `B` isn't a bit-valid value of `A`
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/transmute-padding-ice.rs:11:14