Enable co-induction support for Safe Transmute

This patch adds the `#[rustc_coinductive]` annotation to
`BikeshedIntrinsicFrom`, so that it's possible to compute transmutability for
recursive types.

Author: bryangarza

library/core/src/mem/transmutability.rs

@@ -9,6 +9,7 @@ use crate::marker::ConstParamTy;
 #[lang = "transmute_trait"]
 #[cfg_attr(not(bootstrap), rustc_deny_explicit_impl(implement_via_object = false))]
 #[cfg_attr(bootstrap, rustc_deny_explicit_impl)]
+#[rustc_coinductive]
 pub unsafe trait BikeshedIntrinsicFrom<Src, Context, const ASSUME: Assume = { Assume::NOTHING }>
 where
     Src: ?Sized,

tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.rs

@@ -1,5 +1,4 @@
-// check-fail
-// FIXME(bryangarza): Change to check-pass when coinduction is supported for BikeshedIntrinsicFrom
+// check-pass
 #![feature(transmutability)]
 
 mod assert {
@@ -22,5 +21,5 @@ mod assert {
 fn main() {
     #[repr(C)] struct A(bool, &'static A);
     #[repr(C)] struct B(u8, &'static B);
-    assert::is_maybe_transmutable::<&'static A, &'static B>(); //~ ERROR overflow evaluating the requirement
+    assert::is_maybe_transmutable::<&'static A, &'static B>();
 }

tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.stderr

@@ -1,5 +1,4 @@
-// check-fail
-// FIXME(bryangarza): Change to check-pass when coinduction is supported for BikeshedIntrinsicFrom
+// check-pass
 #![feature(transmutability)]
 
 mod assert {
@@ -22,6 +21,6 @@ mod assert {
 fn main() {
     #[repr(C)] struct A(&'static B);
     #[repr(C)] struct B(&'static A);
-    assert::is_maybe_transmutable::<&'static A, &'static B>(); //~ overflow evaluating the requirement
+    assert::is_maybe_transmutable::<&'static A, &'static B>();
     assert::is_maybe_transmutable::<&'static B, &'static A>();
 }