safe transmute: reference tracking issue

ref: #92268 (comment)

Author: jswrenn

library/core/src/mem/mod.rs

@@ -28,7 +28,7 @@ mod valid_align;
 pub(crate) use valid_align::ValidAlign;
 
 mod transmutability;
-#[unstable(feature = "transmutability", issue = "none")]
+#[unstable(feature = "transmutability", issue = "99571")]
 pub use transmutability::{Assume, BikeshedIntrinsicFrom};
 
 #[stable(feature = "rust1", since = "1.0.0")]

library/core/src/mem/transmutability.rs

@@ -3,7 +3,7 @@
 /// This trait is implemented on-the-fly by the compiler for types `Src` and `Self` when the bits of
 /// any value of type `Self` are safely transmutable into a value of type `Dst`, in a given `Context`,
 /// notwithstanding whatever safety checks you have asked the compiler to [`Assume`] are satisfied.
-#[unstable(feature = "transmutability", issue = "none")]
+#[unstable(feature = "transmutability", issue = "99571")]
 #[cfg_attr(not(bootstrap), lang = "transmute_trait")]
 #[rustc_on_unimplemented(
     message = "`{Src}` cannot be safely transmuted into `{Self}` in the defining scope of `{Context}`.",
@@ -22,7 +22,7 @@ pub unsafe trait BikeshedIntrinsicFrom<
 }
 
 /// What transmutation safety conditions shall the compiler assume that *you* are checking?
-#[unstable(feature = "transmutability", issue = "none")]
+#[unstable(feature = "transmutability", issue = "99571")]
 #[derive(PartialEq, Eq, Clone, Copy, Debug)]
 pub struct Assume {
     /// When `true`, the compiler assumes that *you* are ensuring (either dynamically or statically) that

src/test/ui/transmutability/malformed-program-gracefulness/feature-missing.rs

@@ -4,3 +4,6 @@
 
 use std::mem::BikeshedIntrinsicFrom;
 //~^ ERROR use of unstable library feature 'transmutability' [E0658]
+
+use std::mem::Assume;
+//~^ ERROR use of unstable library feature 'transmutability' [E0658]

src/test/ui/transmutability/malformed-program-gracefulness/feature-missing.stderr

@@ -4,8 +4,18 @@ error[E0658]: use of unstable library feature 'transmutability'
 LL | use std::mem::BikeshedIntrinsicFrom;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
+   = note: see issue #99571 <https://github.com/rust-lang/rust/issues/99571> for more information
    = help: add `#![feature(transmutability)]` to the crate attributes to enable
 
-error: aborting due to previous error
+error[E0658]: use of unstable library feature 'transmutability'
+  --> $DIR/feature-missing.rs:8:5
+   |
+LL | use std::mem::Assume;
+   |     ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #99571 <https://github.com/rust-lang/rust/issues/99571> for more information
+   = help: add `#![feature(transmutability)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0658`.