Auto merge of #113175 - bryangarza:safe-transmute-rustc-coinductive, r=compiler-errors

Enable coinduction support for Safe Transmute

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

## Motivation
Safe Transmute currently already supports references (#110662). However, if a type is implemented recursively, it leads to an infinite loop when we try to check if transmutation is safe.

A couple simple examples that one might want to write, that are currently not possible to check transmutability for:
```rs
#[repr(C)] struct A(&'static B);
#[repr(C)] struct B(&'static A);
```

```rs
#[repr(C)]
enum IList<'a> { Nil, Cons(isize, &'a IList<'a>) }
#[repr(C)]
enum UList<'a> { Nil, Cons(usize, &'a UList<'a>) }
```

Previously, `@jswrenn` was considering writing a co-inductive solver from scratch, just for the `rustc_tranmsute` crate. Later on as I started working on Safe Transmute myself, I came across the `#[rustc_coinductive]` annotation, which is currently only being used for the `Sized` trait. Leveraging this trait actually solved the problem entirely, and it saves a lot of duplicate work that would have had to happen in `rustc_transmute`.

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

@@ -1,25 +0,0 @@
-error[E0275]: overflow evaluating the requirement `B: BikeshedIntrinsicFrom<A, assert::Context, Assume { alignment: true, lifetimes: false, safety: true, validity: false }>`
-  --> $DIR/recursive-wrapper-types-bit-compatible.rs:25:5
-   |
-LL |     assert::is_maybe_transmutable::<&'static A, &'static B>();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/recursive-wrapper-types-bit-compatible.rs:11:14
-   |
-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 E0275`.

tests/ui/transmutability/references/recursive-wrapper-types.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 {
@@ -22,6 +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>();
 }

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

@@ -1,25 +0,0 @@
-error[E0275]: overflow evaluating the requirement `A: BikeshedIntrinsicFrom<B, assert::Context, Assume { alignment: true, lifetimes: false, safety: true, validity: false }>`
-  --> $DIR/recursive-wrapper-types.rs:25:5
-   |
-LL |     assert::is_maybe_transmutable::<&'static A, &'static B>();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/recursive-wrapper-types.rs:11:14
-   |
-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 E0275`.