From 6b214bbc1128aa6bd9ddf732396fda7230dfeb64 Mon Sep 17 00:00:00 2001
From: Bryan Garza <1396101+bryangarza@users.noreply.github.com>
Date: Thu, 29 Jun 2023 15:01:39 -0700
Subject: [PATCH] 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.
---
 library/core/src/mem/transmutability.rs       |  1 +
 .../recursive-wrapper-types-bit-compatible.rs |  5 ++--
 ...ursive-wrapper-types-bit-compatible.stderr | 25 -------------------
 .../references/recursive-wrapper-types.rs     |  5 ++--
 .../references/recursive-wrapper-types.stderr | 25 -------------------
 5 files changed, 5 insertions(+), 56 deletions(-)
 delete mode 100644 tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.stderr
 delete mode 100644 tests/ui/transmutability/references/recursive-wrapper-types.stderr

diff --git a/library/core/src/mem/transmutability.rs b/library/core/src/mem/transmutability.rs
index 3805d149b704e..aed23119fd293 100644
--- a/library/core/src/mem/transmutability.rs
+++ b/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,
diff --git a/tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.rs b/tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.rs
index 3ea80173afaa9..709d8cdc762e9 100644
--- a/tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.rs
+++ b/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>();
 }
diff --git a/tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.stderr b/tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.stderr
deleted file mode 100644
index fae332e6af932..0000000000000
--- a/tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.stderr
+++ /dev/null
@@ -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`.
diff --git a/tests/ui/transmutability/references/recursive-wrapper-types.rs b/tests/ui/transmutability/references/recursive-wrapper-types.rs
index 59d1ad84a5d4c..090c1fea6dbdf 100644
--- a/tests/ui/transmutability/references/recursive-wrapper-types.rs
+++ b/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>();
 }
diff --git a/tests/ui/transmutability/references/recursive-wrapper-types.stderr b/tests/ui/transmutability/references/recursive-wrapper-types.stderr
deleted file mode 100644
index 35a60c226437a..0000000000000
--- a/tests/ui/transmutability/references/recursive-wrapper-types.stderr
+++ /dev/null
@@ -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`.