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

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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`.
This commit is contained in:
bors 2023-07-11 13:48:59 +00:00
commit b3ab80c119
5 changed files with 5 additions and 56 deletions
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1
library/core/src/mem/transmutability.rs
View File

@ -9,6 +9,7 @@
#[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,

5
tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.rs
View File

@ -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 @@ pub fn is_maybe_transmutable<Src, Dst>()
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>();
}

25
tests/ui/transmutability/references/recursive-wrapper-types-bit-compatible.stderr
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@ -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`.

5
tests/ui/transmutability/references/recursive-wrapper-types.rs
View File

@ -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 @@ pub fn is_maybe_transmutable<Src, Dst>()
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>();
}

25
tests/ui/transmutability/references/recursive-wrapper-types.stderr
View File

@ -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`.