Enable coinduction support for Safe Transmute

[bryangarza]

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:

#[repr(C)] struct A(&'static B);
#[repr(C)] struct B(&'static A);

#[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.

[rustbot]

r? @b-naber

(rustbot has picked a reviewer for you, use r? to override)

[bryangarza]

r? @compiler-errors

[compiler-errors]

This PR marks the BikeshedIntrinsicFrom ("the safe transmute") trait as coinductive.

We shouldn't be committing to too much by making this trait coinductive, given that 1. it's not intended to be user implementable and has no user-written implementations, 2. it has no supertraits, and 3. is unstable.

@rfcbot fcp merge

[rfcbot]

Team member @compiler-errors has proposed to merge this. The next step is review by the rest of the tagged team members:

• @BoxyUwU
• @aliemjay
• @compiler-errors
• @jackh726
• @lcnr
• @nikomatsakis
• @oli-obk
• @spastorino

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[BoxyUwU]

I think it would be good for lcnr to have a look at this since afaik they have a good understanding of coinduction.

[lcnr]

@rfcbot reviewed

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[compiler-errors]

@bors r+

[bors]

📌 Commit 6b214bb has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit 6b214bb with merge b3ab80c...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing b3ab80c to master...

[rust-timer]

Finished benchmarking commit (b3ab80c): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.6%	[-3.6%, -3.6%]	1
Improvements ✅ (secondary)	-3.2%	[-3.6%, -2.8%]	2
All ❌✅ (primary)	-3.6%	[-3.6%, -3.6%]	1

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.1%	[-2.2%, -2.0%]	2
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 655.7s -> 656.645s (0.14%)