Check RPIT and AFIT hidden types are well-formed considering regions

[compiler-errors]

Fixes #114728

r? @aliemjay @lcnr or @oli-obk (who's on vacation but he shall see it when he gets back 🌴)

[compiler-errors]

@bors try @rust-timer queue

[bors]

⌛ Trying commit fa1e4ba with merge d809384186eaa72b56b7b956dcf0837de4a00262...

[compiler-errors]

We could, perhaps, define the assumed wf types of a TAIT to be any types that show up in the bounds of a parent TAIT.

Don't think it's worth unless someone can come up with an unsoundness test here, though.

[bors]

☀️ Try build successful - checks-actions
Build commit: d809384186eaa72b56b7b956dcf0837de4a00262 (d809384186eaa72b56b7b956dcf0837de4a00262)

[rust-timer]

Finished benchmarking commit (d809384186eaa72b56b7b956dcf0837de4a00262): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -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)	-5.1%	[-6.0%, -4.6%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-5.1%	[-6.0%, -4.6%]	3

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)	1.3%	[1.3%, 1.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.3%	[1.3%, 1.3%]	1

Binary size

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

Bootstrap: 631.671s -> 631.783s (0.02%)

[compiler-errors]

@craterbot check

[craterbot]

👌 Experiment pr-114740 created and queued.
🤖 Automatically detected try build d809384186eaa72b56b7b956dcf0837de4a00262
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🚧 Experiment pr-114740 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[lcnr]

r? @lcnr
r=me after crater + fcp

@rust-lang/types we previously did not regionck RPITs and async function return types because we assumed that they would already be checked by their parent function.

This is not the case in case we end up "subtyping" into an RPIT, see #114728. This PR causes us to now always check regions for RPITs as well.

@rfcbot fcp merge

[lcnr]

sorry

@rfcbot cancel

[rfcbot]

@lcnr proposal cancelled.

[lcnr]

#114740 (comment)

@rfcbot fcp merge

[rfcbot]

Team member @lcnr 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.

[aliemjay]

Here is a couple of what I think are unnecessary breakages of this implementation:

• The RPIT variant of Incorrect lifetime bound check in async + impl_trait_in_assoc_type #114572:

struct Static<T: 'static>(T);
fn test<' a>(s: Static<&'static u8>) -> impl Sized + 'a { s } 

• Another more contreversial one:

trait Id<X> { type Ty; }
impl<X> Id<X> for () { type Ty = X; }
fn test() -> impl Id<&'static T, Ty = impl Sized> {}

Both are valid patterns that predate #94081. I believe we can keep supporting them by doing the WF check where subtyping is involved, namely in nll_relate and higher-ranked equality (which involves mutual subtyping).

[craterbot]

🎉 Experiment pr-114740 is completed!
📊 298 regressed and 2 fixed (348705 total)
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the blacklist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[compiler-errors]

@craterbot check crates=https://crater-reports.s3.amazonaws.com/pr-114740/retry-regressed-list.txt

[craterbot]

👌 Experiment pr-114740-1 created and queued.
🤖 Automatically detected try build d809384186eaa72b56b7b956dcf0837de4a00262
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🚧 Experiment pr-114740-1 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🎉 Experiment pr-114740-1 is completed!
📊 255 regressed and 0 fixed (298 total)
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the blacklist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[lcnr]

@aliemjay is trying an alternative approach in #114933, still in a crater run. It's definitely worth it to wait until that crater run is finished before making progress here.

[compiler-errors]

I'm gonna close this in favor of the other