check for non-defining uses of RPIT

[lcnr]

This PR requires defining uses of RPIT and the async functions return type to use unique generic parameters as type and const arguments, (mostly) fixing #111935. This changes the following snippet to an error (it compiled since 1.62):

fn foo<T>() -> impl Sized {
    let _: () = foo::<u8>(); //~ ERROR non-defining use of `impl Sized`
}

Since 1.62 we only checked that the generic arguments of opaque types are unique parameters for TAIT and ignored RPITs, so this PR changes the behavior here to be consistent.

For defining uses which do not have unique params as arguments it is unclear how the hidden type should map to the generic params of the opaque. In the following snippet, should the hidden type of foo<T>::opaque be T or u32.

fn foo<T>() -> impl Sized {
    let _: u32 = foo::<u32>();
    foo::<T>()
}

There are no crater regressions caused by this change.

---

The same issue exists for lifetime arguments which is not fixed by this PR, currently resulting in an ICE in mir borrowck (I wasn't able to get an example which didn't ICE, it might be possible):

fn foo<'a: 'a>() -> impl Sized {
    let _: &'static () = foo::<'static>();
    //~^ ICE opaque type with non-universal region substs
    foo::<'a>()
}

Fixing this for lifetimes as well is blocked on #113916. Due to this issue, functions returning an RPIT with lifetime parameters equal in the region constraint graph would always result in an error, resulting in breakage found via crater: #112842 (comment)

trait Trait<'a, 'b> {}
impl Trait<'_, '_> for () {}

struct Type<'a>(&'a ());
impl<'a> Type<'a> {
    // `'b == 'a`
    fn do_stuff<'b: 'a>(&'b self) -> impl Trait<'a, 'b> {
        // This fails as long there is something in the body
        // which adds the outlives constraints to the constraint graph.
        //
        // This is the case for nested closures.
        (|| ())() 
    
    }
}

[lcnr]

@bors try

[bors]

⌛ Trying commit dce9c1768c11854dd47b76ebed02a85029d74010 with merge cc4c4328b2b4da72bc61f8f5984292dd73740b5e...

[bors]

☀️ Try build successful - checks-actions
Build commit: cc4c4328b2b4da72bc61f8f5984292dd73740b5e (cc4c4328b2b4da72bc61f8f5984292dd73740b5e)

[oli-obk]

@craterbot check

[craterbot]

👌 Experiment pr-112842 created and queued.
🤖 Automatically detected try build cc4c4328b2b4da72bc61f8f5984292dd73740b5e
🔍 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-112842 is now running

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

[aliemjay]

continuing on from #112841 (comment)

I think the right way to fix the case of async fn and RRPIT is to allow equal lifetimes to appear in the substs if and only if these lifetimes are assumed equal by the opaque type's definition

trait Trait<'a, 'b> {}
impl Trait<'_, '_> for () {}

type Tait1<'a, 'b> = impl Trait<'a, 'b>;
fn define1<'a>() -> Tait1<'a, 'a> {} // ERROR!


type Tait2<'a:' 'b, 'b: 'a> = impl Trait<'a, 'b>;
fn define2<'a>() -> Tait2<'a, 'a> {} // OK!

Because the user can't reference Tait<'a, 'b> without 'a and 'b being equal, per the well-formedness rules.

[craterbot]

🎉 Experiment pr-112842 is completed!
📊 315 regressed and 9 fixed (303008 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

[aliemjay]

@craterbot run mode=check-only p=1 crates=https://crater-reports.s3.amazonaws.com/pr-112842/retry-regressed-list.txt

[craterbot]

👌 Experiment pr-112842-1 created and queued.
🤖 Automatically detected try build cc4c4328b2b4da72bc61f8f5984292dd73740b5e
🔍 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-112842-1 is now running

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

[craterbot]

🎉 Experiment pr-112842-1 is completed!
📊 229 regressed and 0 fixed (315 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

[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.

[oli-obk]

r=me with @aliemjay's comment addressed

@rustbot author

[lcnr]

@bors r=oli-obk rollup=never

[bors]

📌 Commit e04f582 has been approved by oli-obk

It is now in the queue for this repository.

[bors]

⌛ Testing commit e04f582 with merge e845910...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing e845910 to master...

[rust-timer]

Finished benchmarking commit (e845910): 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)	3.9%	[3.9%, 3.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-2.4%, -2.4%]	1
All ❌✅ (primary)	-	-	0

Cycles

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

Binary size

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

Bootstrap: 633.194s -> 633.099s (-0.02%)
Artifact size: 346.49 MiB -> 346.34 MiB (-0.04%)