[WIP] Support param bounds on non-lifetime binders

[compiler-errors]

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This PR adds resolution and AST lowering for where clauses on binders. The meaning for for<T> now becomes for<T: Sized> like regular generics positions, and you are now able to write for<T: Trait> (and for<T: ?Sized>). Binder predicates are only considered in the new solver today. Since non-lifetime binders is an incomplete feature, I don't think we need to do any messaging to tell people that they don't work correctly in the old solver, but I'll see to that in a follow-up.

This PR then adds a new List<ty::Clause> to binders. Most places in the compiler shouldn't care about them existing, but eventually as support for non-lifetime binders gets fleshed out we should be more careful about asserting that they're handled. This will not happen in this PR.

Tracking:

• Tracking Issue for non-lifetime binders #108185

[compiler-errors]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 3471faa6dbd43943bb8dc2e3bdedf59fab546439 with merge ff357375eca19961c0e26a765585b8cf4b3a1b3c...

[bors]

💔 Test failed - checks-actions

[bors]

☔ The latest upstream changes (presumably #115361) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #115751) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #116885) made this pull request unmergeable. Please resolve the merge conflicts.

[fmease]

Obviously, this is still WIP. Here's one observation I made while playing around with this patch:

trait Trait<T> {}
fn f(_: impl for<T: ?Sized> Trait<T>) {}

This successfully compiles (-Ztrait-solver=next ofc) while I don't think it should. impl for<T: ?Sized> Trait<T> isn't WF since the contained Trait<T> requires T: Sized to hold to be WF which isn't the case here. Cf: fn g<T: ?Sized>(_: impl Trait<T>) (ofc this is far from semantically equivalent) doesn't compile, rightly so.

[compiler-errors]

@fmease: Yeah, that's pre-existing.

#![feature(non_lifetime_binders)]

trait Trait<T> {}
fn f<S: for<T> Trait<T>>(_: S) {}
//~^ Recall that `for<T>` on stable == `for<T: ?Sized>`

This is likely due to one of the many !obligation.has_escaping_bound_vars() in wf code.

[compiler-errors]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 82ae54c with merge 2e849e3...

[lcnr]

I am personally not a big fan of supporting where-bounds inside of types and would like to only allow them in where-bounds. I also believe that this is where pretty much all of the perf regression is coming from.

I do expect that playing around with this feature is more annoying and ugly if we have to split binder for it, so 🤷 i guess i don't mind too much

[lcnr]

Currently don't have the mental capacity to review changes related to this feature in-depth

r? oli-obk maybe 🤔

[nnethercote]

w.r.t. perf: we have a lot of size assertions in the codebase, but the one on TyKind is one of the most important. There are many instances of that type and they are all interned, so if it gets bigger that's more memory usage and more hashing costs. It's up there with ast::Expr at the top of the list of "types I least want to get bigger" :(

[compiler-errors]

Maybe I can intern the pair of clauses lists into one or something 🤔 Ideas would be helpful

[bors]

☔ The latest upstream changes (presumably #138965) made this pull request unmergeable. Please resolve the merge conflicts.

[oli-obk]

Since the type size regression comes solely from FnPtr, maybe just intern the contents of the FnPtr variant?

[compiler-errors]

Well, we also have to intern unsafe binders 🤔

[compiler-errors]

@bors try @rust-timer queue

[bors]

⌛ Trying commit f3652c5 with merge e985804...

[bors]

☀️ Try build successful - checks-actions
Build commit: e985804 (e985804039a6ff5eb9176c17876b1030ef7c149b)

[rust-timer]

Finished benchmarking commit (e985804): comparison URL.

Overall result: ❌ regressions - please read the text below

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.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.1%, 5.0%]	75
Regressions ❌ (secondary)	0.7%	[0.1%, 2.4%]	62
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[0.1%, 5.0%]	75

Max RSS (memory usage)

Results (primary 0.1%, secondary 2.2%)

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%	[0.6%, 3.0%]	6
Regressions ❌ (secondary)	2.9%	[0.8%, 5.3%]	15
Improvements ✅ (primary)	-3.5%	[-3.7%, -3.2%]	2
Improvements ✅ (secondary)	-1.3%	[-1.4%, -1.2%]	3
All ❌✅ (primary)	0.1%	[-3.7%, 3.0%]	8

Cycles

Results (primary 1.4%, secondary 1.9%)

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)	1.4%	[0.8%, 4.0%]	38
Regressions ❌ (secondary)	1.9%	[1.2%, 2.8%]	15
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.4%	[0.8%, 4.0%]	38

Binary size

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

Bootstrap: 777.207s -> 792.224s (1.93%)
Artifact size: 365.95 MiB -> 366.51 MiB (0.15%)

[eericjacobson]

why was this closed?

[oli-obk]

EDIT (previously contained a personal rant):

The PR is something we should revive if we find a dev for it.

[eericjacobson]

how do big companies have an influence over single feature PRs in rust nightly?
also what does the time frame for finding a dev for this look like?