Use ReEmpty(U0) as the implicit region bound in typeck

[matthewjasper]

Fixes #74429

r? @nikomatsakis

[LeSeulArtichaut]

#74429 is a stable regression, so would this PR also deserve a stable backport? cc @Mark-Simulacrum

[Mark-Simulacrum]

Seems reasonable, stable-nominating -- @matthewjasper, does that feel reasonable to you? It looks like this is a relatively small patch so I'd feel relatively comfortable backporting it, but you know better of course :)

[matthewjasper]

This should be fine for a stable backport

[nikomatsakis]

I suspect this field could be made non-optional if we use empty here

[nikomatsakis]

I'm a bit confused as to how this winds up fixing any bug -- reading the code that is using this field, it looks like it tries to make 'empty an implicit bound:

rust/src/librustc_infer/infer/outlives/verify.rs

Lines 73 to 99 in 7138410

	fn param_bound(&self, param_ty: ty::ParamTy) -> VerifyBound<'tcx> {
	debug!("param_bound(param_ty={:?})", param_ty);
	// Start with anything like `T: 'a` we can scrape from the
	// environment
	let param_bounds = self
	.declared_generic_bounds_from_env(GenericKind::Param(param_ty))
	.into_iter()
	.map(|outlives| outlives.1);
	// Add in the default bound of fn body that applies to all in
	// scope type parameters:
	let param_bounds = param_bounds.chain(self.implicit_region_bound);
	let any_bounds: Vec<_> = param_bounds.map(|r| VerifyBound::OutlivedBy(r)).collect();
	if any_bounds.is_empty() {
	// We know that all types `T` outlive `'empty`, so if we
	// can find no other bound, then check that the region
	// being tested is `'empty`.
	VerifyBound::IsEmpty
	} else {
	// If we can find any other bound `R` such that `T: R`, then
	// we don't need to check for `'empty`, because `R: 'empty`.
	VerifyBound::AnyBound(any_bounds)
	}
	}

maybe VerifyBound::IsEmpty can be removed after this change, even?

[matthewjasper]

The issue is that it's possible that we have T: ReEmpty(Un) as an implied bound from a closure, which causes us to not generate VerifyBound::IsEmpty.

[nikomatsakis]

@bors r+

[bors]

📌 Commit 45c01ed has been approved by nikomatsakis

[Mark-Simulacrum]

@bors p=1

We are most likely backporting to stable so would be good to get it in nightly as quickly as we can for additional testing.

[bors]

⌛ Testing commit 45c01ed with merge e89f905f89a47aac35739b2a36afde253162449f...

[rust-highfive]

The job mingw-check of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

[TIMING] Std { stage: 0, target: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } } -- 44.625
Build completed successfully in 0:01:49
+ /scripts/validate-toolstate.sh
Cloning into 'rust-toolstate'...
<Nothing changed>
Traceback (most recent call last):
  File "../../src/tools/publish_toolstate.py", line 286, in <module>
    validate_maintainers(repo, github_token)
  File "../../src/tools/publish_toolstate.py", line 87, in validate_maintainers
    'Accept': 'application/vnd.github.hellcat-preview+json',
  File "/usr/lib/python3.6/urllib/request.py", line 223, in urlopen
    return opener.open(url, data, timeout)
  File "/usr/lib/python3.6/urllib/request.py", line 532, in open
    response = meth(req, response)
  File "/usr/lib/python3.6/urllib/request.py", line 642, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python3.6/urllib/request.py", line 570, in error
    return self._call_chain(*args)
  File "/usr/lib/python3.6/urllib/request.py", line 504, in _call_chain
    result = func(*args)
  File "/usr/lib/python3.6/urllib/request.py", line 650, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 403: Forbidden
  local time: Wed Jul 22 12:55:34 UTC 2020
  network time: Wed, 22 Jul 2020 12:55:34 GMT
== end clock drift check ==
== end clock drift check ==
##[error]Process completed with exit code 1.
Terminate orphan process: pid (2900) (node)
Terminate orphan process: pid (2927) (python)

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @rust-lang/infra. (Feature Requests)

[bors]

💔 Test failed - checks-actions

[matthewjasper]

@bors retry

[bors]

⌛ Testing commit 45c01ed with merge 39a295f...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: nikomatsakis
Pushing 39a295f to master...

[pnkfelix]

discussed at weekly T-compiler triage meeting.

beta-accepted

[pnkfelix]

the stable-nomination was also discussed (separately) at the weekly T-compiler triage meeting.

We are a little nervous that this only landed so recently, and yet we have a (stable) point release expected next week.

Still, this is pretty low risk, we think. We will keep our eye out collectively for suspicious behavior on nightly, and revert (at least from stable) if we see anything fishy.

stable-accepted.