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Warning period for detecting nested impl trait #58608
Warning period for detecting nested impl trait #58608
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(rust_highfive has picked a reviewer for you, use r? to override) |
nominating for discussion at T-compiler meeting, in terms of evaluating the cost/benefit of doing a warning cycle in this case. |
I think the benefit of a warning cycle is low (don't think this is too common) but I think so is the cost since we don't want to use this syntax for anything right now. That said, I'm wary of the number of C-future-compatibility lints that we have right now and that some of them have existed for so long. To make sure these don't spiral out of control and to bring them down to a more manageable number, I'd like us to set stricter timelines and deadlines for turning these warnings into deny-lints and then to hard errors. The timelines can be set on a case-by-case basis when we make a future-compat lint. E.g. if we merge this before 1.34 goes into beta, in this instance I think it should become |
@Centril note that the PR as written does not make use of the lint infrastructure. I suppose I should interpret your comment as pushing for the PR to be changed to be a proper lint, yes? |
@pnkfelix I assumed this was just a quick hack and that you'd convert it to the infrastructure we have for C-future-compatibility warnings (and with a dedicated issue). You should interpret my comment as "we should schedule times for when we progress C-future-compatibility to deny to errors" starting with this one. |
Okay. I'll wait until we discuss this at T-compiler meeting; if we collectively decide that we will do a warning-cycle, then I'll implement a proper lint. If we decide that we won't, then I'll just close this PR then. |
This is changes existing errors into warnings too though, instead of just the edge case which was allowed. |
My feeling:
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Oh, if we were going to do it, I would prefer to do it as a "real future compat lint" |
@Zoxc wrote:
Hmm, my intention was to only catch the cases where we went injected fresh errors. I may have messed that up (as in, I may have misread the original code prior to PR #57730 and thought it was missing more than it was in its recursive traversal). I'll try to double-check the situation there. I certainly don't want to downgrade more errors to warnings than necessary. |
@@ -36,6 +36,11 @@ struct AstValidator<'a> { | |||
// Used to ban `impl Trait` in path projections like `<impl Iterator>::Item` | |||
// or `Foo::Bar<impl Trait>` | |||
is_impl_trait_banned: bool, | |||
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// rust-lang/rust#57979: the ban of nested `impl Trait` was incomplete | |||
// until sometime after PR #57730 landed. We track whether we should |
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I think should say bugged until PR #57730 landed
.
I'd suggest bringing back |
yes okay I think I see where I went wrong now. I was doing this too late at night. |
r? @Zoxc |
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I wrote a long comment on one of the commits, but this is the important bit:
Oh, also: it still doesn't use the lint infrastructure. I'll work on that next. But I'm nervous about investing much more effort in this; I was already on the fence about whether code for a warning cycle was worthwhile here, and that was before I put in the more complicated version encoded in the current PR. |
also: It is of course a red flag that I have added two different pieces of boolean state for tracking warning cycle state, but only one diagnostic has changed in our test output. My current hypothesis is that this is a reflection of weakness in our test suite, rather than a redundancy in the approach. But it would be good to actually come up with a concrete test ilustrating the point. |
Here is a concrete example illustrating the case I am trying to catch with the second boolean flag: pub trait Bar { }
pub trait Quux<T> { type Assoc; }
pub fn demo(_: impl Quux<(), Assoc=<() as Quux<impl Bar>>::Assoc>) { }
impl<T> Quux<T> for () { type Assoc = u32; }
fn main() { } |
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Okay, I think I'm done working on this. However I believe I have also missed the cut-off for beta-next (see #58747).
Anyway, assuming that we are not planning to recut beta, there isn't much point in landing this unless we also plan to evenutally beta-backport it. So, nominating for discussion (again) at T-compiler meeting. |
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This looks good to me. One thing to note is that the |
(We should not turn errors on stable into warnings...) |
I did try (in an earlier draft of this branch) to avoid making the booleans sticky; in particular, I had tried to reset them back to However, that attempt at a reset back to (also, my attempts to examples illustrating the stickiness of the current code were unsuccessful, which is another reason why I stopped investigating the matter.) I'll look again and try to make the fields not-sticky. |
@pnkfelix I think you'll need to bring back |
I just want to note, we have a set of potentially conflicting factors here:
Similarly, we should not inject hard-errors into stable without a warning cycle (which is what motivated this PR in the first place). And we should not overly complicate our own code base (which is why I've been trying to avoid solutions like bringing back I don't know how to weigh the relative risk of the two options I see here; that is, the risk of:
Neither of the two options is great. But also, neither option is ... that bad (?), given that we don't expect the given coding pattern to be very common in the first place. Plus I think the outcome that result from a downgrade from stable-error-to-warning are not unsoundness, but rather either fragile-dependence on type-inference details, or ICE'ing? I haven't come to any concrete conclusions yet. I'm going to spend a little bit of time investigating (again) the option of making Update: well, after further review I at least now believe I understand why my prior attempts to make the flag non-sticky failed (I had misunderstood how subtle the bug actually was, in terms of what the original use of |
Instead of a sticky-boolean flag that would downgrade errors to warnings during further recursion into the type (which is overly broad because we were not missing errors at arbitrarily deep levels), this instead tracks state closer to what the original bug actually was. In particular, the actual original bug was that we were failing to record the existence of an outer `impl Trait` solely when it occurred as an *immediate child* during the walk of the child types in `visit_generic_args`. Therefore, the correct way to precisely model when that bug would manifest itself (and thus downgrade the error-to-warning accordingly) is to track when those outer `impl Trait` cases were previously unrecorded. That's what this code does, by storing a flag with the recorded outer `impl Trait` indicating at which point in the compiler's control flow it had been stored. I will note that this commit passes the current test suite. A follow-up commit will also include tests illustrating the cases that this commit gets right (and were handled incorrectly by the previous sticky boolean).
@Zoxc you mind taking a look at this revised (non-sticky) approach for the nested |
This looks good to me now, though there's still the incorrect comment I noted earlier. |
Discussed in zulip; I will address. |
@bors r=zoxc |
📌 Commit 0a03ca7 has been approved by |
…mpl-trait, r=zoxc Warning period for detecting nested impl trait Here is some proposed code for making a warning period for the new checking of nested impl trait. It undoes some of the corrective effects of PR #57730, by using boolean flags to track parts of the analysis that were previously skipped prior to PRs #57730 and #57981 landing. Cc #57979
☀️ Test successful - checks-travis, status-appveyor |
Tested on commit rust-lang/rust@8f4c226. Direct link to PR: <rust-lang/rust#58608> 🎉 rls on windows: test-fail → test-pass (cc @nrc @Xanewok, @rust-lang/infra).
[beta] Rollup backports Cherry-picked: * Include bounds from promoted constants in NLL #57202 * Warning period for detecting nested impl trait #58608 * Don't promote function calls to nonpromotable things #58784 * Make migrate mode work at item level granularity #58788 * Expand where negative supertrait specific error is shown #58861 * Expand where negative supertrait specific error is shown #58861 Rolled up: * [BETA] Update cargo #59217 r? @ghost
Here is some proposed code for making a warning period for the new checking of nested impl trait.
It undoes some of the corrective effects of PR #57730, by using boolean flags to track parts of the analysis that were previously skipped prior to PRs #57730 and #57981 landing.
Cc #57979