Add a HIR pass to check consts for `if`, `loop`, etc. #66170

ecstatic-morse · 2019-11-06T20:56:17Z

Resolves #66125.

This PR adds a HIR pass to check for high-level control flow constructs that are forbidden in a const-context. The MIR const-checker is unable to provide good spans for these since they are lowered to control flow primitives (e.g., Goto and SwitchInt), and these often don't map back to the underlying statement as a whole. This PR is intended only to improve diagnostics once if and match become commonplace in constants (behind a feature flag). The MIR const-checker will continue to operate unchanged, and will catch anything this check might miss.

In this implementation, the HIR const-checking pass is run much earlier than the MIR one, so it will supersede any errors from the latter. I will need some mentoring if we wish to change this, since I'm not familiar with the diagnostics system. Moving this pass into the same phase as the MIR const-checker could also help keep backwards compatibility for items like const _: () = loop { break; };, which are currently (erroneously?) accepted by the MIR const-checker (see #62272).

r? @Centril
cc @eddyb (since they filed #62272)

eddyb · 2019-11-06T21:32:00Z

cc @rust-lang/compiler I was hoping we would implement const-checking for arbitrary CFGs and never have to deal with a HIR check on constants again, but I guess I was wrong.

Centril

The checker code looks good aside from some nits and some thoughts for the future (cleanup tasks mostly unrelated to the PR but which I noticed along the way).

src/librustc_passes/error_codes.rs

src/librustc/hir/map/mod.rs

Centril · 2019-11-07T00:06:14Z

src/librustc/query/mod.rs

@@ -329,6 +329,11 @@ rustc_queries! {
            desc { |tcx| "checking for unstable API usage in {}", key.describe_as_module(tcx) }
        }

+        /// Checks the const bodies in the module for illegal operations (e.g. `if` or `loop`).
+        query check_mod_const_bodies(key: DefId) -> () {


Aside: I noticed that the categorization in this module wrt. Other, TypeChecking, and friends is rather messy and there doesn't seem to be a lot of logic to it? -- Maybe another C-cleanup issue...? (cc @Zoxc & @nikomatsakis)

Oh, this categorization was added as a very coarse grouping for profiling purposes IIRC, we can certainly clean things up here. cc @wesleywiser

Cleanups wrt categorization are definitely appreciated. I did the initial pass ~1yr ago and I don't think they've changed substantially since that time. Feel free to assign me to any such cleanup PRs.

src/librustc_passes/check_const.rs

src/test/ui/consts/min_const_fn/min_const_fn.rs

Centril · 2019-11-07T00:39:24Z

In this implementation, the HIR const-checking pass is run much earlier than the MIR one, so it will supersede any errors from the latter. I will need some mentoring if we wish to change this, since I'm not familiar with the diagnostics system. Moving this pass into the same phase as the MIR const-checker could also help keep backwards compatibility for items like const _: () = loop { break; };, which are currently (erroneously?) accepted by the MIR const-checker (see #62272).

I wouldn't expend any energy on making #62272 work; it's not very useful in any case and was never intended to be allowed on stable other than as a side-effect of how the compiler is structured. The check as written in this PR is good. :)

src/test/ui/consts/const-loop.rs

src/test/ui/consts/const-eval/issue-62272.rs

ecstatic-morse · 2019-11-07T02:00:03Z

Okay, I fixed most of @Centril's suggestions, blessed the back-compat breakages, and added an explanation to them. Let me know if you're okay with the new error messages.

This should pass CI now. I'd like people to weigh in on where to put this in the compilation pipeline; I think it should go alongside the MIR const-checker if possible. ~~I'll also look into checking array length expressions.~~ (done)

Centril · 2019-11-07T02:04:24Z

On my end, the code & diagnostics look good, so r=me on that aspect when you feel comfortable.

src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs

ecstatic-morse · 2019-11-07T18:13:06Z

src/test/ui/borrowck/issue-64453.rs

@@ -13,7 +11,6 @@ fn set_editor(_: Value) {}

 fn main() {
    let settings_data = from_string(settings_dir);
-    //~^ ERROR cannot move out of static item `settings_dir` [E0507]


It's unfortunate that this error no longer occurs. I believe it's simply because the compiler stops working before the move checker runs?

ecstatic-morse · 2019-11-07T18:17:29Z

src/test/ui/consts/const-eval/issue-62272.rs


 fn main() {
-    [FOO; { let x; loop { x = 5; break; } x }];
+    [FOO; { let x; loop { x = 5; break; } x }]; //~ ERROR `loop` is not allowed in a `const`


How do we feel about the term const for an array initializer? Is there a preexisting convention here? It's pretty easy to change.

Maybe @estebank has thoughts?

I would prefer us to be correct mentioning the array length is a const context, but this should be ok for now, we have plenty of other places where we are inconsistent about calling this out.

Oh, also, I think const generic arguments are also anon_const, so this isn't that easy. Probably the right way is to add an AnonConstKind to AnonConst in hir/mod.rs. With that in mind, I'm inclined to fix this later :).

ecstatic-morse · 2019-11-07T18:24:30Z

I've rebased to clean up the history a bit and (hopefully) blessed the remaining tests. I've left some comments where the new error messages aren't a clear improvement.

This is still waiting for @rust-lang/compiler to weigh in on whether breaking #62272 is okay. It might need an FCP/crater run?

Also, I would still like for someone to review the relative order of the check_mod_const_bodies query. It would be nice run this at the same time as the MIR const-checker instead of superseding it, so we could leave the min_const_fn tests unchanged. We could then change the if/match errors in the MIR const-checker to delay_span_bugs (I think).

Centril · 2019-11-08T07:46:31Z

src/librustc_passes/error_codes.rs

+Control-flow expressions are not allowed inside a const context.
+
+At the moment, `if` and `match`, as well as the looping constructs `for`,
+`while`, and `loop`, are forbidden inside a `const`, `static`, or `const fn`.


Upon re-reading this it occurred to me that we could link to the tracking issues... should we perhaps do that?
cc @estebank @oli-obk

I'll do this in a follow-up PR since the bulk of this one is ready.

Centril · 2019-11-08T07:49:08Z

Also, I would still like for someone to review the relative order of the check_mod_const_bodies query. It would be nice run this at the same time as the MIR const-checker instead of superseding it, so we could leave the min_const_fn tests unchanged. We could then change the if/match errors in the MIR const-checker to delay_span_bugs (I think).

This part is unfortunately out of my expertise to mentor; I'll defer to folks on T-Compiler.

The MIR const-checker errors for if/match/loop are now delay span bugs, so nothing will be emitted unless the HIR checker misses something.

ecstatic-morse · 2019-11-13T18:49:28Z

Once again broken by a refactoring. @Centril can I make this p=1? I guess it won't matter since this is old enough to be at the top of the queue now.

@bors r=Centril,oli-obk

bors · 2019-11-13T18:49:29Z

📌 Commit 7552bd6 has been approved by Centril,oli-obk

bors · 2019-11-13T20:11:00Z

⌛ Testing commit 7552bd6 with merge ded5ee0...

@Centril

Add a HIR pass to check consts for `if`, `loop`, etc. Resolves #66125. This PR adds a HIR pass to check for high-level control flow constructs that are forbidden in a const-context. The MIR const-checker is unable to provide good spans for these since they are lowered to control flow primitives (e.g., `Goto` and `SwitchInt`), and these often don't map back to the underlying statement as a whole. This PR is intended only to improve diagnostics once `if` and `match` become commonplace in constants (behind a feature flag). The MIR const-checker will continue to operate unchanged, and will catch anything this check might miss. In this implementation, the HIR const-checking pass is run much earlier than the MIR one, so it will supersede any errors from the latter. I will need some mentoring if we wish to change this, since I'm not familiar with the diagnostics system. Moving this pass into the same phase as the MIR const-checker could also help keep backwards compatibility for items like `const _: () = loop { break; };`, which are currently (erroneously?) accepted by the MIR const-checker (see #62272). r? @Centril cc @eddyb (since they filed #62272)

bors · 2019-11-13T23:21:49Z

☀️ Test successful - checks-azure
Approved by: Centril,oli-obk
Pushing ded5ee0 to master...

@GuillaumeGomez

…msg, r=estebank Link to tracking issue in HIR const-check error code description This is a follow up to rust-lang#66170 that addresses [this comment](rust-lang#66170 (comment)). As an aside, the only other error code whose description uses the phrase "tracking issue" is `E0202`. We may want to come up with standards around this, especially since error codes are now centralized and easier to keep track of (thanks @GuillaumeGomez). r? @estebank (since they +1'ed the comment)

@GuillaumeGomez

…msg, r=estebank Link to tracking issue in HIR const-check error code description This is a follow up to rust-lang#66170 that addresses [this comment](rust-lang#66170 (comment)). As an aside, the only other error code whose description uses the phrase "tracking issue" is `E0202`. We may want to come up with standards around this, especially since error codes are now centralized and easier to keep track of (thanks @GuillaumeGomez). r? @estebank (since they +1'ed the comment)

@GuillaumeGomez

…msg, r=estebank Link to tracking issue in HIR const-check error code description This is a follow up to rust-lang#66170 that addresses [this comment](rust-lang#66170 (comment)). As an aside, the only other error code whose description uses the phrase "tracking issue" is `E0202`. We may want to come up with standards around this, especially since error codes are now centralized and easier to keep track of (thanks @GuillaumeGomez). r? @estebank (since they +1'ed the comment)

@GuillaumeGomez

…msg, r=estebank Link to tracking issue in HIR const-check error code description This is a follow up to rust-lang#66170 that addresses [this comment](rust-lang#66170 (comment)). As an aside, the only other error code whose description uses the phrase "tracking issue" is `E0202`. We may want to come up with standards around this, especially since error codes are now centralized and easier to keep track of (thanks @GuillaumeGomez). r? @estebank (since they +1'ed the comment)

…leywiser Get rid of HIR const checker As far as I can tell, the HIR const checker was implemented in rust-lang#66170 because we were not able to issue useful const error messages in the MIR const checker. This seems to have changed in the last 5 years, probably due to work like rust-lang#90532. I've tweaked the diagnostics slightly and think the error messages have gotten *better* in fact. Thus I think the HIR const checker has reached the end of its usefulness, and we can retire it. cc `@RalfJung`

Get rid of HIR const checker As far as I can tell, the HIR const checker was implemented in rust-lang/rust#66170 because we were not able to issue useful const error messages in the MIR const checker. This seems to have changed in the last 5 years, probably due to work like #90532. I've tweaked the diagnostics slightly and think the error messages have gotten *better* in fact. Thus I think the HIR const checker has reached the end of its usefulness, and we can retire it. cc `@RalfJung`

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ecstatic-morse added 3 commits November 13, 2019 10:44

Bless less verbose error messages

86734b1

The MIR const-checker errors for if/match/loop are now delay span bugs, so nothing will be emitted unless the HIR checker misses something.

Bless miri unleashed test now that errors are mandatory

5e048da

Use ast::Mutability

7552bd6

ecstatic-morse force-pushed the hir-const-check branch from 16c674f to 7552bd6 Compare November 13, 2019 18:47

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Nov 13, 2019

ecstatic-morse mentioned this pull request Nov 13, 2019

Make dataflow-based const qualification the canonical one #66385

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bors added the merged-by-bors This PR was explicitly merged by bors. label Nov 13, 2019

bors merged commit 7552bd6 into rust-lang:master Nov 13, 2019

This was referenced Nov 13, 2019

Move error codes #66314

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Adjust cannot move out of static error in prep for removing Statics from Place #66328

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Cleanup rmeta::MacroDef #66364

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ecstatic-morse mentioned this pull request Nov 14, 2019

Link to tracking issue in HIR const-check error code description #66418

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compiler-errors mentioned this pull request Nov 22, 2024

Get rid of HIR const checker #133321

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Add a HIR pass to check consts for `if`, `loop`, etc. #66170

Add a HIR pass to check consts for `if`, `loop`, etc. #66170

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Add a HIR pass to check consts for if, loop, etc. #66170

Add a HIR pass to check consts for if, loop, etc. #66170

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Add a HIR pass to check consts for `if`, `loop`, etc. #66170

Add a HIR pass to check consts for `if`, `loop`, etc. #66170

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