typeck: always expose explicit enum discriminant `AnonConst`s' parent in `generics_of`. #70825

eddyb · 2020-04-05T21:28:29Z

This is similar to #70452 but for explicit enum discriminant constant expressions.
However, unlike #70452, this PR should have no effect on stable code, as while it alleviates #43408 errors, there is no way to actually compile an enum with generic parameters and explicit discriminants, without #![feature(arbitrary_enum_discriminant)], as explicit discriminant expression don't count as uses of parameters (if they did, they would count as invariant uses).

There's also 2 other commits here, both related to #70453:

"ty: use delay_span_bug in ty::AdtDef::eval_explicit_discr." - hides the ICEs demonstrated on Should enum discriminants have generics in scope? #70453, when there are other errors (which the next commit adds)
"typeck/wfcheck: require that explicit enum discriminants const-evaluate succesfully." - closes Should enum discriminants have generics in scope? #70453 by picking alternative "2", i.e. erroring when a discriminant doesn't fully const-evaluate from the perspective of the enum definition

In the future, it might be possible to allow enum discriminants to actually depend on parameters, but that will likely require #68436 + some way to restrict the values so no two variants can end up with overlapping discriminants.

As this PR would close #70453, it shouldn't be merged until a decision is reached there.

r? @nikomatsakis

Centril · 2020-04-05T21:35:52Z

src/librustc_typeck/check/wfcheck.rs

@@ -411,6 +411,22 @@ fn check_type_defn<'tcx, F>(
                    ObligationCauseCode::MiscObligation,
                )
            }
+
+            // Explicit `enum` discriminant values must const-evaluate successfully.


Comment could use some elaboration regarding the "why" as well as a link towards #70453.

Just to make sure I'm following along, this concept of "must be evaluatable" is going to ultimately be true for any const expression, right? I think this is the basic WF criteria we are working towards?

Yes, at the minimum any ty::Const used in the typesystem (array lengths and args to const params). While enum discrminants aren't that, they do require strict checks (no overlap), for which successful evaluatability is a bare minimum.

Centril · 2020-04-05T21:48:42Z

src/librustc_typeck/collect.rs

@@ -1182,14 +1182,28 @@ fn generics_of(tcx: TyCtxt<'_>, def_id: DefId) -> &ty::Generics {
        }
        // FIXME(#43408) enable this always when we get lazy normalization.
        Node::AnonConst(_) => {
+            let parent_id = tcx.hir().get_parent_item(hir_id);


Nit: bind let hir = tcx.hir(); earlier in the function to reduce repetition.

What's funny is that I copy-pasted these two lines from #70452 - but also I'm not fond of binding tcx.hir() due to the ideal future in which we request the tcx.hir(...) for a given LocalDefId and then use that, binding it wouldn't help with that AFAICT.

Well it's just a nit, but are there plans for that future though?

nikomatsakis · 2020-04-06T13:27:02Z

FYI: I'm planning to review this this week.

nikomatsakis

r=me, left a few nits

src/librustc_typeck/check/wfcheck.rs

nikomatsakis · 2020-04-14T14:52:23Z

src/librustc_typeck/check/wfcheck.rs

@@ -411,6 +411,22 @@ fn check_type_defn<'tcx, F>(
                    ObligationCauseCode::MiscObligation,
                )
            }
+
+            // Explicit `enum` discriminant values must const-evaluate successfully.


Just to make sure I'm following along, this concept of "must be evaluatable" is going to ultimately be true for any const expression, right? I think this is the basic WF criteria we are working towards?

@Skinny121

…rent, r=nikomatsakis typeck: always expose repeat count `AnonConst`s' parent in `generics_of`. This should reduce some of the confusion around rust-lang#43408, although, if you look at the changed test outputs (for the last commit), they all hit rust-lang#68436, so nothing new will start compiling. We can let counts of "repeat expressions" (`N` in `[x; N]`) always have the correct generics parenting, because they're always in a body, so nothing in the `where` clauses or `impl` trait/type of the parent can use it, and therefore no query cycles can occur. <hr/> Other potential candidates we might want to apply the same approach to, are: * ~~(easy) `enum` discriminants (see also rust-lang#70453)~~ opened rust-lang#70825 * (trickier) array *type* (not *expression*) lengths nested in: * bodies * types of (associated or not) `const`/`static` * RHS of `type` aliases and associated `type`s * `fn` signatures We should've done so from the start, the only reason we haven't is because I was squeamish about blacklisting some of the cases, but if we whitelist instead we should be fine. Also, lazy normalization is taking forever 😞. <hr/> There's also 5 other commits here: * "typeck: track any errors injected during writeback and taint tables appropriately." - fixes rust-lang#66706, as the next commit would otherwise trigger an ICE again * "typeck: workaround WF hole in `to_const`." - its purpose is to emulate most of rust-lang#70107's direct effect, at least in the case of repeat expressions, where the count always goes through `to_const` * this is the reason no new code can really compile, as the WF checks require rust-lang#68436 to bypass * however, this has more test changes than I hoped, so it should be reviewed separately, and maybe even landed separately (as rust-lang#70107 might take a while, as it's blocked on a few of my PRs) * "ty: erase lifetimes early in `ty::Const::eval`." - first attempt at fixing rust-lang#70773 * still useful, I believe the new approach is less likely to cause issues long-term * I could take this out or move it into another PR if desired or someone else could take over (cc @Skinny121) * "traits/query/normalize: add some `debug!` logging for the result." - debugging aid for rust-lang#70773 * "borrow_check/type_check: normalize `Aggregate` and `Call` operands." - actually fixes rust-lang#70773 r? @nikomatsakis cc @pnkfelix @varkor @yodaldevoid @oli-obk @estebank

eddyb · 2020-05-02T14:39:36Z

Oops, I missed that FCP completed (#70453 (comment)).

…te succesfully.

… in `generics_of`.

eddyb · 2020-05-02T18:30:08Z

@bors r=nikomatsakis

bors · 2020-05-02T18:30:10Z

📌 Commit 926c7a2 has been approved by nikomatsakis

bors · 2020-05-03T06:24:18Z

⌛ Testing commit 926c7a2 with merge 97fb24cfbf1ff1cb855868b9209675f7c5031abd...

Dylan-DPC-zz · 2020-05-03T08:52:57Z

@bors retry yield

eddyb · 2020-05-03T11:49:54Z

@bors rollup=never p=1

bors · 2020-05-03T12:05:52Z

⌛ Testing commit 926c7a2 with merge e5f35df...

bors · 2020-05-03T15:11:37Z

☀️ Test successful - checks-azure
Approved by: nikomatsakis
Pushing e5f35df to master...

rust-highfive assigned nikomatsakis Apr 5, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 5, 2020

This was referenced Apr 5, 2020

Should enum discriminants have generics in scope? #70453

Closed

typeck: always expose repeat count AnonConsts' parent in generics_of. #70452

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Centril suggested changes Apr 5, 2020

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eddyb mentioned this pull request Apr 16, 2020

Do not ICE in the face of invalid enum discriminant #69903

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joelpalmer added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 23, 2020

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eddyb added 3 commits May 2, 2020 18:58

ty: use delay_span_bug in ty::AdtDef::eval_explicit_discr.

50c63e4

typeck/wfcheck: require that explicit enum discriminants const-evalua…

2c29f0c

…te succesfully.

typeck: always expose explicit enum discriminant AnonConsts' parent…

926c7a2

… in `generics_of`.

eddyb force-pushed the enum-discr-correct-generics-parent branch from 1dcd516 to 926c7a2 Compare May 2, 2020 17:44

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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels May 2, 2020

bors mentioned this pull request May 3, 2020

Rollup of 5 pull requests #71828

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bors added the merged-by-bors This PR was explicitly merged by bors. label May 3, 2020

bors merged commit e5f35df into rust-lang:master May 3, 2020

JohnTitor mentioned this pull request May 6, 2020

ICE: index out of bounds: the len is 0 but the index is 0 #67945

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eddyb deleted the enum-discr-correct-generics-parent branch June 25, 2020 18:15

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typeck: always expose explicit enum discriminant `AnonConst`s' parent in `generics_of`. #70825

typeck: always expose explicit enum discriminant `AnonConst`s' parent in `generics_of`. #70825

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typeck: always expose explicit enum discriminant AnonConsts' parent in generics_of. #70825

typeck: always expose explicit enum discriminant AnonConsts' parent in generics_of. #70825

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typeck: always expose explicit enum discriminant `AnonConst`s' parent in `generics_of`. #70825

typeck: always expose explicit enum discriminant `AnonConst`s' parent in `generics_of`. #70825