user0/rust
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

Auto merge of #54199 - nikomatsakis:predicate_may_hold-failure, r=eddyb

Browse source 
overlook overflows in rustdoc trait solving

Context:

The new rustdoc "auto trait" feature walks across impls and tries to run trait solving on them with a lot of unconstrained variables. This is prone to overflows. These overflows used to cause an ICE because of a caching bug (fixed in this PR). But even once that is fixed, it means that rustdoc causes an overflow rather than generating docs.

This PR therefore adds a new helper that propagates the overflow error out. This requires rustdoc to then decide what to do when it encounters such an overflow: technically, an overflow represents neither "yes" nor "no", but rather a failure to make a decision. I've decided to opt on the side of treating this as "yes, implemented", since rustdoc already takes an optimistic view. This may prove to include too many items, but I *suspect* not.

We could probably reduce the rate of overflows by unifying more of the parameters from the impl -- right now we only seem to consider the self type. Moreover, in the future, as we transition to Chalk, overflow errors are expected to just "go away" (in some cases, though, queries might return an ambiguous result).

Fixes #52873

cc @QuietMisdreavus -- this is the stuff we were talking about earlier
cc @GuillaumeGomez -- this supersedes #53687
This commit is contained in:
bors 2018-09-26 12:39:20 +00:00
commit e783d2be40
5 changed files with 214 additions and 15 deletions
Download patch file Download diff file Expand all files Collapse all files

19
src/librustdoc/clean/blanket_impl.rs
View file

@ -103,11 +103,20 @@ impl<'a, 'tcx, 'rcx, 'cstore> BlanketImplFinder <'a, 'tcx, 'rcx, 'cstore> {
// FIXME(eddyb) ignoring `obligations` might cause false positives.
drop(obligations);
let may_apply = infcx.predicate_may_hold(&traits::Obligation::new(
cause.clone(),
param_env,
trait_ref.to_predicate(),
));
debug!(
"invoking predicate_may_hold: {:?}",
trait_ref,
);
let may_apply = match infcx.evaluate_obligation(
&traits::Obligation::new(
cause.clone(),
param_env,
trait_ref.to_predicate(),
),
) {
Ok(eval_result) => eval_result.may_apply(),
Err(traits::OverflowError) => true, // overflow doesn't mean yes *or* no
};
if !may_apply {
return
}