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Perform obligation deduplication to avoid buggy ExistentialMismatch #73485

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Jun 27, 2020
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Perform obligation deduplication to avoid buggy ExistentialMismatch #73485

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16 changes: 13 additions & 3 deletions src/librustc_middle/ty/relate.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -617,12 +617,22 @@ impl<'tcx> Relate<'tcx> for &'tcx ty::List<ty::ExistentialPredicate<'tcx>> {
a: &Self,
b: &Self,
) -> RelateResult<'tcx, Self> {
if a.len() != b.len() {
let tcx = relation.tcx();

// FIXME: this is wasteful, but want to do a perf run to see how slow it is.
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I wonder if we should open up a FIXME issue for this..? To try and prevent the duplicates in the first place and figure out where they come from?

// We need to perform this deduplication as we sometimes generate duplicate projections
// in `a`.
let mut a_v: Vec<_> = a.into_iter().collect();
let mut b_v: Vec<_> = b.into_iter().collect();
a_v.sort_by(|a, b| a.stable_cmp(tcx, b));
a_v.dedup();
Comment on lines +627 to +628
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Is it correct, that the first one is a sort_by, but the next one a plain dedup, instead of dedup_by?

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You're right, that is probably a potential bug. On the repro case that I posted it didn't cause problems, but I'll change it after the perf run has completed (don't want to break anything by force-pushing to this branch before then).

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I'm not sure if dedup will cause trouble in practice and don't know whether dedup_by_key might introduce a slight slowdown. Either way, this PR seems to solve bad rejection part of the ticket.

b_v.sort_by(|a, b| a.stable_cmp(tcx, b));
b_v.dedup();
if a_v.len() != b_v.len() {
return Err(TypeError::ExistentialMismatch(expected_found(relation, a, b)));
}

let tcx = relation.tcx();
let v = a.iter().zip(b.iter()).map(|(ep_a, ep_b)| {
let v = a_v.into_iter().zip(b_v.into_iter()).map(|(ep_a, ep_b)| {
use crate::ty::ExistentialPredicate::*;
match (ep_a, ep_b) {
(Trait(ref a), Trait(ref b)) => Ok(Trait(relation.relate(a, b)?)),
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26 changes: 26 additions & 0 deletions src/test/ui/issues/issue-59326.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,26 @@
// check-pass
trait Service {
type S;
}

trait Framing {
type F;
}

impl Framing for () {
type F = ();
}

trait HttpService<F: Framing>: Service<S = F::F> {}

type BoxService = Box<dyn HttpService<(), S = ()>>;

fn build_server<F: FnOnce() -> BoxService>(_: F) {}

fn make_server<F: Framing>() -> Box<dyn HttpService<F, S = F::F>> {
unimplemented!()
}

fn main() {
build_server(|| make_server())
}