bjoernager/rust
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Fix ICE from projection cycle

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Cycles in normalization can cause evaluations to change from Unknown to
Err. This means that some selection that were applicable no longer are.

To avoid this:
* Selection candidates that are known to be applicable are prefered
  over candidates that are not.
* We don't ICE if a candidate is no longer applicable.
This commit is contained in:
Matthew Jasper 2020-10-08 21:49:36 +01:00
parent 22e6b9c689
commit 6c43a64931
3 changed files with 34 additions and 20 deletions
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18
compiler/rustc_trait_selection/src/traits/select/confirmation.rs
View file

@ -69,7 +69,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
}
ProjectionCandidate(idx) => {
let obligations = self.confirm_projection_candidate(obligation, idx);
let obligations = self.confirm_projection_candidate(obligation, idx)?;
Ok(ImplSource::Param(obligations))
}
@ -120,7 +120,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
&mut self,
obligation: &TraitObligation<'tcx>,
idx: usize,
) -> Vec<PredicateObligation<'tcx>> {
) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
self.infcx.commit_unconditionally(|_| {
let tcx = self.tcx();
@ -148,19 +148,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
&mut obligations,
);
obligations.extend(
obligations.extend(self.infcx.commit_if_ok(|_| {
self.infcx
.at(&obligation.cause, obligation.param_env)
.sup(placeholder_trait_predicate.trait_ref.to_poly_trait_ref(), candidate)
.map(|InferOk { obligations, .. }| obligations)
.unwrap_or_else(|_| {
bug!(
"Projection bound `{:?}` was applicable to `{:?}` but now is not",
candidate,
obligation
);
}),
);
.map_err(|_| Unimplemented)
})?);
if let ty::Projection(..) = placeholder_self_ty.kind() {
for predicate in tcx.predicates_of(def_id).instantiate_own(tcx, substs).predicates {
@ -181,7 +175,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
}
}
obligations
Ok(obligations)
})
}