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Fix Rustdoc ICE when checking blanket impls

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Fixes #55001, #54744

Previously, SelectionContext would unconditionally cache the selection
result for an obligation. This worked fine for most users of
SelectionContext, but it caused an issue when used by Rustdoc's blanket
impl finder.

The issue occured when SelectionContext chose a ParamCandidate which
contained inference variables. Since inference variables can change
between calls to select(), it's not safe to cache the selection result -
the chosen candidate might not be applicable for future results, leading
to an ICE when we try to run confirmation.

This commit prevents SelectionContext from caching any ParamCandidate
that contains inference variables. This should always be completely
safe, as trait selection should never depend on a particular result
being cached.

I've also added some extra debug!() statements, which I found helpful in
tracking down this bug.
This commit is contained in:
Aaron Hill 2018-10-22 00:12:16 -04:00
parent ca2639e82e
commit 4f2624cac9
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GPG key ID: B4087E510E98B164
5 changed files with 87 additions and 3 deletions
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11
src/librustdoc/clean/blanket_impl.rs
View file

@ -50,6 +50,7 @@ impl<'a, 'tcx, 'rcx, 'cstore> BlanketImplFinder <'a, 'tcx, 'rcx, 'cstore> {
name: Option<String>,
) -> Vec<Item>
where F: Fn(DefId) -> Def {
debug!("get_blanket_impls(def_id={:?}, ...)", def_id);
let mut impls = Vec::new();
if self.cx
.tcx
@ -78,6 +79,8 @@ impl<'a, 'tcx, 'rcx, 'cstore> BlanketImplFinder <'a, 'tcx, 'rcx, 'cstore> {
}
self.cx.tcx.for_each_relevant_impl(trait_def_id, ty, |impl_def_id| {
self.cx.tcx.infer_ctxt().enter(|infcx| {
debug!("get_blanet_impls: Considering impl for trait '{:?}' {:?}",
trait_def_id, impl_def_id);
let t_generics = infcx.tcx.generics_of(impl_def_id);
let trait_ref = infcx.tcx.impl_trait_ref(impl_def_id)
.expect("Cannot get impl trait");
@ -104,8 +107,8 @@ impl<'a, 'tcx, 'rcx, 'cstore> BlanketImplFinder <'a, 'tcx, 'rcx, 'cstore> {
drop(obligations);
debug!(
"invoking predicate_may_hold: {:?}",
trait_ref,
"invoking predicate_may_hold: param_env={:?}, trait_ref={:?}, ty={:?}",
param_env, trait_ref, ty
);
let may_apply = match infcx.evaluate_obligation(
&traits::Obligation::new(
@ -117,6 +120,10 @@ impl<'a, 'tcx, 'rcx, 'cstore> BlanketImplFinder <'a, 'tcx, 'rcx, 'cstore> {
Ok(eval_result) => eval_result.may_apply(),
Err(traits::OverflowError) => true, // overflow doesn't mean yes *or* no
};
debug!("get_blanket_impls: found applicable impl: {}\
for trait_ref={:?}, ty={:?}",
may_apply, trait_ref, ty);
if !may_apply {
return
}