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Rollup merge of #108319 - compiler-errors:dont-project-to-specializable-rpitits, r=lcnr

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Don't project specializable RPITIT projection

This effective rejects specialization + RPITIT/AFIT (usages of `impl Trait` in traits) because the implementation is significantly complicated over making regular "default" trait method bodies work.

I have another PR that experimentally fixes all this, but the code may not be worth investing in.
This commit is contained in:
Matthias Krüger 2023-02-27 06:11:51 +01:00 • committed by GitHub
commit 3a6c5429c2
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4 changed files with 126 additions and 5 deletions
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27
compiler/rustc_trait_selection/src/traits/project.rs
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@ -1307,21 +1307,38 @@ fn assemble_candidate_for_impl_trait_in_trait<'cx, 'tcx>(
let _ = selcx.infcx.commit_if_ok(|_| {
match selcx.select(&obligation.with(tcx, trait_predicate)) {
Ok(Some(super::ImplSource::UserDefined(data))) => {
candidate_set.push_candidate(ProjectionCandidate::ImplTraitInTrait(data));
Ok(())
let Ok(leaf_def) = specialization_graph::assoc_def(tcx, data.impl_def_id, trait_fn_def_id) else {
return Err(());
};
// Only reveal a specializable default if we're past type-checking
// and the obligation is monomorphic, otherwise passes such as
// transmute checking and polymorphic MIR optimizations could
// get a result which isn't correct for all monomorphizations.
if leaf_def.is_final()
|| (obligation.param_env.reveal() == Reveal::All
&& !selcx
.infcx
.resolve_vars_if_possible(obligation.predicate.trait_ref(tcx))
.still_further_specializable())
{
candidate_set.push_candidate(ProjectionCandidate::ImplTraitInTrait(data));
Ok(())
} else {
Err(())
}
}
Ok(None) => {
candidate_set.mark_ambiguous();
return Err(());
Err(())
}
Ok(Some(_)) => {
// Don't know enough about the impl to provide a useful signature
return Err(());
Err(())
}
Err(e) => {
debug!(error = ?e, "selection error");
candidate_set.mark_error(e);
return Err(());
Err(())
}
}
});