Specialization does not work correctly during constant evaluation.

[Lymia]

A crate using specialization I've been working on seems to have broken sometime between nightly-2019-11-19 and nightly-2019-11-20. After a bit of debugging, it seems the issue is that specialization doesn't always work correctly in const contexts.

This code, for example, returns the following:

const_bool::<TypeA>: in default impl
const_bool::<TypeB>: in default impl
const_str::<TypeA>: in specialized impl
const_str::<TypeB>: in default impl
run_method::<TypeA>: in specialized impl
run_method::<TypeB>: in default impl

On earlier versions, this would have returned the following:

const_bool::<TypeA>: in specialized impl
const_bool::<TypeB>: in default impl
const_str::<TypeA>: in specialized impl
const_str::<TypeB>: in default impl
run_method::<TypeA>: in specialized impl
run_method::<TypeB>: in default impl

This bug does not happen when the -Z mir-opt-level=0 flag is set, and it behaves the same as prior versions.

[oli-obk]

cc @wesleywiser const prop is evaluating polymorphic code and evaluating to a default instead of bailing out due to being too generic

[eddyb]

I'm guessing Instance::resolve is missing this bit of logic (NB "polymorphic MIR optimizations"):

rust/src/librustc/traits/project.rs

Lines 1075 to 1090 in 8f1bbd6

	// Only reveal a specializable default if we're past type-checking
	// and the obligations is monomorphic, otherwise passes such as
	// transmute checking and polymorphic MIR optimizations could
	// get a result which isn't correct for all monomorphizations.
	if !is_default {
	true
	} else if obligation.param_env.reveal == Reveal::All {
	debug_assert!(!poly_trait_ref.needs_infer());
	if !poly_trait_ref.needs_subst() {
	true
	} else {
	false
	}
	} else {
	false
	}

EDIT: that is, this should perform the same checks to pick Some or None:

rust/src/librustc/ty/instance.rs

Lines 427 to 432 in 8f1bbd6

	traits::VtableImpl(impl_data) => {
	let (def_id, substs) = traits::find_associated_item(
	tcx, param_env, trait_item, rcvr_substs, &impl_data);
	let substs = tcx.erase_regions(&substs);
	Some(ty::Instance::new(def_id, substs))
	}