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fix universe map in ifcx.instantiate_canonical_* #99814

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Nov 30, 2022
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fix universe map in ifcx.instantiate_canonical_* #99814

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f87953f
fix universe map in ifcx.instantiate_canonical_*
aliemjay Jul 27, 2022
70200ac
Update compiler/rustc_infer/src/infer/canonical/mod.rs
nikomatsakis Oct 10, 2022
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9 changes: 6 additions & 3 deletions compiler/rustc_infer/src/infer/canonical/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -41,7 +41,7 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
/// inference variables and applies it to the canonical value.
/// Returns both the instantiated result *and* the substitution S.
///
/// This is only meant to be invoked as part of constructing an
/// This can be invoked as part of constructing an
/// inference context at the start of a query (see
/// `InferCtxtBuilder::enter_with_canonical`). It basically
/// brings the canonical value "into scope" within your new infcx.
Expand All @@ -63,8 +63,11 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
// in them, so this code has no effect, but it is looking
// forward to the day when we *do* want to carry universes
// through into queries.
let universes: IndexVec<ty::UniverseIndex, _> = std::iter::once(ty::UniverseIndex::ROOT)
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Currently reading about Universes/Placeholders and am curious, why can we do that? Why can't we encounter canonicalized variables with UniverseIndex::Root inside instantiate_canonical_vars?

.chain((0..canonical.max_universe.as_u32()).map(|_| self.create_next_universe()))
//
// Instantiate the root-universe content into the current universe,
// and create fresh universes for the higher universes.
let universes: IndexVec<ty::UniverseIndex, _> = std::iter::once(self.universe())
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.chain((1..=canonical.max_universe.as_u32()).map(|_| self.create_next_universe()))
.collect();

let canonical_inference_vars =
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