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Resurrect some comments, remove a pub

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This commit addresses @lqd's code review and resurrects
a lost comment and removes some dead code.
This commit is contained in:
Amanda Stjerna 2024-05-21 10:42:36 +02:00
parent 7782d2023b
commit e6eb63d4b9
1 changed files with 42 additions and 1 deletions
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43
compiler/rustc_borrowck/src/region_infer/mod.rs
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@ -72,7 +72,7 @@ pub struct RegionInferenceContext<'tcx> {
/// The SCC computed from `constraints` and the constraint
/// graph. We have an edge from SCC A to SCC B if `A: B`. Used to
/// compute the values of each region.
pub constraint_sccs: Rc<ConstraintSccs>,
constraint_sccs: Rc<ConstraintSccs>,
/// Reverse of the SCC constraint graph -- i.e., an edge `A -> B` exists if
/// `B: A`. This is used to compute the universal regions that are required
@ -397,6 +397,39 @@ impl<'tcx> RegionInferenceContext<'tcx> {
/// and (b) any universally quantified regions that it outlives,
/// which in this case is just itself. R1 (`'b`) in contrast also
/// outlives `'a` and hence contains R0 and R1.
///
/// This bit of logic also handles invalid universe relations
/// for higher-kinded types.
///
// We Walk each SCC `A` and `B` such that `A: B`
// and ensure that universe(A) can see universe(B).
//
// This serves to enforce the 'empty/placeholder' hierarchy
// (described in more detail on `RegionKind`):
//
// ```
// static -----+
// | |
// empty(U0) placeholder(U1)
// | /
// empty(U1)
// ```
//
// In particular, imagine we have variables R0 in U0 and R1
// created in U1, and constraints like this;
//
// ```
// R1: !1 // R1 outlives the placeholder in U1
// R1: R0 // R1 outlives R0
// ```
//
// Here, we wish for R1 to be `'static`, because it
// cannot outlive `placeholder(U1)` and `empty(U0)` any other way.
//
// Thanks to this loop, what happens is that the `R1: R0`
// constraint has lowered the universe of `R1` to `U0`, which in turn
// means that the `R1: !1` constraint here will cause
// `R1` to become `'static`.
fn init_free_and_bound_regions(&mut self) {
// Update the names (if any)
// This iterator has unstable order but we collect it all into an IndexVec
@ -1412,6 +1445,8 @@ impl<'tcx> RegionInferenceContext<'tcx> {
}
}
/// The minimum universe of any variable reachable from this
/// SCC, inside or outside of it.
fn scc_universe(&self, scc: ConstraintSccIndex) -> UniverseIndex {
self.constraint_sccs().annotation(scc).universe()
}
@ -2151,6 +2186,12 @@ impl<'tcx> RegionInferenceContext<'tcx> {
/// Returns the representative `RegionVid` for a given SCC.
/// See `RegionTracker` for how a region variable ID is chosen.
///
/// It is a hacky way to manage checking regions for equality,
/// since we can 'canonicalize' each region to the representative
/// of its SCC and be sure that -- if they have the same repr --
/// they *must* be equal (though not having the same repr does not
/// mean they are unequal).
fn scc_representative(&self, scc: ConstraintSccIndex) -> RegionVid {
self.constraint_sccs.annotation(scc).representative
}