More code review (will squash later)

Co-authored-by: lcnr <rust@lcnr.de>

Author: amandasystems

compiler/rustc_borrowck/src/eliminate_placeholders.rs

@@ -1,9 +1,7 @@
 //! Logic for lowering higher-kinded outlives constraints
 //! (with placeholders and universes) and turn them into regular
 //! outlives constraints.
-//!
-//! This logic is provisional and should be removed once the trait
-//! solver can handle this kind of constraint.
+
 use rustc_data_structures::frozen::Frozen;
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::graph::scc;
@@ -197,7 +195,7 @@ fn region_definitions<'tcx>(
 /// This code is a stop-gap measure in preparation for the future trait solver.
 ///
 /// Every constraint added by this method is an internal `IllegalUniverse` constraint.
-pub(crate) fn rewrite_higher_kinded_outlives_as_constraints<'tcx>(
+pub(crate) fn compute_sccs_applying_placeholder_outlives_constraints<'tcx>(
     constraints: MirTypeckRegionConstraints<'tcx>,
     universal_region_relations: &Frozen<UniversalRegionRelations<'tcx>>,
     infcx: &BorrowckInferCtxt<'tcx>,

compiler/rustc_borrowck/src/nll.rs

@@ -22,7 +22,7 @@ use tracing::{debug, instrument};
 use crate::borrow_set::BorrowSet;
 use crate::consumers::ConsumerOptions;
 use crate::diagnostics::RegionErrors;
-use crate::eliminate_placeholders::rewrite_higher_kinded_outlives_as_constraints;
+use crate::eliminate_placeholders::compute_sccs_applying_placeholder_outlives_constraints;
 use crate::polonius::PoloniusDiagnosticsContext;
 use crate::polonius::legacy::{
     PoloniusFacts, PoloniusFactsExt, PoloniusLocationTable, PoloniusOutput,
@@ -118,7 +118,7 @@ pub(crate) fn compute_regions<'a, 'tcx>(
         Rc::clone(&location_map),
     );
 
-    let lowered_constraints = rewrite_higher_kinded_outlives_as_constraints(
+    let lowered_constraints = compute_sccs_applying_placeholder_outlives_constraints(
         constraints,
         &universal_region_relations,
         infcx,