From 4b46271841d75ee8b45b7b4db2ce531a764785fa Mon Sep 17 00:00:00 2001
From: Mark Mansi <markm@cs.wisc.edu>
Date: Thu, 12 Mar 2020 19:14:13 -0500
Subject: [PATCH] Remove a couple of Rc's from RegionInferenceContext

---
 src/librustc_mir/borrow_check/mod.rs          | 28 +++++++++++++++----
 .../borrow_check/region_infer/mod.rs          |  9 +++---
 2 files changed, 28 insertions(+), 9 deletions(-)

diff --git a/src/librustc_mir/borrow_check/mod.rs b/src/librustc_mir/borrow_check/mod.rs
index a61d00b0120..8dc6ee7ea3b 100644
--- a/src/librustc_mir/borrow_check/mod.rs
+++ b/src/librustc_mir/borrow_check/mod.rs
@@ -73,6 +73,24 @@ crate use place_ext::PlaceExt;
 crate use places_conflict::{places_conflict, PlaceConflictBias};
 crate use region_infer::RegionInferenceContext;
 
+/// An owned immutable value.
+#[derive(Debug)]
+struct Frozen<T>(T);
+
+impl<T> Frozen<T> {
+    pub fn freeze(val: T) -> Self {
+        Frozen(val)
+    }
+}
+
+impl<T> std::ops::Deref for Frozen<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &self.0
+    }
+}
+
 // FIXME(eddyb) perhaps move this somewhere more centrally.
 #[derive(Debug)]
 crate struct Upvar {
@@ -1577,11 +1595,11 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 mpi,
             );
         } // Only query longest prefix with a MovePath, not further
-        // ancestors; dataflow recurs on children when parents
-        // move (to support partial (re)inits).
-        //
-        // (I.e., querying parents breaks scenario 7; but may want
-        // to do such a query based on partial-init feature-gate.)
+          // ancestors; dataflow recurs on children when parents
+          // move (to support partial (re)inits).
+          //
+          // (I.e., querying parents breaks scenario 7; but may want
+          // to do such a query based on partial-init feature-gate.)
     }
 
     /// Subslices correspond to multiple move paths, so we iterate through the
diff --git a/src/librustc_mir/borrow_check/region_infer/mod.rs b/src/librustc_mir/borrow_check/region_infer/mod.rs
index 144f655420b..43744d78219 100644
--- a/src/librustc_mir/borrow_check/region_infer/mod.rs
+++ b/src/librustc_mir/borrow_check/region_infer/mod.rs
@@ -31,6 +31,7 @@ use crate::borrow_check::{
     },
     type_check::{free_region_relations::UniversalRegionRelations, Locations},
     universal_regions::UniversalRegions,
+    Frozen,
 };
 
 mod dump_mir;
@@ -54,12 +55,12 @@ pub struct RegionInferenceContext<'tcx> {
     liveness_constraints: LivenessValues<RegionVid>,
 
     /// The outlives constraints computed by the type-check.
-    constraints: Rc<OutlivesConstraintSet>,
+    constraints: Frozen<OutlivesConstraintSet>,
 
     /// The constraint-set, but in graph form, making it easy to traverse
     /// the constraints adjacent to a particular region. Used to construct
     /// the SCC (see `constraint_sccs`) and for error reporting.
-    constraint_graph: Rc<NormalConstraintGraph>,
+    constraint_graph: Frozen<NormalConstraintGraph>,
 
     /// The SCC computed from `constraints` and the constraint
     /// graph. We have an edge from SCC A to SCC B if `A: B`. Used to
@@ -263,8 +264,8 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             .map(|info| RegionDefinition::new(info.universe, info.origin))
             .collect();
 
-        let constraints = Rc::new(outlives_constraints); // freeze constraints
-        let constraint_graph = Rc::new(constraints.graph(definitions.len()));
+        let constraints = Frozen::freeze(outlives_constraints);
+        let constraint_graph = Frozen::freeze(constraints.graph(definitions.len()));
         let fr_static = universal_regions.fr_static;
         let constraint_sccs = Rc::new(constraints.compute_sccs(&constraint_graph, fr_static));