Matthew's work on improving NLL's "higher-ranked subtype error"s

Cargo.lock

@@ -4055,6 +4055,7 @@ dependencies = [
  "rustc_span",
  "rustc_target",
  "rustc_trait_selection",
+ "rustc_traits",
  "smallvec",
  "tracing",
 ]

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -1191,16 +1191,26 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     //         |   |
                     //         |   elided as they were the same
                     //         not elided, they were different, but irrelevant
+                    //
+                    // For bound lifetimes, keep the names of the lifetimes,
+                    // even if they are the same so that it's clear what's happening
+                    // if we have something like
+                    //
+                    // for<'r, 's> fn(Inv<'r>, Inv<'s>)
+                    // for<'r> fn(Inv<'r>, Inv<'r>)
                     let lifetimes = sub1.regions().zip(sub2.regions());
                     for (i, lifetimes) in lifetimes.enumerate() {
                         let l1 = lifetime_display(lifetimes.0);
                         let l2 = lifetime_display(lifetimes.1);
-                        if lifetimes.0 == lifetimes.1 {
-                            values.0.push_normal("'_");
-                            values.1.push_normal("'_");
-                        } else {
+                        if lifetimes.0 != lifetimes.1 {
                             values.0.push_highlighted(l1);
                             values.1.push_highlighted(l2);
+                        } else if lifetimes.0.is_late_bound() {
+                            values.0.push_normal(l1);
+                            values.1.push_normal(l2);
+                        } else {
+                            values.0.push_normal("'_");
+                            values.1.push_normal("'_");
                         }
                         self.push_comma(&mut values.0, &mut values.1, len, i);
                     }

compiler/rustc_infer/src/infer/mod.rs

@@ -1,4 +1,5 @@
 pub use self::freshen::TypeFreshener;
+pub use self::lexical_region_resolve::RegionResolutionError;
 pub use self::LateBoundRegionConversionTime::*;
 pub use self::RegionVariableOrigin::*;
 pub use self::SubregionOrigin::*;
@@ -1110,7 +1111,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     /// etc) this is the root universe U0. For inference variables or
     /// placeholders, however, it will return the universe which which
     /// they are associated.
-    fn universe_of_region(&self, r: ty::Region<'tcx>) -> ty::UniverseIndex {
+    pub fn universe_of_region(&self, r: ty::Region<'tcx>) -> ty::UniverseIndex {
         self.inner.borrow_mut().unwrap_region_constraints().universe(r)
     }
 
@@ -1288,6 +1289,17 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         op(inner.unwrap_region_constraints().data())
     }
 
+    pub fn region_var_origin(&self, vid: ty::RegionVid) -> RegionVariableOrigin {
+        let mut inner = self.inner.borrow_mut();
+        let inner = &mut *inner;
+        inner
+            .region_constraint_storage
+            .as_mut()
+            .expect("regions already resolved")
+            .with_log(&mut inner.undo_log)
+            .var_origin(vid)
+    }
+
     /// Takes ownership of the list of variable regions. This implies
     /// that all the region constraints have already been taken, and
     /// hence that `resolve_regions_and_report_errors` can never be
@@ -1505,7 +1517,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         self.inner.borrow_mut().projection_cache().clear();
     }
 
-    fn universe(&self) -> ty::UniverseIndex {
+    pub fn universe(&self) -> ty::UniverseIndex {
         self.universe.get()
     }
 

compiler/rustc_infer/src/infer/region_constraints/mod.rs

@@ -445,6 +445,11 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
         self.var_infos[vid].universe
     }
 
+    /// Returns the origin for the given variable.
+    pub fn var_origin(&self, vid: RegionVid) -> RegionVariableOrigin {
+        self.var_infos[vid].origin
+    }
+
     fn add_constraint(&mut self, constraint: Constraint<'tcx>, origin: SubregionOrigin<'tcx>) {
         // cannot add constraints once regions are resolved
         debug!("RegionConstraintCollector: add_constraint({:?})", constraint);

compiler/rustc_mir/Cargo.toml

@@ -27,6 +27,7 @@ rustc_serialize = { path = "../rustc_serialize" }
 rustc_session = { path = "../rustc_session" }
 rustc_target = { path = "../rustc_target" }
 rustc_trait_selection = { path = "../rustc_trait_selection" }
+rustc_traits = { path = "../rustc_traits" }
 rustc_ast = { path = "../rustc_ast" }
 rustc_span = { path = "../rustc_span" }
 rustc_apfloat = { path = "../rustc_apfloat" }