diff --git a/src/librustc/infer/canonical/query_response.rs b/src/librustc/infer/canonical/query_response.rs
index f0ea61cf9b4..d00da11d17d 100644
--- a/src/librustc/infer/canonical/query_response.rs
+++ b/src/librustc/infer/canonical/query_response.rs
@@ -654,14 +654,14 @@ pub fn make_query_outlives<'tcx>(
         constraints,
         verifys,
         givens,
-        in_constraints,
+        pick_constraints,
     } = region_constraints;
 
     assert!(verifys.is_empty());
     assert!(givens.is_empty());
 
     // FIXME(ndm) -- we have to think about what to do here, perhaps
-    assert!(in_constraints.is_empty());
+    assert!(pick_constraints.is_empty());
 
     let outlives: Vec<_> = constraints
         .into_iter()
diff --git a/src/librustc/infer/lexical_region_resolve/mod.rs b/src/librustc/infer/lexical_region_resolve/mod.rs
index 61c711967ad..a37eff82e6e 100644
--- a/src/librustc/infer/lexical_region_resolve/mod.rs
+++ b/src/librustc/infer/lexical_region_resolve/mod.rs
@@ -2,7 +2,7 @@
 
 use crate::infer::region_constraints::Constraint;
 use crate::infer::region_constraints::GenericKind;
-use crate::infer::region_constraints::InConstraint;
+use crate::infer::region_constraints::PickConstraint;
 use crate::infer::region_constraints::RegionConstraintData;
 use crate::infer::region_constraints::VarInfos;
 use crate::infer::region_constraints::VerifyBound;
@@ -118,7 +118,7 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
 
         let graph = self.construct_graph();
         self.expand_givens(&graph);
-        self.enforce_in_constraints(&graph, &mut var_data);
+        self.enforce_pick_constraints(&graph, &mut var_data);
         self.expansion(&mut var_data);
         self.collect_errors(&mut var_data, errors);
         self.collect_var_errors(&var_data, &graph, errors);
@@ -200,63 +200,63 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
     ///     - if `'o[i] <= 'b` is false, then `'o[i]` is not an option
     ///
     /// Hopefully this narrows it down to just one option.
-    fn enforce_in_constraints(
+    fn enforce_pick_constraints(
         &self,
         graph: &RegionGraph<'tcx>,
         var_values: &mut LexicalRegionResolutions<'tcx>,
     ) {
-        for in_constraint in &self.data.in_constraints {
-            let _ = self.enforce_in_constraint(graph, in_constraint, var_values);
+        for pick_constraint in &self.data.pick_constraints {
+            let _ = self.enforce_pick_constraint(graph, pick_constraint, var_values);
         }
     }
 
-    fn enforce_in_constraint(
+    fn enforce_pick_constraint(
         &self,
         graph: &RegionGraph<'tcx>,
-        in_constraint: &InConstraint<'tcx>,
+        pick_constraint: &PickConstraint<'tcx>,
         var_values: &mut LexicalRegionResolutions<'tcx>,
     ) -> Result<(), ()> {
-        debug!("enforce_in_constraint(in_constraint={:#?})", in_constraint);
+        debug!("enforce_pick_constraint(pick_constraint={:#?})", pick_constraint);
 
         // the constraint is some inference variable (`vid`) which
         // must be equal to one of the options
-        let vid = match in_constraint.region {
+        let pick_vid = match pick_constraint.pick_region {
             ty::ReVar(vid) => *vid,
             _ => return Err(()),
         };
 
         // find all the "bounds" -- that is, each region `b` such that
         // `r0 <= b` must hold.
-        let (bounds, _) = self.collect_concrete_regions(graph, vid, OUTGOING, None);
+        let (pick_bounds, _) = self.collect_concrete_regions(graph, pick_vid, OUTGOING, None);
 
         // get an iterator over the *available options* -- that is,
         // each constraint regions `o` where `o <= b` for all the
         // bounds `b`.
-        debug!("enforce_in_constraint: bounds={:#?}", bounds);
-        let mut options = in_constraint.in_regions.iter().filter(|option| {
-            bounds.iter().all(|bound| self.sub_concrete_regions(option, bound.region))
+        debug!("enforce_pick_constraint: bounds={:#?}", pick_bounds);
+        let mut options = pick_constraint.option_regions.iter().filter(|option| {
+            pick_bounds.iter().all(|bound| self.sub_concrete_regions(option, bound.region))
         });
 
         // if there >1 option, we only make a choice if there is a
         // single *least* choice -- i.e., some available region that
         // is `<=` all the others.
         let mut least_choice = options.next().ok_or(())?;
-        debug!("enforce_in_constraint: least_choice={:?}", least_choice);
+        debug!("enforce_pick_constraint: least_choice={:?}", least_choice);
         for option in options {
-            debug!("enforce_in_constraint: option={:?}", option);
+            debug!("enforce_pick_constraint: option={:?}", option);
             if !self.sub_concrete_regions(least_choice, option) {
                 if self.sub_concrete_regions(option, least_choice) {
-                    debug!("enforce_in_constraint: new least choice");
+                    debug!("enforce_pick_constraint: new least choice");
                     least_choice = option;
                 } else {
-                    debug!("enforce_in_constraint: no least choice");
+                    debug!("enforce_pick_constraint: no least choice");
                     return Err(());
                 }
             }
         }
 
-        debug!("enforce_in_constraint: final least choice = {:?}", least_choice);
-        *var_values.value_mut(vid) = VarValue::Value(least_choice);
+        debug!("enforce_pick_constraint: final least choice = {:?}", least_choice);
+        *var_values.value_mut(pick_vid) = VarValue::Value(least_choice);
 
         Ok(())
     }
diff --git a/src/librustc/infer/mod.rs b/src/librustc/infer/mod.rs
index d4386b321e3..ba42c7a5441 100644
--- a/src/librustc/infer/mod.rs
+++ b/src/librustc/infer/mod.rs
@@ -907,7 +907,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
     /// Require that the region `r` be equal to one of the regions in
     /// the set `regions`.
-    pub fn in_constraint(
+    pub fn pick_constraint(
         &self,
         origin: SubregionOrigin<'tcx>,
         region: ty::Region<'tcx>,
@@ -915,7 +915,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     ) {
         debug!("sub_regions({:?} <: {:?})", region, in_regions);
         self.borrow_region_constraints()
-            .in_constraint(origin, region, in_regions);
+            .pick_constraint(origin, region, in_regions);
     }
 
     pub fn subtype_predicate(
diff --git a/src/librustc/infer/opaque_types/mod.rs b/src/librustc/infer/opaque_types/mod.rs
index d4c6c8b99b5..733155ad501 100644
--- a/src/librustc/infer/opaque_types/mod.rs
+++ b/src/librustc/infer/opaque_types/mod.rs
@@ -421,7 +421,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
         concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
             tcx: self.tcx,
-            op: |r| self.in_constraint(infer::CallReturn(span), r, &in_regions),
+            op: |r| self.pick_constraint(infer::CallReturn(span), r, &in_regions),
         });
     }
 
diff --git a/src/librustc/infer/region_constraints/mod.rs b/src/librustc/infer/region_constraints/mod.rs
index 14572c050b0..fa64690ded4 100644
--- a/src/librustc/infer/region_constraints/mod.rs
+++ b/src/librustc/infer/region_constraints/mod.rs
@@ -79,10 +79,10 @@ pub struct RegionConstraintData<'tcx> {
     /// be a region variable (or neither, as it happens).
     pub constraints: BTreeMap<Constraint<'tcx>, SubregionOrigin<'tcx>>,
 
-    /// Constraints of the form `R0 in [R1, ..., Rn]`, meaning that
+    /// Constraints of the form `pick R0 from [R1, ..., Rn]`, meaning that
     /// `R0` must be equal to one of the regions `R1..Rn`. These occur
     /// with `impl Trait` quite frequently.
-    pub in_constraints: Vec<InConstraint<'tcx>>,
+    pub pick_constraints: Vec<PickConstraint<'tcx>>,
 
     /// A "verify" is something that we need to verify after inference
     /// is done, but which does not directly affect inference in any
@@ -143,12 +143,17 @@ impl Constraint<'_> {
     }
 }
 
-/// Requires that `region` must be equal to one of the regions in `in_regions`.
+/// Requires that `region` must be equal to one of the regions in `option_regions`.
+/// We often denote this using the syntax:
+///
+/// ```
+/// pick R0 from [O1..On]
+/// ```
 #[derive(Debug, Clone)]
-pub struct InConstraint<'tcx> {
+pub struct PickConstraint<'tcx> {
     pub origin: SubregionOrigin<'tcx>,
-    pub region: Region<'tcx>,
-    pub in_regions: Rc<Vec<Region<'tcx>>>,
+    pub pick_region: Region<'tcx>,
+    pub option_regions: Rc<Vec<Region<'tcx>>>,
 }
 
 /// `VerifyGenericBound(T, _, R, RS)`: the parameter type `T` (or
@@ -657,20 +662,20 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
         }
     }
 
-    pub fn in_constraint(
+    pub fn pick_constraint(
         &mut self,
         origin: SubregionOrigin<'tcx>,
-        region: ty::Region<'tcx>,
-        in_regions: &Rc<Vec<ty::Region<'tcx>>>,
+        pick_region: ty::Region<'tcx>,
+        option_regions: &Rc<Vec<ty::Region<'tcx>>>,
     ) {
-        debug!("in_constraint({:?} in {:#?})", region, in_regions);
+        debug!("pick_constraint({:?} in {:#?})", pick_region, option_regions);
 
-        if in_regions.iter().any(|&r| r == region) {
+        if option_regions.iter().any(|&r| r == pick_region) {
             return;
         }
 
-        self.data.in_constraints.push(InConstraint {
-            origin, region, in_regions: in_regions.clone()
+        self.data.pick_constraints.push(PickConstraint {
+            origin, pick_region, option_regions: option_regions.clone()
         });
 
     }
@@ -938,10 +943,10 @@ impl<'tcx> RegionConstraintData<'tcx> {
     pub fn is_empty(&self) -> bool {
         let RegionConstraintData {
             constraints,
-            in_constraints,
+            pick_constraints,
             verifys,
             givens,
         } = self;
-        constraints.is_empty() && in_constraints.is_empty() && verifys.is_empty() && givens.is_empty()
+        constraints.is_empty() && pick_constraints.is_empty() && verifys.is_empty() && givens.is_empty()
     }
 }