diff --git a/src/librustc_mir/borrow_check/nll/mod.rs b/src/librustc_mir/borrow_check/nll/mod.rs
index 5dd7b745273..364d12aa395 100644
--- a/src/librustc_mir/borrow_check/nll/mod.rs
+++ b/src/librustc_mir/borrow_check/nll/mod.rs
@@ -37,6 +37,7 @@ crate mod type_check;
 mod universal_regions;
 
 mod constraints;
+mod pick_constraints;
 
 use self::facts::AllFacts;
 use self::region_infer::RegionInferenceContext;
diff --git a/src/librustc_mir/borrow_check/nll/pick_constraints.rs b/src/librustc_mir/borrow_check/nll/pick_constraints.rs
new file mode 100644
index 00000000000..6bbd2e80527
--- /dev/null
+++ b/src/librustc_mir/borrow_check/nll/pick_constraints.rs
@@ -0,0 +1,210 @@
+#![allow(dead_code)] // TODO
+
+use crate::rustc::ty::{self, Ty};
+use rustc::hir::def_id::DefId;
+use rustc::infer::region_constraints::PickConstraint;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::indexed_vec::{Idx, IndexVec};
+use std::hash::Hash;
+use std::ops::Index;
+
+/// Compactly stores a set of `pick R0 in [R1...Rn]` constraints,
+/// indexed by the region R0.
+crate struct PickConstraintSet<'tcx, R>
+where
+    R: Copy + Hash + Eq,
+{
+    /// Stores the first "pick" constraint for a given R0. This is an
+    /// index into the `constraints` vector below.
+    first_constraints: FxHashMap<R, NllPickConstraintIndex>,
+
+    /// Stores the data about each `pick R0 from [R1..Rn]` constraint.
+    /// These are organized into a linked list, so each constraint
+    /// contains the index of the next constraint with the same R0.
+    constraints: IndexVec<NllPickConstraintIndex, NllPickConstraint<'tcx>>,
+
+    /// Stores the `R1..Rn` regions for *all* sets. For any given
+    /// constraint, we keep two indices so that we can pull out a
+    /// slice.
+    option_regions: Vec<ty::RegionVid>,
+}
+
+/// Represents a `pick R0 in [R1..Rn]` constraint
+crate struct NllPickConstraint<'tcx> {
+    next_constraint: Option<NllPickConstraintIndex>,
+
+    /// The opaque type whose hidden type is being inferred. (Used in error reporting.)
+    crate opaque_type_def_id: DefId,
+
+    /// The hidden type in which R0 appears. (Used in error reporting.)
+    crate hidden_ty: Ty<'tcx>,
+
+    /// The region R0.
+    crate pick_region_vid: ty::RegionVid,
+
+    /// Index of `R1` in `option_regions` vector from `PickConstraintSet`.
+    start_index: usize,
+
+    /// Index of `Rn` in `option_regions` vector from `PickConstraintSet`.
+    end_index: usize,
+}
+
+newtype_index! {
+    crate struct NllPickConstraintIndex {
+        DEBUG_FORMAT = "PickConstraintIndex({})"
+    }
+}
+
+impl<'tcx> PickConstraintSet<'tcx, ty::RegionVid> {
+    crate fn new() -> Self {
+        Self {
+            first_constraints: Default::default(),
+            constraints: Default::default(),
+            option_regions: Default::default(),
+        }
+    }
+
+    crate fn push_constraint(
+        &mut self,
+        p_c: PickConstraint<'tcx>,
+        mut to_region_vid: impl FnMut(ty::Region<'tcx>) -> ty::RegionVid,
+    ) {
+        let pick_region_vid: ty::RegionVid = to_region_vid(p_c.pick_region);
+        let next_constraint = self.first_constraints.get(&pick_region_vid).cloned();
+        let start_index = self.option_regions.len();
+        let end_index = start_index + p_c.option_regions.len();
+        let constraint_index = self.constraints.push(NllPickConstraint {
+            next_constraint,
+            pick_region_vid,
+            opaque_type_def_id: p_c.opaque_type_def_id,
+            hidden_ty: p_c.hidden_ty,
+            start_index,
+            end_index,
+        });
+        self.first_constraints.insert(pick_region_vid, constraint_index);
+        self.option_regions.extend(p_c.option_regions.iter().map(|&r| to_region_vid(r)));
+    }
+}
+
+impl<'tcx, R1> PickConstraintSet<'tcx, R1>
+where
+    R1: Copy + Hash + Eq,
+{
+    /// Remap the "pick region" key using `map_fn`, producing a new
+    /// pick-constraint set.  This is used in the NLL code to map from
+    /// the original `RegionVid` to an scc index. In some cases, we
+    /// may have multiple R1 values mapping to the same R2 key -- that
+    /// is ok, the two sets will be merged.
+    crate fn into_mapped<R2>(self, mut map_fn: impl FnMut(R1) -> R2) -> PickConstraintSet<'tcx, R2>
+    where
+        R2: Copy + Hash + Eq,
+    {
+        // We can re-use most of the original data, just tweaking the
+        // linked list links a bit.
+        //
+        // For example if we had two keys Ra and Rb that both now wind
+        // up mapped to the same key S, we would append the linked
+        // list for Ra onto the end of the linked list for Rb (or vice
+        // versa) -- this basically just requires rewriting the final
+        // link from one list to point at the othe other (see
+        // `append_list`).
+
+        let PickConstraintSet { first_constraints, mut constraints, option_regions } = self;
+
+        let mut first_constraints2 = FxHashMap::default();
+        first_constraints2.reserve(first_constraints.len());
+
+        for (r1, start1) in first_constraints {
+            let r2 = map_fn(r1);
+            if let Some(&start2) = first_constraints2.get(&r2) {
+                append_list(&mut constraints, start1, start2);
+            }
+            first_constraints2.insert(r2, start1);
+        }
+
+        PickConstraintSet {
+            first_constraints: first_constraints2,
+            constraints,
+            option_regions,
+        }
+    }
+}
+
+impl<'tcx, R> PickConstraintSet<'tcx, R>
+where
+    R: Copy + Hash + Eq,
+{
+    /// Iterate down the constraint indices associated with a given
+    /// peek-region.  You can then use `option_regions` and other
+    /// methods to access data.
+    crate fn indices(
+        &self,
+        pick_region_vid: R,
+    ) -> impl Iterator<Item = NllPickConstraintIndex> + '_ {
+        let mut next = self.first_constraints.get(&pick_region_vid).cloned();
+        std::iter::from_fn(move || -> Option<NllPickConstraintIndex> {
+            if let Some(current) = next {
+                next = self.constraints[current].next_constraint;
+                Some(current)
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Returns the "option regions" for a given pick constraint. This is the R1..Rn from
+    /// a constraint like:
+    ///
+    /// ```
+    /// pick R0 in [R1..Rn]
+    /// ```
+    crate fn option_regions(&self, pci: NllPickConstraintIndex) -> &[ty::RegionVid] {
+        let NllPickConstraint { start_index, end_index, .. } = &self.constraints[pci];
+        &self.option_regions[*start_index..*end_index]
+    }
+}
+
+impl<'tcx, R> Index<NllPickConstraintIndex> for PickConstraintSet<'tcx, R>
+where
+    R: Copy + Hash + Eq,
+{
+    type Output = NllPickConstraint<'tcx>;
+
+    fn index(&self, i: NllPickConstraintIndex) -> &NllPickConstraint<'tcx> {
+        &self.constraints[i]
+    }
+}
+
+/// Given a linked list starting at `source_list` and another linked
+/// list starting at `target_list`, modify `target_list` so that it is
+/// followed by `source_list`.
+///
+/// Before:
+///
+/// ```
+/// target_list: A -> B -> C -> (None)
+/// source_list: D -> E -> F -> (None)
+/// ```
+///
+/// After:
+///
+/// ```
+/// target_list: A -> B -> C -> D -> E -> F -> (None)
+/// ```
+fn append_list(
+    constraints: &mut IndexVec<NllPickConstraintIndex, NllPickConstraint<'_>>,
+    target_list: NllPickConstraintIndex,
+    source_list: NllPickConstraintIndex,
+) {
+    let mut p = target_list;
+    loop {
+        let mut r = &mut constraints[p];
+        match r.next_constraint {
+            Some(q) => p = q,
+            None => {
+                r.next_constraint = Some(source_list);
+                return;
+            }
+        }
+    }
+}