introduce localized outlives constraints

these are the basic blocks of the naive polonius constraint graph
implementation.

compiler/rustc_borrowck/src/polonius/constraints.rs

@@ -0,0 +1,45 @@
+use rustc_middle::ty::RegionVid;
+use rustc_mir_dataflow::points::PointIndex;
+
+/// A localized outlives constraint reifies the CFG location where the outlives constraint holds,
+/// within the origins themselves as if they were different from point to point: from `a: b`
+/// outlives constraints to `a@p: b@p`, where `p` is the point in the CFG.
+///
+/// This models two sources of constraints:
+/// - constraints that traverse the subsets between regions at a given point, `a@p: b@p`. These
+///   depend on typeck constraints generated via assignments, calls, etc. (In practice there are
+///   subtleties where a statement's effect only starts being visible at the successor point, via
+///   the "result" of that statement).
+/// - constraints that traverse the CFG via the same region, `a@p: a@q`, where `p` is a predecessor
+///   of `q`. These depend on the liveness of the regions at these points, as well as their
+///   variance.
+///
+/// The `source` origin at `from` flows into the `target` origin at `to`.
+///
+/// This dual of NLL's [crate::constraints::OutlivesConstraint] therefore encodes the
+/// position-dependent outlives constraints used by Polonius, to model the flow-sensitive loan
+/// propagation via reachability within a graph of localized constraints.
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
+pub(crate) struct LocalizedOutlivesConstraint {
+    pub source: RegionVid,
+    pub from: PointIndex,
+    pub target: RegionVid,
+    pub to: PointIndex,
+}
+
+/// A container of [LocalizedOutlivesConstraint]s that can be turned into a traversable
+/// `rustc_data_structures` graph.
+#[derive(Clone, Default, Debug)]
+pub(crate) struct LocalizedOutlivesConstraintSet {
+    pub outlives: Vec<LocalizedOutlivesConstraint>,
+}
+
+impl LocalizedOutlivesConstraintSet {
+    pub(crate) fn push(&mut self, constraint: LocalizedOutlivesConstraint) {
+        if constraint.source == constraint.target && constraint.from == constraint.to {
+            // 'a@p: 'a@p is pretty uninteresting
+            return;
+        }
+        self.outlives.push(constraint);
+    }
+}

compiler/rustc_borrowck/src/polonius/mod.rs

@@ -1 +1,4 @@
+mod constraints;
+pub(crate) use constraints::*;
+
 pub(crate) mod legacy;