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+//! Traits used to represent [lattices] for use as the domain of a dataflow analysis.
+//!
+//! ## Implementation Notes
+//!
+//! Given that they represent partially ordered sets, you may be surprised that [`MeetSemiLattice`]
+//! and [`JoinSemiLattice`] do not have [`PartialOrd`][std::cmp::PartialOrd] as a supertrait. This
+//! is because most standard library types use lexicographic ordering instead of [set inclusion]
+//! for their `PartialOrd` impl. Since we do not actually need to compare lattice elements to run a
+//! dataflow analysis, there's no need for a hypothetical `SetInclusion` newtype with a custom
+//! `PartialOrd` impl.  The only benefit would be the ability to check (in debug mode) that the
+//! least upper (or greatest lower) bound returned by the lattice join (or meet) operator was in
+//! fact greater (or lower) than the inputs.
+//!
+//! [lattices]: https://en.wikipedia.org/wiki/Lattice_(order)
+//! [set inclusion]: https://en.wikipedia.org/wiki/Subset
+
+use rustc_index::bit_set::BitSet;
+use rustc_index::vec::{Idx, IndexVec};
+
+/// A [partially ordered set][poset] that has a [least upper bound][lub] for any pair of elements
+/// in the set.
+///
+/// [lub]: https://en.wikipedia.org/wiki/Infimum_and_supremum
+/// [poset]: https://en.wikipedia.org/wiki/Partially_ordered_set
+pub trait JoinSemiLattice: Eq {
+    /// Computes the least upper bound of two elements, storing the result in `self` and returning
+    /// `true` if `self` has changed.
+    ///
+    /// The lattice join operator is abbreviated as `∨`.
+    fn join(&mut self, other: &Self) -> bool;
+}
+
+/// A [partially ordered set][poset] that has a [greatest lower bound][glb] for any pair of
+/// elements in the set.
+///
+/// Dataflow analyses only require that their domains implement [`JoinSemiLattice`], not
+/// `MeetSemiLattice`. However, types that will be used as dataflow domains should implement both
+/// so that they can be used with [`Dual`].
+///
+/// [glb]: https://en.wikipedia.org/wiki/Infimum_and_supremum
+/// [poset]: https://en.wikipedia.org/wiki/Partially_ordered_set
+pub trait MeetSemiLattice: Eq {
+    /// Computes the greatest lower bound of two elements, storing the result in `self` and
+    /// returning `true` if `self` has changed.
+    ///
+    /// The lattice meet operator is abbreviated as `∧`.
+    fn meet(&mut self, other: &Self) -> bool;
+}
+
+/// A `bool` is a "two-point" lattice with `true` as the top element and `false` as the bottom.
+impl JoinSemiLattice for bool {
+    fn join(&mut self, other: &Self) -> bool {
+        if let (false, true) = (*self, *other) {
+            *self = true;
+            return true;
+        }
+
+        false
+    }
+}
+
+impl MeetSemiLattice for bool {
+    fn meet(&mut self, other: &Self) -> bool {
+        if let (true, false) = (*self, *other) {
+            *self = false;
+            return true;
+        }
+
+        false
+    }
+}
+
+/// A tuple or list of lattices is itself a lattice whose least upper bound is the concatenation of
+/// the least upper bounds of each element of the tuple or list.
+impl<I: Idx, T: JoinSemiLattice> JoinSemiLattice for IndexVec<I, T> {
+    fn join(&mut self, other: &Self) -> bool {
+        assert_eq!(self.len(), other.len());
+
+        let mut changed = false;
+        for (a, b) in self.iter_mut().zip(other.iter()) {
+            changed |= a.join(b);
+        }
+        changed
+    }
+}
+
+impl<I: Idx, T: MeetSemiLattice> MeetSemiLattice for IndexVec<I, T> {
+    fn meet(&mut self, other: &Self) -> bool {
+        assert_eq!(self.len(), other.len());
+
+        let mut changed = false;
+        for (a, b) in self.iter_mut().zip(other.iter()) {
+            changed |= a.meet(b);
+        }
+        changed
+    }
+}
+
+/// A `BitSet` is an efficent way to store a tuple of "two-point" lattices. Equivalently, it is the
+/// lattice corresponding to the powerset of the set of all possibe values of the index type `T`
+/// ordered by inclusion.
+impl<T: Idx> JoinSemiLattice for BitSet<T> {
+    fn join(&mut self, other: &Self) -> bool {
+        self.union(other)
+    }
+}
+
+impl<T: Idx> MeetSemiLattice for BitSet<T> {
+    fn meet(&mut self, other: &Self) -> bool {
+        self.intersect(other)
+    }
+}
+
+/// The counterpart of a given semilattice `T` using the [inverse order].
+///
+/// The dual of a join-semilattice is a meet-semilattice and vice versa. For example, the dual of a
+/// powerset has the empty set as its top element and the full set as its bottom element and uses
+/// set *intersection* as its join operator.
+///
+/// [inverse order]: https://en.wikipedia.org/wiki/Duality_(order_theory)
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct Dual<T>(pub T);
+
+impl<T> std::borrow::Borrow<T> for Dual<T> {
+    fn borrow(&self) -> &T {
+        &self.0
+    }
+}
+
+impl<T> std::borrow::BorrowMut<T> for Dual<T> {
+    fn borrow_mut(&mut self) -> &mut T {
+        &mut self.0
+    }
+}
+
+impl<T: MeetSemiLattice> JoinSemiLattice for Dual<T> {
+    fn join(&mut self, other: &Self) -> bool {
+        self.0.meet(&other.0)
+    }
+}
+
+impl<T: JoinSemiLattice> MeetSemiLattice for Dual<T> {
+    fn meet(&mut self, other: &Self) -> bool {
+        self.0.join(&other.0)
+    }
+}
+
+/// Extends a type `T` with top and bottom elements to make it a partially ordered set in which no
+/// value of `T` is comparable with any other. A flat set has the following [Hasse
+/// diagram](https://en.wikipedia.org/wiki/Hasse_diagram):
+///
+/// ```text
+///         top
+///       / /  \ \
+/// all possible values of `T`
+///       \ \  / /
+///        bottom
+/// ```
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum FlatSet<T> {
+    Bottom,
+    Elem(T),
+    Top,
+}
+
+impl<T: Clone + Eq> JoinSemiLattice for FlatSet<T> {
+    fn join(&mut self, other: &Self) -> bool {
+        let result = match (&*self, other) {
+            (Self::Top, _) | (_, Self::Bottom) => return false,
+            (Self::Elem(a), Self::Elem(b)) if a == b => return false,
+
+            (Self::Bottom, Self::Elem(x)) => Self::Elem(x.clone()),
+
+            _ => Self::Top,
+        };
+
+        *self = result;
+        true
+    }
+}
+
+impl<T: Clone + Eq> MeetSemiLattice for FlatSet<T> {
+    fn meet(&mut self, other: &Self) -> bool {
+        let result = match (&*self, other) {
+            (Self::Bottom, _) | (_, Self::Top) => return false,
+            (Self::Elem(ref a), Self::Elem(ref b)) if a == b => return false,
+
+            (Self::Top, Self::Elem(ref x)) => Self::Elem(x.clone()),
+
+            _ => Self::Bottom,
+        };
+
+        *self = result;
+        true
+    }
+}