Move HasTop and HasBottom into lattice.rs

compiler/rustc_mir_dataflow/src/framework/lattice.rs

@@ -73,6 +73,16 @@ pub trait MeetSemiLattice: Eq {
     fn meet(&mut self, other: &Self) -> bool;
 }
 
+/// A set that has a "bottom" element, which is less than or equal to any other element.
+pub trait HasBottom {
+    fn bottom() -> Self;
+}
+
+/// A set that has a "top" element, which is greater than or equal to any other element.
+pub trait HasTop {
+    fn top() -> Self;
+}
+
 /// A `bool` is a "two-point" lattice with `true` as the top element and `false` as the bottom:
 ///
 /// ```text
@@ -102,6 +112,18 @@ impl MeetSemiLattice for bool {
     }
 }
 
+impl HasBottom for bool {
+    fn bottom() -> Self {
+        false
+    }
+}
+
+impl HasTop for bool {
+    fn top() -> Self {
+        true
+    }
+}
+
 /// 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).
 ///
@@ -250,3 +272,15 @@ impl<T: Clone + Eq> MeetSemiLattice for FlatSet<T> {
         true
     }
 }
+
+impl<T> HasBottom for FlatSet<T> {
+    fn bottom() -> Self {
+        Self::Bottom
+    }
+}
+
+impl<T> HasTop for FlatSet<T> {
+    fn top() -> Self {
+        Self::Top
+    }
+}