Move the dataflow framework to its own crate.

compiler/rustc_mir_dataflow/src/storage.rs

@@ -0,0 +1,41 @@
+use rustc_index::bit_set::BitSet;
+use rustc_middle::mir::{self, Local};
+
+/// The set of locals in a MIR body that do not have `StorageLive`/`StorageDead` annotations.
+///
+/// These locals have fixed storage for the duration of the body.
+//
+// FIXME: Currently, we need to traverse the entire MIR to compute this. We should instead store it
+// as a field in the `LocalDecl` for each `Local`.
+#[derive(Debug, Clone)]
+pub struct AlwaysLiveLocals(BitSet<Local>);
+
+impl AlwaysLiveLocals {
+    pub fn new(body: &mir::Body<'tcx>) -> Self {
+        let mut always_live_locals = AlwaysLiveLocals(BitSet::new_filled(body.local_decls.len()));
+
+        for block in body.basic_blocks() {
+            for statement in &block.statements {
+                use mir::StatementKind::{StorageDead, StorageLive};
+                if let StorageLive(l) | StorageDead(l) = statement.kind {
+                    always_live_locals.0.remove(l);
+                }
+            }
+        }
+
+        always_live_locals
+    }
+
+    pub fn into_inner(self) -> BitSet<Local> {
+        self.0
+    }
+}
+
+impl std::ops::Deref for AlwaysLiveLocals {
+    type Target = BitSet<Local>;
+
+    #[inline]
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}