Add utility to find locals that don't use Storage* annotations

Author: ecstatic-morse

src/librustc_mir/util/mod.rs

@@ -3,6 +3,7 @@ pub mod borrowck_errors;
 pub mod def_use;
 pub mod elaborate_drops;
 pub mod patch;
+pub mod storage;
 
 mod alignment;
 pub mod collect_writes;

src/librustc_mir/util/storage.rs

@@ -0,0 +1,42 @@
+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 locals = BitSet::new_filled(body.local_decls.len());
+
+        // FIXME: Use a visitor for this when `visit_body` can take a plain `Body`.
+        for block in body.basic_blocks().iter() {
+            for stmt in &block.statements {
+                if let mir::StatementKind::StorageLive(l) | mir::StatementKind::StorageDead(l) =
+                    stmt.kind
+                {
+                    locals.remove(l);
+                }
+            }
+        }
+
+        AlwaysLiveLocals(locals)
+    }
+
+    pub fn into_inner(self) -> BitSet<Local> {
+        self.0
+    }
+}
+
+impl std::ops::Deref for AlwaysLiveLocals {
+    type Target = BitSet<Local>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}