Change how always_live_locals is stored.

In `MaybeStorage{Live,Dead}` just store a reference. No need for a
`Cow`.

Author: nnethercote

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -1,7 +1,6 @@
 //! The `Visitor` responsible for actually checking a `mir::Body` for invalid operations.
 
 use std::assert_matches::assert_matches;
-use std::borrow::Cow;
 use std::mem;
 use std::num::NonZero;
 use std::ops::Deref;
@@ -236,10 +235,9 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
                 // A local is "transient" if it is guaranteed dead at all `Return`.
                 // So first compute the say of "maybe live" locals at each program point.
                 let always_live_locals = &always_storage_live_locals(&ccx.body);
-                let mut maybe_storage_live =
-                    MaybeStorageLive::new(Cow::Borrowed(always_live_locals))
-                        .iterate_to_fixpoint(ccx.tcx, &ccx.body, None)
-                        .into_results_cursor(&ccx.body);
+                let mut maybe_storage_live = MaybeStorageLive::new(&always_live_locals)
+                    .iterate_to_fixpoint(ccx.tcx, &ccx.body, None)
+                    .into_results_cursor(&ccx.body);
 
                 // And then check all `Return` in the MIR, and if a local is "maybe live" at a
                 // `Return` then it is definitely not transient.

compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs

@@ -1,5 +1,3 @@
-use std::borrow::Cow;
-
 use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::visit::{NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
@@ -8,11 +6,11 @@ use super::MaybeBorrowedLocals;
 use crate::{Analysis, GenKill, ResultsCursor};
 
 pub struct MaybeStorageLive<'a> {
-    always_live_locals: Cow<'a, BitSet<Local>>,
+    always_live_locals: &'a BitSet<Local>,
 }
 
 impl<'a> MaybeStorageLive<'a> {
-    pub fn new(always_live_locals: Cow<'a, BitSet<Local>>) -> Self {
+    pub fn new(always_live_locals: &'a BitSet<Local>) -> Self {
         MaybeStorageLive { always_live_locals }
     }
 }
@@ -53,11 +51,11 @@ impl<'a, 'tcx> Analysis<'tcx> for MaybeStorageLive<'a> {
 }
 
 pub struct MaybeStorageDead<'a> {
-    always_live_locals: Cow<'a, BitSet<Local>>,
+    always_live_locals: &'a BitSet<Local>,
 }
 
 impl<'a> MaybeStorageDead<'a> {
-    pub fn new(always_live_locals: Cow<'a, BitSet<Local>>) -> Self {
+    pub fn new(always_live_locals: &'a BitSet<Local>) -> Self {
         MaybeStorageDead { always_live_locals }
     }
 }

compiler/rustc_mir_transform/src/coroutine.rs

@@ -665,7 +665,7 @@ fn locals_live_across_suspend_points<'tcx>(
 ) -> LivenessInfo {
     // Calculate when MIR locals have live storage. This gives us an upper bound of their
     // lifetimes.
-    let mut storage_live = MaybeStorageLive::new(std::borrow::Cow::Borrowed(always_live_locals))
+    let mut storage_live = MaybeStorageLive::new(always_live_locals)
         .iterate_to_fixpoint(tcx, body, None)
         .into_results_cursor(body);
 

compiler/rustc_mir_transform/src/lint.rs

@@ -2,8 +2,6 @@
 //! It can be used to locate problems in MIR building or optimizations. It assumes that all code
 //! can be executed, so it has false positives.
 
-use std::borrow::Cow;
-
 use rustc_data_structures::fx::FxHashSet;
 use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::visit::{PlaceContext, Visitor};
@@ -16,11 +14,11 @@ use rustc_mir_dataflow::{Analysis, ResultsCursor};
 pub(super) fn lint_body<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, when: String) {
     let always_live_locals = &always_storage_live_locals(body);
 
-    let maybe_storage_live = MaybeStorageLive::new(Cow::Borrowed(always_live_locals))
+    let maybe_storage_live = MaybeStorageLive::new(always_live_locals)
         .iterate_to_fixpoint(tcx, body, None)
         .into_results_cursor(body);
 
-    let maybe_storage_dead = MaybeStorageDead::new(Cow::Borrowed(always_live_locals))
+    let maybe_storage_dead = MaybeStorageDead::new(always_live_locals)
         .iterate_to_fixpoint(tcx, body, None)
         .into_results_cursor(body);
 

compiler/rustc_mir_transform/src/ref_prop.rs

@@ -1,5 +1,3 @@
-use std::borrow::Cow;
-
 use rustc_data_structures::fx::FxHashSet;
 use rustc_index::IndexVec;
 use rustc_index::bit_set::BitSet;
@@ -125,7 +123,7 @@ fn compute_replacement<'tcx>(
 
     // Compute `MaybeStorageDead` dataflow to check that we only replace when the pointee is
     // definitely live.
-    let mut maybe_dead = MaybeStorageDead::new(Cow::Owned(always_live_locals))
+    let mut maybe_dead = MaybeStorageDead::new(&always_live_locals)
         .iterate_to_fixpoint(tcx, body, None)
         .into_results_cursor(body);