Auto merge of rust-lang#112279 - nbdd0121:generator, r=<try>

Stop considering moved-out locals when computing auto traits for generators

Addresses rust-lang#94067 (but does not fix it since drop-tracking-mir is unstable)

This PR, unlike rust-lang#110420 or rust-lang#112050, does not attempt to reduce the number of live locals across suspension points. It however ignores moved-out locals for trait purposes. So this PR solves the non-send issue, but not the size issue.

Suggested by `@RalfJung` in [rust-lang/unsafe-code-guidelines#188](rust-lang/unsafe-code-guidelines#188 (comment))

cc `@b-naber` who's working on this from a different perspective.

r? `@cjgillot`

Author: bors

compiler/rustc_mir_transform/src/generator.rs

@@ -70,10 +70,12 @@ use rustc_middle::ty::InstanceDef;
 use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt};
 use rustc_middle::ty::{GeneratorArgs, GenericArgsRef};
 use rustc_mir_dataflow::impls::{
-    MaybeBorrowedLocals, MaybeLiveLocals, MaybeRequiresStorage, MaybeStorageLive,
+    MaybeBorrowedLocals, MaybeInitializedPlaces, MaybeLiveLocals, MaybeRequiresStorage,
+    MaybeStorageLive,
 };
+use rustc_mir_dataflow::move_paths::MoveData;
 use rustc_mir_dataflow::storage::always_storage_live_locals;
-use rustc_mir_dataflow::{self, Analysis};
+use rustc_mir_dataflow::{self, Analysis, MaybeReachable, MoveDataParamEnv};
 use rustc_span::def_id::{DefId, LocalDefId};
 use rustc_span::symbol::sym;
 use rustc_span::Span;
@@ -567,6 +569,10 @@ struct LivenessInfo {
     /// Which locals are live across any suspension point.
     saved_locals: GeneratorSavedLocals,
 
+    /// Which locals are live *and* initialized across any suspension point.
+    /// A local that is live but is not initialized does not need to accounted in auto trait checking.
+    init_locals: BitSet<Local>,
+
     /// The set of saved locals live at each suspension point.
     live_locals_at_suspension_points: Vec<BitSet<GeneratorSavedLocal>>,
 
@@ -620,10 +626,25 @@ fn locals_live_across_suspend_points<'tcx>(
         .iterate_to_fixpoint()
         .into_results_cursor(body_ref);
 
+    let param_env = tcx.param_env(body.source.def_id());
+    let move_data =
+        MoveData::gather_moves(body, tcx, param_env).unwrap_or_else(|(move_data, _)| {
+            tcx.sess.delay_span_bug(body.span, "gather_moves failed");
+            move_data
+        });
+    let mdpe = MoveDataParamEnv { move_data, param_env };
+
+    // Calculate the set of locals which are initialized
+    let mut inits = MaybeInitializedPlaces::new(tcx, body, &mdpe)
+        .into_engine(tcx, body)
+        .iterate_to_fixpoint()
+        .into_results_cursor(body_ref);
+
     let mut storage_liveness_map = IndexVec::from_elem(None, &body.basic_blocks);
     let mut live_locals_at_suspension_points = Vec::new();
     let mut source_info_at_suspension_points = Vec::new();
     let mut live_locals_at_any_suspension_point = BitSet::new_empty(body.local_decls.len());
+    let mut init_locals_at_any_suspension_point = BitSet::new_empty(body.local_decls.len());
 
     for (block, data) in body.basic_blocks.iter_enumerated() {
         if let TerminatorKind::Yield { .. } = data.terminator().kind {
@@ -662,12 +683,27 @@ fn locals_live_across_suspend_points<'tcx>(
             // The generator argument is ignored.
             live_locals.remove(SELF_ARG);
 
-            debug!("loc = {:?}, live_locals = {:?}", loc, live_locals);
+            inits.seek_to_block_end(block);
+            let mut init_locals: BitSet<_> = BitSet::new_empty(body.local_decls.len());
+            if let MaybeReachable::Reachable(bitset) = inits.get() {
+                for move_path_index in bitset.iter() {
+                    if let Some(local) = mdpe.move_data.move_paths[move_path_index].place.as_local()
+                    {
+                        init_locals.insert(local);
+                    }
+                }
+            }
+            init_locals.intersect(&live_locals);
+
+            debug!(
+                "loc = {:?}, live_locals = {:?}, init_locals = {:?}",
+                loc, live_locals, init_locals
+            );
 
             // Add the locals live at this suspension point to the set of locals which live across
             // any suspension points
             live_locals_at_any_suspension_point.union(&live_locals);
-
+            init_locals_at_any_suspension_point.union(&init_locals);
             live_locals_at_suspension_points.push(live_locals);
             source_info_at_suspension_points.push(data.terminator().source_info);
         }
@@ -692,6 +728,7 @@ fn locals_live_across_suspend_points<'tcx>(
 
     LivenessInfo {
         saved_locals,
+        init_locals: init_locals_at_any_suspension_point,
         live_locals_at_suspension_points,
         source_info_at_suspension_points,
         storage_conflicts,
@@ -863,6 +900,7 @@ fn compute_layout<'tcx>(
 ) {
     let LivenessInfo {
         saved_locals,
+        init_locals,
         live_locals_at_suspension_points,
         source_info_at_suspension_points,
         storage_conflicts,
@@ -879,20 +917,26 @@ fn compute_layout<'tcx>(
         let decl = &body.local_decls[local];
         debug!(?decl);
 
-        // Do not `assert_crate_local` here, as post-borrowck cleanup may have already cleared
-        // the information. This is alright, since `ignore_for_traits` is only relevant when
-        // this code runs on pre-cleanup MIR, and `ignore_for_traits = false` is the safer
-        // default.
-        let ignore_for_traits = match decl.local_info {
-            // Do not include raw pointers created from accessing `static` items, as those could
-            // well be re-created by another access to the same static.
-            ClearCrossCrate::Set(box LocalInfo::StaticRef { is_thread_local, .. }) => {
-                !is_thread_local
+        let ignore_for_traits = if !init_locals.contains(local) {
+            // If only the storage is required to be live, but local is not initialized, then we can
+            // ignore such type for auto trait purposes.
+            true
+        } else {
+            // Do not `assert_crate_local` here, as post-borrowck cleanup may have already cleared
+            // the information. This is alright, since `ignore_for_traits` is only relevant when
+            // this code runs on pre-cleanup MIR, and `ignore_for_traits = false` is the safer
+            // default.
+            match decl.local_info {
+                // Do not include raw pointers created from accessing `static` items, as those could
+                // well be re-created by another access to the same static.
+                ClearCrossCrate::Set(box LocalInfo::StaticRef { is_thread_local, .. }) => {
+                    !is_thread_local
+                }
+                // Fake borrows are only read by fake reads, so do not have any reality in
+                // post-analysis MIR.
+                ClearCrossCrate::Set(box LocalInfo::FakeBorrow) => true,
+                _ => false,
             }
-            // Fake borrows are only read by fake reads, so do not have any reality in
-            // post-analysis MIR.
-            ClearCrossCrate::Set(box LocalInfo::FakeBorrow) => true,
-            _ => false,
         };
         let decl =
             GeneratorSavedTy { ty: decl.ty, source_info: decl.source_info, ignore_for_traits };

tests/ui/async-await/drop-track-field-assign-nonsend.rs

@@ -5,14 +5,11 @@ LL |     assert_send(agent.handle());
    |                 ^^^^^^^^^^^^^^ future returned by `handle` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<String>`
-note: future is not `Send` as this value is used across an await
-  --> $DIR/field-assign-nonsend.rs:20:39
+note: captured value is not `Send` because `&mut` references cannot be sent unless their referent is `Send`
+  --> $DIR/field-assign-nonsend.rs:16:21
    |
-LL |         let mut info = self.info_result.clone();
-   |             -------- has type `InfoResult` which is not `Send`
-...
-LL |         let _ = send_element(element).await;
-   |                                       ^^^^^ await occurs here, with `mut info` maybe used later
+LL |     async fn handle(&mut self) {
+   |                     ^^^^^^^^^ has type `&mut Agent` which is not `Send`, because `Agent` is not `Send`
 note: required by a bound in `assert_send`
   --> $DIR/field-assign-nonsend.rs:37:19
    |

tests/ui/async-await/drop-track-field-assign-nonsend.stderr

@@ -0,0 +1,23 @@
+// check-pass
+// edition:2021
+
+use core::marker::PhantomData;
+
+struct B(PhantomData<*const ()>);
+
+fn do_sth(_: &B) {}
+
+async fn foo() {}
+
+async fn test() {
+    let b = B(PhantomData);
+    do_sth(&b);
+    drop(b);
+    foo().await;
+}
+
+fn assert_send<T: Send>(_: T) {}
+
+fn main() {
+    assert_send(test());
+}