Auto merge of rust-lang#128846 - compiler-errors:drop-liveness, r=<try>

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

This PR revives rust-lang#112279. I wanted to reopen this to gauge `@cjgillot's` thoughts about this, since it's been a while since `-Zdrop-tracking-mir` has landed.

If this PR looks OK, I can flesh it out and write up an FCP report -- I think this is a T-types FCP, since this has to do with the types that are considered live for auto traits, and doesn't otherwise affect the layout of coroutines.

Open questions:
* **"Why do we require storage to be live if the locals is not initialized?"** (rust-lang#112279 (comment))
    * I agree that we should eventually fix the storage analysis for coroutines, but this seems like a problem that can be fixed after we fix witnesses for *the purposes of traits* here.
    * As far as I could tell, fixing the problem of moved locals for *storage* would require insertion of additional `StorageDead` statements, or would require further entangling the type system with the operational semantics in ways that T-opsem seemed uncomfortable with [when I asked](https://rust-lang.zulipchat.com/#narrow/stream/189540-t-compiler.2Fwg-mir-opt/topic/Inserting.20.60StorageDrop.60.20after.20unconditional.20moves). cc `@RalfJung`
    * Relevant: rust-lang/unsafe-code-guidelines#188

Fixes rust-lang#128095
Fixes rust-lang#94067

r? `@cjgillot`

Author: bors

compiler/rustc_mir_transform/src/coroutine.rs

@@ -66,10 +66,12 @@ use rustc_middle::mir::*;
 use rustc_middle::ty::{self, CoroutineArgs, CoroutineArgsExt, InstanceKind, Ty, TyCtxt};
 use rustc_middle::{bug, span_bug};
 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::Analysis;
+use rustc_mir_dataflow::{self, Analysis, MaybeReachable};
 use rustc_span::def_id::{DefId, LocalDefId};
 use rustc_span::symbol::sym;
 use rustc_span::Span;
@@ -725,6 +727,10 @@ struct LivenessInfo {
     /// Which locals are live across any suspension point.
     saved_locals: CoroutineSavedLocals,
 
+    /// 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<CoroutineSavedLocal>>,
 
@@ -784,10 +790,20 @@ fn locals_live_across_suspend_points<'tcx>(
         .iterate_to_fixpoint()
         .into_results_cursor(body);
 
+    let param_env = tcx.param_env(body.source.def_id());
+    let move_data = MoveData::gather_moves(body, tcx, param_env, |_| true);
+
+    // Calculate the set of locals which are initialized
+    let mut inits = MaybeInitializedPlaces::new(tcx, body, &move_data)
+        .into_engine(tcx, body)
+        .iterate_to_fixpoint()
+        .into_results_cursor(body);
+
     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 {
@@ -826,12 +842,26 @@ fn locals_live_across_suspend_points<'tcx>(
             // The coroutine 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) = 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);
         }
@@ -856,6 +886,7 @@ 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,
@@ -1030,6 +1061,7 @@ fn compute_layout<'tcx>(
 ) {
     let LivenessInfo {
         saved_locals,
+        init_locals,
         live_locals_at_suspension_points,
         source_info_at_suspension_points,
         storage_conflicts,
@@ -1046,20 +1078,26 @@ 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 =
             CoroutineSavedTy { 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>`, which is required by `impl Future<Output = ()>: Send`
-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());
+}