Consider alias bounds when computing liveness in NLL (but this time sound hopefully)

compiler/rustc_borrowck/src/type_check/liveness/trace.rs

@@ -3,6 +3,7 @@ use rustc_data_structures::graph::WithSuccessors;
 use rustc_index::bit_set::{HybridBitSet, SparseBitMatrix};
 use rustc_index::interval::IntervalSet;
 use rustc_infer::infer::canonical::QueryRegionConstraints;
+use rustc_infer::infer::outlives::for_liveness;
 use rustc_middle::mir::{BasicBlock, Body, ConstraintCategory, Local, Location};
 use rustc_middle::traits::query::DropckOutlivesResult;
 use rustc_middle::ty::{RegionVid, Ty, TyCtxt, TypeVisitable, TypeVisitableExt};
@@ -601,34 +602,36 @@ impl<'tcx> LivenessContext<'_, '_, '_, 'tcx> {
             values::location_set_str(elements, live_at.iter()),
         );
 
-        let tcx = typeck.tcx();
-        let borrowck_context = &mut typeck.borrowck_context;
-
         // When using `-Zpolonius=next`, we want to record the loans that flow into this value's
         // regions as being live at the given `live_at` points: this will be used to compute the
         // location where a loan goes out of scope.
-        let num_loans = borrowck_context.borrow_set.len();
-        let mut value_loans = HybridBitSet::new_empty(num_loans);
-
-        tcx.for_each_free_region(&value, |live_region| {
-            let live_region_vid = borrowck_context.universal_regions.to_region_vid(live_region);
-
-            borrowck_context
-                .constraints
-                .liveness_constraints
-                .add_elements(live_region_vid, live_at);
-
-            // There can only be inflowing loans for this region when we are using
-            // `-Zpolonius=next`.
-            if let Some(inflowing) = inflowing_loans.row(live_region_vid) {
-                value_loans.union(inflowing);
-            }
+        let num_loans = typeck.borrowck_context.borrow_set.len();
+        let value_loans = &mut HybridBitSet::new_empty(num_loans);
+
+        value.visit_with(&mut for_liveness::FreeRegionsVisitor {
+            tcx: typeck.tcx(),
+            param_env: typeck.param_env,
+            op: |r| {
+                let live_region_vid = typeck.borrowck_context.universal_regions.to_region_vid(r);
+
+                typeck
+                    .borrowck_context
+                    .constraints
+                    .liveness_constraints
+                    .add_elements(live_region_vid, live_at);
+
+                // There can only be inflowing loans for this region when we are using
+                // `-Zpolonius=next`.
+                if let Some(inflowing) = inflowing_loans.row(live_region_vid) {
+                    value_loans.union(inflowing);
+                }
+            },
         });
 
         // Record the loans reaching the value.
         if !value_loans.is_empty() {
             for point in live_at.iter() {
-                borrowck_context.live_loans.union_row(point, &value_loans);
+                typeck.borrowck_context.live_loans.union_row(point, value_loans);
             }
         }
     }

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -380,6 +380,8 @@ impl<'tcx> InferCtxt<'tcx> {
                 .collect(),
         );
 
+        // FIXME(#42940): This should use the `FreeRegionsVisitor`, but that's
+        // not currently sound until we have existential regions.
         concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
             tcx: self.tcx,
             op: |r| self.member_constraint(opaque_type_key, span, concrete_ty, r, &choice_regions),

compiler/rustc_infer/src/infer/outlives/for_liveness.rs

@@ -0,0 +1,121 @@
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitor};
+
+use std::ops::ControlFlow;
+
+use crate::infer::outlives::test_type_match;
+use crate::infer::region_constraints::VerifyIfEq;
+
+/// Visits free regions in the type that are relevant for liveness computation.
+/// These regions are passed to `OP`.
+///
+/// Specifically, we visit all of the regions of types recursively, except if
+/// the type is an alias, we look at the outlives bounds in the param-env
+/// and alias's item bounds. If there is a unique outlives bound, then visit
+/// that instead. If there is not a unique but there is a `'static` outlives
+/// bound, then don't visit anything. Otherwise, walk through the opaque's
+/// regions structurally.
+pub struct FreeRegionsVisitor<'tcx, OP: FnMut(ty::Region<'tcx>)> {
+    pub tcx: TyCtxt<'tcx>,
+    pub param_env: ty::ParamEnv<'tcx>,
+    pub op: OP,
+}
+
+impl<'tcx, OP> TypeVisitor<TyCtxt<'tcx>> for FreeRegionsVisitor<'tcx, OP>
+where
+    OP: FnMut(ty::Region<'tcx>),
+{
+    fn visit_binder<T: TypeVisitable<TyCtxt<'tcx>>>(
+        &mut self,
+        t: &ty::Binder<'tcx, T>,
+    ) -> ControlFlow<Self::BreakTy> {
+        t.super_visit_with(self);
+        ControlFlow::Continue(())
+    }
+
+    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+        match *r {
+            // ignore bound regions, keep visiting
+            ty::ReLateBound(_, _) => ControlFlow::Continue(()),
+            _ => {
+                (self.op)(r);
+                ControlFlow::Continue(())
+            }
+        }
+    }
+
+    fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+        // We're only interested in types involving regions
+        if !ty.flags().intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
+            return ControlFlow::Continue(());
+        }
+
+        match ty.kind() {
+            // We can prove that an alias is live two ways:
+            // 1. All the components are live.
+            //
+            // 2. There is a known outlives bound or where-clause, and that
+            //    region is live.
+            //
+            // We search through the item bounds and where clauses for
+            // either `'static` or a unique outlives region, and if one is
+            // found, we just need to prove that that region is still live.
+            // If one is not found, then we continue to walk through the alias.
+            ty::Alias(kind, ty::AliasTy { def_id, args, .. }) => {
+                let tcx = self.tcx;
+                let param_env = self.param_env;
+                let outlives_bounds: Vec<_> = tcx
+                    .item_bounds(def_id)
+                    .iter_instantiated(tcx, args)
+                    .chain(param_env.caller_bounds())
+                    .filter_map(|clause| {
+                        let outlives = clause.as_type_outlives_clause()?;
+                        if let Some(outlives) = outlives.no_bound_vars()
+                            && outlives.0 == ty
+                        {
+                            Some(outlives.1)
+                        } else {
+                            test_type_match::extract_verify_if_eq(
+                                tcx,
+                                param_env,
+                                &outlives.map_bound(|ty::OutlivesPredicate(ty, bound)| {
+                                    VerifyIfEq { ty, bound }
+                                }),
+                                ty,
+                            )
+                        }
+                    })
+                    .collect();
+                // If we find `'static`, then we know the alias doesn't capture *any* regions.
+                // Otherwise, all of the outlives regions should be equal -- if they're not,
+                // we don't really know how to proceed, so we continue recursing through the
+                // alias.
+                if outlives_bounds.contains(&tcx.lifetimes.re_static) {
+                    // no
+                } else if let Some(r) = outlives_bounds.first()
+                    && outlives_bounds[1..].iter().all(|other_r| other_r == r)
+                {
+                    assert!(r.type_flags().intersects(ty::TypeFlags::HAS_FREE_REGIONS));
+                    r.visit_with(self)?;
+                } else {
+                    // Skip lifetime parameters that are not captures.
+                    let variances = match kind {
+                        ty::Opaque => Some(self.tcx.variances_of(*def_id)),
+                        _ => None,
+                    };
+
+                    for (idx, s) in args.iter().enumerate() {
+                        if variances.map(|variances| variances[idx]) != Some(ty::Variance::Bivariant) {
+                            s.visit_with(self)?;
+                        }
+                    }
+                }
+            }
+
+            _ => {
+                ty.super_visit_with(self)?;
+            }
+        }
+
+        ControlFlow::Continue(())
+    }
+}

compiler/rustc_infer/src/infer/outlives/mod.rs

@@ -9,6 +9,7 @@ use rustc_middle::ty;
 
 pub mod components;
 pub mod env;
+pub mod for_liveness;
 pub mod obligations;
 pub mod test_type_match;
 pub mod verify;

tests/ui/borrowck/alias-liveness/gat-static.rs

@@ -0,0 +1,29 @@
+// check-pass
+
+trait Foo {
+    type Assoc<'a>
+    where
+        Self: 'a;
+
+    fn assoc(&mut self) -> Self::Assoc<'_>;
+}
+
+fn overlapping_mut<T>(mut t: T)
+where
+    T: Foo,
+    for<'a> T::Assoc<'a>: 'static,
+{
+    let a = t.assoc();
+    let b = t.assoc();
+}
+
+fn live_past_borrow<T>(mut t: T)
+where
+    T: Foo,
+    for<'a> T::Assoc<'a>: 'static {
+    let x = t.assoc();
+    drop(t);
+    drop(x);
+}
+
+fn main() {}

tests/ui/borrowck/alias-liveness/higher-ranked-outlives-for-capture.rs

@@ -0,0 +1,16 @@
+// known-bug: #42940
+
+trait Captures<'a> {}
+impl<T> Captures<'_> for T {}
+
+trait Outlives<'a>: 'a {}
+impl<'a, T: 'a> Outlives<'a> for T {}
+
+// Test that we treat `for<'a> Opaque: 'a` as `Opaque: 'static`
+fn test<'o>(v: &'o Vec<i32>) -> impl Captures<'o> + for<'a> Outlives<'a> {}
+
+fn statik() -> impl Sized {
+    test(&vec![])
+}
+
+fn main() {}

tests/ui/borrowck/alias-liveness/higher-ranked-outlives-for-capture.stderr

@@ -0,0 +1,16 @@
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/higher-ranked-outlives-for-capture.rs:13:11
+   |
+LL |     test(&vec![])
+   |     ------^^^^^^-
+   |     |     |
+   |     |     creates a temporary value which is freed while still in use
+   |     argument requires that borrow lasts for `'static`
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: this error originates in the macro `vec` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0716`.

tests/ui/borrowck/alias-liveness/higher-ranked.rs

@@ -0,0 +1,16 @@
+// check-pass
+
+trait Captures<'a> {}
+impl<T> Captures<'_> for T {}
+
+trait Outlives<'a>: 'a {}
+impl<'a, T: 'a> Outlives<'a> for T {}
+
+// Test that we treat `for<'a> Opaque: 'a` as `Opaque: 'static`
+fn test<'o>(v: &'o Vec<i32>) -> impl Captures<'o> + for<'a> Outlives<'a> {}
+
+fn opaque_doesnt_use_temporary() {
+    let a = test(&vec![]);
+}
+
+fn main() {}

tests/ui/borrowck/alias-liveness/opaque-capture.rs

@@ -0,0 +1,17 @@
+// check-pass
+
+// Check that opaques capturing early and late-bound vars correctly mark
+// regions required to be live using the item bounds.
+
+trait Captures<'a> {}
+impl<T> Captures<'_> for T {}
+
+fn captures_temp_late<'a>(x: &'a Vec<i32>) -> impl Sized + Captures<'a> + 'static {}
+fn captures_temp_early<'a: 'a>(x: &'a Vec<i32>) -> impl Sized + Captures<'a> + 'static {}
+
+fn test() {
+    let x = captures_temp_early(&vec![]);
+    let y = captures_temp_late(&vec![]);
+}
+
+fn main() {}

tests/ui/borrowck/alias-liveness/opaque-type-param.rs

@@ -0,0 +1,14 @@
+// known-bug: #42940
+
+trait Trait {}
+impl Trait for () {}
+
+fn foo<'a>(s: &'a str) -> impl Trait + 'static {
+    bar(s)
+}
+
+fn bar<P: AsRef<str>>(s: P) -> impl Trait + 'static {
+    ()
+}
+
+fn main() {}

tests/ui/borrowck/alias-liveness/opaque-type-param.stderr

@@ -0,0 +1,13 @@
+error[E0700]: hidden type for `impl Trait + 'static` captures lifetime that does not appear in bounds
+  --> $DIR/opaque-type-param.rs:7:5
+   |
+LL | fn foo<'a>(s: &'a str) -> impl Trait + 'static {
+   |        --                 -------------------- opaque type defined here
+   |        |
+   |        hidden type `impl Trait + 'static` captures the lifetime `'a` as defined here
+LL |     bar(s)
+   |     ^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0700`.

tests/ui/borrowck/alias-liveness/rpit-static.rs

@@ -0,0 +1,22 @@
+// check-pass
+
+trait Captures<'a> {}
+impl<T> Captures<'_> for T {}
+
+fn foo(x: &mut i32) -> impl Sized + Captures<'_> + 'static {}
+
+fn overlapping_mut() {
+    let i = &mut 1;
+    let x = foo(i);
+    let y = foo(i);
+}
+
+fn live_past_borrow() {
+    let y;
+    {
+        let x = &mut 1;
+        y = foo(x);
+    }
+}
+
+fn main() {}

tests/ui/borrowck/alias-liveness/rpitit-static.rs

@@ -0,0 +1,18 @@
+// check-pass
+
+trait Foo {
+    fn rpitit(&mut self) -> impl Sized + 'static;
+}
+
+fn live_past_borrow<T: Foo>(mut t: T) {
+    let x = t.rpitit();
+    drop(t);
+    drop(x);
+}
+
+fn overlapping_mut<T: Foo>(mut t: T) {
+    let a = t.rpitit();
+    let b = t.rpitit();
+}
+
+fn main() {}

tests/ui/borrowck/alias-liveness/rtn-static.rs

@@ -0,0 +1,23 @@
+// check-pass
+
+#![feature(return_type_notation)]
+//~^ WARN the feature `return_type_notation` is incomplete
+
+trait Foo {
+    fn borrow(&mut self) -> impl Sized + '_;
+}
+
+fn live_past_borrow<T: Foo<borrow(): 'static>>(mut t: T) {
+    let x = t.borrow();
+    drop(t);
+    drop(x);
+}
+
+// Test that the `'_` item bound in `borrow` does not cause us to
+// overlook the `'static` RTN bound.
+fn overlapping_mut<T: Foo<borrow(): 'static>>(mut t: T) {
+    let x = t.borrow();
+    let x = t.borrow();
+}
+
+fn main() {}