Auto merge of #57202 - matthewjasper:nll-typeck-promoteds, r=pnkfelix

Include bounds from promoted constants in NLL

Previously a promoted function wouldn't have its bound propagated out to
the main function body.

When we visit a promoted, we now type check the MIR of the promoted
and transfer any lifetime constraints to back to the main function's MIR.

Fixes #57170

r? @nikomatsakis

12 files changed, 229 insertions, 37 deletions

diff --git a/src/librustc_mir/borrow_check/nll/region_infer/mod.rs b/src/librustc_mir/borrow_check/nll/region_infer/mod.rs
index 785f810d941..f0d3a0d2986 100644
--- a/src/librustc_mir/borrow_check/nll/region_infer/mod.rs
+++ b/src/librustc_mir/borrow_check/nll/region_infer/mod.rs
@@ -1357,7 +1357,6 @@ pub trait ClosureRegionRequirementsExt<'gcx, 'tcx> {
     fn apply_requirements(
         &self,
         tcx: TyCtxt<'_, 'gcx, 'tcx>,
-        location: Location,
         closure_def_id: DefId,
         closure_substs: SubstsRef<'tcx>,
     ) -> Vec<QueryRegionConstraint<'tcx>>;
@@ -1388,13 +1387,12 @@ impl<'gcx, 'tcx> ClosureRegionRequirementsExt<'gcx, 'tcx> for ClosureRegionRequi
     fn apply_requirements(
         &self,
         tcx: TyCtxt<'_, 'gcx, 'tcx>,
-        location: Location,
         closure_def_id: DefId,
         closure_substs: SubstsRef<'tcx>,
     ) -> Vec<QueryRegionConstraint<'tcx>> {
         debug!(
-            "apply_requirements(location={:?}, closure_def_id={:?}, closure_substs={:?})",
-            location, closure_def_id, closure_substs
+            "apply_requirements(closure_def_id={:?}, closure_substs={:?})",
+            closure_def_id, closure_substs
         );
 
         // Extract the values of the free regions in `closure_substs`
diff --git a/src/librustc_mir/borrow_check/nll/region_infer/values.rs b/src/librustc_mir/borrow_check/nll/region_infer/values.rs
index ef27fdbde38..c4491778162 100644
--- a/src/librustc_mir/borrow_check/nll/region_infer/values.rs
+++ b/src/librustc_mir/borrow_check/nll/region_infer/values.rs
@@ -154,10 +154,10 @@ impl<N: Idx> LivenessValues<N> {
     /// Creates a new set of "region values" that tracks causal information.
     /// Each of the regions in num_region_variables will be initialized with an
     /// empty set of points and no causal information.
-    crate fn new(elements: &Rc<RegionValueElements>) -> Self {
+    crate fn new(elements: Rc<RegionValueElements>) -> Self {
         Self {
-            elements: elements.clone(),
             points: SparseBitMatrix::new(elements.num_points),
+            elements: elements,
         }
     }
 
diff --git a/src/librustc_mir/borrow_check/nll/renumber.rs b/src/librustc_mir/borrow_check/nll/renumber.rs
index eab9e0ae171..0a0a88e6942 100644
--- a/src/librustc_mir/borrow_check/nll/renumber.rs
+++ b/src/librustc_mir/borrow_check/nll/renumber.rs
@@ -47,6 +47,14 @@ impl<'a, 'gcx, 'tcx> NLLVisitor<'a, 'gcx, 'tcx> {
 }
 
 impl<'a, 'gcx, 'tcx> MutVisitor<'tcx> for NLLVisitor<'a, 'gcx, 'tcx> {
+    fn visit_mir(&mut self, mir: &mut Mir<'tcx>) {
+        for promoted in mir.promoted.iter_mut() {
+            self.visit_mir(promoted);
+        }
+
+        self.super_mir(mir);
+    }
+
     fn visit_ty(&mut self, ty: &mut Ty<'tcx>, ty_context: TyContext) {
         debug!("visit_ty(ty={:?}, ty_context={:?})", ty, ty_context);
 
diff --git a/src/librustc_mir/borrow_check/nll/type_check/mod.rs b/src/librustc_mir/borrow_check/nll/type_check/mod.rs
index cf054c2d055..9fa3bf55247 100644
--- a/src/librustc_mir/borrow_check/nll/type_check/mod.rs
+++ b/src/librustc_mir/borrow_check/nll/type_check/mod.rs
@@ -39,13 +39,14 @@ use rustc::ty::fold::TypeFoldable;
 use rustc::ty::subst::{Subst, SubstsRef, UnpackedKind, UserSubsts};
 use rustc::ty::{
     self, RegionVid, ToPolyTraitRef, Ty, TyCtxt, TyKind, UserType,
-    CanonicalUserTypeAnnotation, UserTypeAnnotationIndex,
+    CanonicalUserTypeAnnotation, CanonicalUserTypeAnnotations,
+    UserTypeAnnotationIndex,
 };
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::indexed_vec::{IndexVec, Idx};
 use rustc::ty::layout::VariantIdx;
 use std::rc::Rc;
-use std::{fmt, iter};
+use std::{fmt, iter, mem};
 use syntax_pos::{Span, DUMMY_SP};
 
 macro_rules! span_mirbug {
@@ -124,7 +125,7 @@ pub(crate) fn type_check<'gcx, 'tcx>(
     let mut constraints = MirTypeckRegionConstraints {
         placeholder_indices: PlaceholderIndices::default(),
         placeholder_index_to_region: IndexVec::default(),
-        liveness_constraints: LivenessValues::new(elements),
+        liveness_constraints: LivenessValues::new(elements.clone()),
         outlives_constraints: ConstraintSet::default(),
         closure_bounds_mapping: Default::default(),
         type_tests: Vec::default(),
@@ -253,7 +254,7 @@ enum FieldAccessError {
 /// is a problem.
 struct TypeVerifier<'a, 'b: 'a, 'gcx: 'tcx, 'tcx: 'b> {
     cx: &'a mut TypeChecker<'b, 'gcx, 'tcx>,
-    mir: &'a Mir<'tcx>,
+    mir: &'b Mir<'tcx>,
     last_span: Span,
     mir_def_id: DefId,
     errors_reported: bool,
@@ -283,7 +284,7 @@ impl<'a, 'b, 'gcx, 'tcx> Visitor<'tcx> for TypeVerifier<'a, 'b, 'gcx, 'tcx> {
                 location.to_locations(),
                 ConstraintCategory::Boring,
             ) {
-                let annotation = &self.mir.user_type_annotations[annotation_index];
+                let annotation = &self.cx.user_type_annotations[annotation_index];
                 span_mirbug!(
                     self,
                     constant,
@@ -385,7 +386,7 @@ impl<'a, 'b, 'gcx, 'tcx> Visitor<'tcx> for TypeVerifier<'a, 'b, 'gcx, 'tcx> {
 }
 
 impl<'a, 'b, 'gcx, 'tcx> TypeVerifier<'a, 'b, 'gcx, 'tcx> {
-    fn new(cx: &'a mut TypeChecker<'b, 'gcx, 'tcx>, mir: &'a Mir<'tcx>) -> Self {
+    fn new(cx: &'a mut TypeChecker<'b, 'gcx, 'tcx>, mir: &'b Mir<'tcx>) -> Self {
         TypeVerifier {
             mir,
             mir_def_id: cx.mir_def_id,
@@ -454,19 +455,31 @@ impl<'a, 'b, 'gcx, 'tcx> TypeVerifier<'a, 'b, 'gcx, 'tcx> {
             Place::Base(PlaceBase::Local(index)) => PlaceTy::Ty {
                 ty: self.mir.local_decls[index].ty,
             },
-            Place::Base(PlaceBase::Promoted(box (_index, sty))) => {
+            Place::Base(PlaceBase::Promoted(box (index, sty))) => {
                 let sty = self.sanitize_type(place, sty);
-                // FIXME -- promoted MIR return types reference
-                // various "free regions" (e.g., scopes and things)
-                // that they ought not to do. We have to figure out
-                // how best to handle that -- probably we want treat
-                // promoted MIR much like closures, renumbering all
-                // their free regions and propagating constraints
-                // upwards. We have the same acyclic guarantees, so
-                // that should be possible. But for now, ignore them.
-                //
-                // let promoted_mir = &self.mir.promoted[index];
-                // promoted_mir.return_ty()
+
+                if !self.errors_reported {
+                    let promoted_mir = &self.mir.promoted[index];
+                    self.sanitize_promoted(promoted_mir, location);
+
+                    let promoted_ty = promoted_mir.return_ty();
+
+                    if let Err(terr) = self.cx.eq_types(
+                        sty,
+                        promoted_ty,
+                        location.to_locations(),
+                        ConstraintCategory::Boring,
+                    ) {
+                        span_mirbug!(
+                            self,
+                            place,
+                            "bad promoted type ({:?}: {:?}): {:?}",
+                            promoted_ty,
+                            sty,
+                            terr
+                        );
+                    };
+                }
                 PlaceTy::Ty { ty: sty }
             }
             Place::Base(PlaceBase::Static(box Static { def_id, ty: sty })) => {
@@ -533,6 +546,72 @@ impl<'a, 'b, 'gcx, 'tcx> TypeVerifier<'a, 'b, 'gcx, 'tcx> {
         place_ty
     }
 
+    fn sanitize_promoted(&mut self, promoted_mir: &'b Mir<'tcx>, location: Location) {
+        // Determine the constraints from the promoted MIR by running the type
+        // checker on the promoted MIR, then transfer the constraints back to
+        // the main MIR, changing the locations to the provided location.
+
+        let parent_mir = mem::replace(&mut self.mir, promoted_mir);
+
+        let all_facts = &mut None;
+        let mut constraints = Default::default();
+        let mut closure_bounds = Default::default();
+        if let Some(ref mut bcx) = self.cx.borrowck_context {
+            // Don't try to add borrow_region facts for the promoted MIR
+            mem::swap(bcx.all_facts, all_facts);
+
+            // Use a new sets of constraints and closure bounds so that we can
+            // modify their locations.
+            mem::swap(&mut bcx.constraints.outlives_constraints, &mut constraints);
+            mem::swap(&mut bcx.constraints.closure_bounds_mapping, &mut closure_bounds);
+        };
+
+        self.visit_mir(promoted_mir);
+
+        if !self.errors_reported {
+            // if verifier failed, don't do further checks to avoid ICEs
+            self.cx.typeck_mir(promoted_mir);
+        }
+
+        self.mir = parent_mir;
+        // Merge the outlives constraints back in, at the given location.
+        if let Some(ref mut base_bcx) = self.cx.borrowck_context {
+            mem::swap(base_bcx.all_facts, all_facts);
+            mem::swap(&mut base_bcx.constraints.outlives_constraints, &mut constraints);
+            mem::swap(&mut base_bcx.constraints.closure_bounds_mapping, &mut closure_bounds);
+
+            let locations = location.to_locations();
+            for constraint in constraints.iter() {
+                let mut constraint = *constraint;
+                constraint.locations = locations;
+                if let ConstraintCategory::Return
+                    | ConstraintCategory::UseAsConst
+                    | ConstraintCategory::UseAsStatic = constraint.category
+                {
+                    // "Returning" from a promoted is an assigment to a
+                    // temporary from the user's point of view.
+                    constraint.category = ConstraintCategory::Boring;
+                }
+                base_bcx.constraints.outlives_constraints.push(constraint)
+            }
+
+            if !closure_bounds.is_empty() {
+                let combined_bounds_mapping = closure_bounds
+                    .into_iter()
+                    .flat_map(|(_, value)| value)
+                    .collect();
+                let existing = base_bcx
+                    .constraints
+                    .closure_bounds_mapping
+                    .insert(location, combined_bounds_mapping);
+                assert!(
+                    existing.is_none(),
+                    "Multiple promoteds/closures at the same location."
+                );
+            }
+        }
+    }
+
     fn sanitize_projection(
         &mut self,
         base: PlaceTy<'tcx>,
@@ -738,7 +817,9 @@ struct TypeChecker<'a, 'gcx: 'tcx, 'tcx: 'a> {
     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
     param_env: ty::ParamEnv<'gcx>,
     last_span: Span,
-    mir: &'a Mir<'tcx>,
+    /// User type annotations are shared between the main MIR and the MIR of
+    /// all of the promoted items.
+    user_type_annotations: &'a CanonicalUserTypeAnnotations<'tcx>,
     mir_def_id: DefId,
     region_bound_pairs: &'a RegionBoundPairs<'tcx>,
     implicit_region_bound: Option<ty::Region<'tcx>>,
@@ -893,8 +974,8 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
         let mut checker = Self {
             infcx,
             last_span: DUMMY_SP,
-            mir,
             mir_def_id,
+            user_type_annotations: &mir.user_type_annotations,
             param_env,
             region_bound_pairs,
             implicit_region_bound,
@@ -910,9 +991,9 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
     fn check_user_type_annotations(&mut self) {
         debug!(
             "check_user_type_annotations: user_type_annotations={:?}",
-             self.mir.user_type_annotations
+             self.user_type_annotations
         );
-        for user_annotation in &self.mir.user_type_annotations {
+        for user_annotation in self.user_type_annotations {
             let CanonicalUserTypeAnnotation { span, ref user_ty, inferred_ty } = *user_annotation;
             let (annotation, _) = self.infcx.instantiate_canonical_with_fresh_inference_vars(
                 span, user_ty
@@ -1095,7 +1176,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
             a, v, user_ty, locations,
         );
 
-        let annotated_type = self.mir.user_type_annotations[user_ty.base].inferred_ty;
+        let annotated_type = self.user_type_annotations[user_ty.base].inferred_ty;
         let mut curr_projected_ty = PlaceTy::from_ty(annotated_type);
 
         let tcx = self.infcx.tcx;
@@ -1281,7 +1362,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
                         location.to_locations(),
                         ConstraintCategory::Boring,
                     ) {
-                        let annotation = &mir.user_type_annotations[annotation_index];
+                        let annotation = &self.user_type_annotations[annotation_index];
                         span_mirbug!(
                             self,
                             stmt,
@@ -1340,7 +1421,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
                     Locations::All(stmt.source_info.span),
                     ConstraintCategory::TypeAnnotation,
                 ) {
-                    let annotation = &mir.user_type_annotations[projection.base];
+                    let annotation = &self.user_type_annotations[projection.base];
                     span_mirbug!(
                         self,
                         stmt,
@@ -1998,7 +2079,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
             }
 
             Rvalue::Ref(region, _borrow_kind, borrowed_place) => {
-                self.add_reborrow_constraint(location, region, borrowed_place);
+                self.add_reborrow_constraint(mir, location, region, borrowed_place);
             }
 
             // FIXME: These other cases have to be implemented in future PRs
@@ -2097,6 +2178,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
     /// - `borrowed_place`: the place `P` being borrowed
     fn add_reborrow_constraint(
         &mut self,
+        mir: &Mir<'tcx>,
         location: Location,
         borrow_region: ty::Region<'tcx>,
         borrowed_place: &Place<'tcx>,
@@ -2146,7 +2228,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
             match *elem {
                 ProjectionElem::Deref => {
                     let tcx = self.infcx.tcx;
-                    let base_ty = base.ty(self.mir, tcx).to_ty(tcx);
+                    let base_ty = base.ty(mir, tcx).to_ty(tcx);
 
                     debug!("add_reborrow_constraint - base_ty = {:?}", base_ty);
                     match base_ty.sty {
@@ -2275,7 +2357,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
     ) -> ty::InstantiatedPredicates<'tcx> {
         if let Some(closure_region_requirements) = tcx.mir_borrowck(def_id).closure_requirements {
             let closure_constraints =
-                closure_region_requirements.apply_requirements(tcx, location, def_id, substs);
+                closure_region_requirements.apply_requirements(tcx, def_id, substs);
 
             if let Some(ref mut borrowck_context) = self.borrowck_context {
                 let bounds_mapping = closure_constraints
diff --git a/src/test/ui/nll/issue-48697.rs b/src/test/ui/nll/issue-48697.rs
index c60c265b813..ececd6fccd8 100644
--- a/src/test/ui/nll/issue-48697.rs
+++ b/src/test/ui/nll/issue-48697.rs
@@ -1,14 +1,12 @@
 // Regression test for #48697
 
-// compile-pass
-
 #![feature(nll)]
 
 fn foo(x: &i32) -> &i32 {
     let z = 4;
     let f = &|y| y;
     let k = f(&z);
-    f(x)
+    f(x) //~ cannot return value referencing local variable
 }
 
 fn main() {}
diff --git a/src/test/ui/nll/issue-48697.stderr b/src/test/ui/nll/issue-48697.stderr
new file mode 100644
index 00000000000..16b46c15b90
--- /dev/null
+++ b/src/test/ui/nll/issue-48697.stderr
@@ -0,0 +1,11 @@
+error[E0515]: cannot return value referencing local variable `z`
+  --> $DIR/issue-48697.rs:9:5
+   |
+LL |     let k = f(&z);
+   |               -- `z` is borrowed here
+LL |     f(x) //~ cannot return value referencing local variable
+   |     ^^^^ returns a value referencing data owned by the current function
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0515`.
diff --git a/src/test/ui/nll/promoted-bounds.rs b/src/test/ui/nll/promoted-bounds.rs
new file mode 100644
index 00000000000..59b21cf9ac2
--- /dev/null
+++ b/src/test/ui/nll/promoted-bounds.rs
@@ -0,0 +1,27 @@
+#![feature(nll)]
+
+fn shorten_lifetime<'a, 'b, 'min>(a: &'a i32, b: &'b i32) -> &'min i32
+where
+    'a: 'min,
+    'b: 'min,
+{
+    if *a < *b {
+        &a
+    } else {
+        &b
+    }
+}
+
+fn main() {
+    let promoted_fn_item_ref = &shorten_lifetime;
+
+    let a = &5;
+    let ptr = {
+        let l = 3;
+        let b = &l; //~ ERROR does not live long enough
+        let c = promoted_fn_item_ref(a, b);
+        c
+    };
+
+    println!("ptr = {:?}", ptr);
+}
diff --git a/src/test/ui/nll/promoted-bounds.stderr b/src/test/ui/nll/promoted-bounds.stderr
new file mode 100644
index 00000000000..9798f238fc4
--- /dev/null
+++ b/src/test/ui/nll/promoted-bounds.stderr
@@ -0,0 +1,15 @@
+error[E0597]: `l` does not live long enough
+  --> $DIR/promoted-bounds.rs:21:17
+   |
+LL |     let ptr = {
+   |         --- borrow later stored here
+LL |         let l = 3;
+LL |         let b = &l; //~ ERROR does not live long enough
+   |                 ^^ borrowed value does not live long enough
+...
+LL |     };
+   |     - `l` dropped here while still borrowed
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.
diff --git a/src/test/ui/nll/promoted-closure-pair.rs b/src/test/ui/nll/promoted-closure-pair.rs
new file mode 100644
index 00000000000..7b3bbad4c1e
--- /dev/null
+++ b/src/test/ui/nll/promoted-closure-pair.rs
@@ -0,0 +1,12 @@
+// Check that we handle multiple closures in the same promoted constant.
+
+#![feature(nll)]
+
+fn foo() -> &'static i32 {
+    let z = 0;
+    let p = &(|y| y, |y| y);
+    p.0(&z);
+    p.1(&z)         //~ ERROR cannot return
+}
+
+fn main() {}
diff --git a/src/test/ui/nll/promoted-closure-pair.stderr b/src/test/ui/nll/promoted-closure-pair.stderr
new file mode 100644
index 00000000000..5f4a6037668
--- /dev/null
+++ b/src/test/ui/nll/promoted-closure-pair.stderr
@@ -0,0 +1,12 @@
+error[E0515]: cannot return value referencing local variable `z`
+  --> $DIR/promoted-closure-pair.rs:9:5
+   |
+LL |     p.1(&z)         //~ ERROR cannot return
+   |     ^^^^--^
+   |     |   |
+   |     |   `z` is borrowed here
+   |     returns a value referencing data owned by the current function
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0515`.
diff --git a/src/test/ui/nll/user-annotations/promoted-annotation.rs b/src/test/ui/nll/user-annotations/promoted-annotation.rs
new file mode 100644
index 00000000000..fa2d2fb8118
--- /dev/null
+++ b/src/test/ui/nll/user-annotations/promoted-annotation.rs
@@ -0,0 +1,12 @@
+// Test that type annotations are checked in promoted constants correctly.
+
+#![feature(nll)]
+
+fn foo<'a>() {
+    let x = 0;
+    let f = &drop::<&'a i32>;
+    f(&x);
+    //~^ ERROR `x` does not live long enough
+}
+
+fn main() {}
diff --git a/src/test/ui/nll/user-annotations/promoted-annotation.stderr b/src/test/ui/nll/user-annotations/promoted-annotation.stderr
new file mode 100644
index 00000000000..144af1e0ec1
--- /dev/null
+++ b/src/test/ui/nll/user-annotations/promoted-annotation.stderr
@@ -0,0 +1,17 @@
+error[E0597]: `x` does not live long enough
+  --> $DIR/promoted-annotation.rs:8:7
+   |
+LL | fn foo<'a>() {
+   |        -- lifetime `'a` defined here
+LL |     let x = 0;
+LL |     let f = &drop::<&'a i32>;
+   |             ---------------- assignment requires that `x` is borrowed for `'a`
+LL |     f(&x);
+   |       ^^ borrowed value does not live long enough
+LL |     //~^ ERROR `x` does not live long enough
+LL | }
+   | - `x` dropped here while still borrowed
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.