Rollup merge of rust-lang#62736 - lqd:polonius_tests3, r=matthewjasper

Polonius: fix some cases of `killed` fact generation, and most of the `ui` test suite

Since basic Polonius functionality was re-enabled by @matthewjasper in rust-lang#54468, some tests were still failing in the polonius compare-mode.

This PR fixes all but one test in the `ui` suite by:
- fixing some bugs in the fact generation code, related to the `killed` relation: Polonius would incorrectly reject some NLL-accepted code, because of these missing `killed` facts.
- ignoring some tests in the polonius compare-mode: a lot of those manually test the NLL or migrate mode, and the failures were mostly artifacts of the test revisions, e.g. that `-Z polonius` requires full NLLs. Some others were also both failing with NLL and succeeding with Polonius, which we can't encode in tests at the moment.
- blessing the output of some tests: whenever Polonius and NLL have basically the same errors, except for diagnostics differences, the Polonius output is blessed. Whenever we've advanced into a less experimental phase, we'll want to revisit these cases (much like we did on the NLL test suite last year) to specifically work on diagnostics.

Fact generation changes:
- we now kill loans on the destination place of `Call` terminators
- we now kill loans on the locals destroyed by `StorageDead`
- we now also handle assignments to projections: killing the loans on a either a deref-ed local, or the ones whose `borrowed_place` conflicts with the current place.

One failing test remains: an overflow during fact generation, on a case of polymorphic recursion (and which I'll continue investigating later).

This adds some tests for the fact generation changes, with some simple Polonius cases similar to the existing smoke tests, but also for some cases encountered in the wild (in the `rand` crate for example).

A more detailed write-up is available [here](https://hackmd.io/CjYB0fs4Q9CweyeTdKWyEg?view) with an explanation for each test failure, the steps taken to resolve it (as a commit in the current PR), NLL and Polonius outputs (and diff), etc.

Since they've worked on this before, and we've discussed some of these failures together:

r? @matthewjasper

src/librustc_mir/borrow_check/nll/constraint_generation.rs

@@ -3,12 +3,15 @@ use crate::borrow_check::location::LocationTable;
 use crate::borrow_check::nll::ToRegionVid;
 use crate::borrow_check::nll::facts::AllFacts;
 use crate::borrow_check::nll::region_infer::values::LivenessValues;
+use crate::borrow_check::places_conflict;
 use rustc::infer::InferCtxt;
 use rustc::mir::visit::TyContext;
 use rustc::mir::visit::Visitor;
-use rustc::mir::{BasicBlock, BasicBlockData, Location, Body, Place, PlaceBase, Rvalue};
-use rustc::mir::{SourceInfo, Statement, Terminator};
-use rustc::mir::UserTypeProjection;
+use rustc::mir::{
+    BasicBlock, BasicBlockData, Body, Local, Location, Place, PlaceBase, Projection,
+    ProjectionElem, Rvalue, SourceInfo, Statement, StatementKind, Terminator, TerminatorKind,
+    UserTypeProjection,
+};
 use rustc::ty::fold::TypeFoldable;
 use rustc::ty::{self, ClosureSubsts, GeneratorSubsts, RegionVid, Ty};
 use rustc::ty::subst::SubstsRef;
@@ -27,6 +30,7 @@ pub(super) fn generate_constraints<'cx, 'tcx>(
         liveness_constraints,
         location_table,
         all_facts,
+        body,
     };
 
     for (bb, data) in body.basic_blocks().iter_enumerated() {
@@ -41,6 +45,7 @@ struct ConstraintGeneration<'cg, 'cx, 'tcx> {
     location_table: &'cg LocationTable,
     liveness_constraints: &'cg mut LivenessValues<RegionVid>,
     borrow_set: &'cg BorrowSet<'tcx>,
+    body: &'cg Body<'tcx>,
 }
 
 impl<'cg, 'cx, 'tcx> Visitor<'tcx> for ConstraintGeneration<'cg, 'cx, 'tcx> {
@@ -114,6 +119,17 @@ impl<'cg, 'cx, 'tcx> Visitor<'tcx> for ConstraintGeneration<'cg, 'cx, 'tcx> {
                 self.location_table
                     .start_index(location.successor_within_block()),
             ));
+
+            // If there are borrows on this now dead local, we need to record them as `killed`.
+            if let StatementKind::StorageDead(ref local) = statement.kind {
+                record_killed_borrows_for_local(
+                    all_facts,
+                    self.borrow_set,
+                    self.location_table,
+                    local,
+                    location,
+                );
+            }
         }
 
         self.super_statement(statement, location);
@@ -127,20 +143,7 @@ impl<'cg, 'cx, 'tcx> Visitor<'tcx> for ConstraintGeneration<'cg, 'cx, 'tcx> {
     ) {
         // When we see `X = ...`, then kill borrows of
         // `(*X).foo` and so forth.
-        if let Some(all_facts) = self.all_facts {
-            if let Place {
-                base: PlaceBase::Local(temp),
-                projection: None,
-            } = place {
-                if let Some(borrow_indices) = self.borrow_set.local_map.get(temp) {
-                    all_facts.killed.reserve(borrow_indices.len());
-                    for &borrow_index in borrow_indices {
-                        let location_index = self.location_table.mid_index(location);
-                        all_facts.killed.push((borrow_index, location_index));
-                    }
-                }
-            }
-        }
+        self.record_killed_borrows_for_place(place, location);
 
         self.super_assign(place, rvalue, location);
     }
@@ -167,6 +170,14 @@ impl<'cg, 'cx, 'tcx> Visitor<'tcx> for ConstraintGeneration<'cg, 'cx, 'tcx> {
             }
         }
 
+        // A `Call` terminator's return value can be a local which has borrows,
+        // so we need to record those as `killed` as well.
+        if let TerminatorKind::Call { ref destination, .. } = terminator.kind {
+            if let Some((place, _)) = destination {
+                self.record_killed_borrows_for_place(place, location);
+            }
+        }
+
         self.super_terminator(terminator, location);
     }
 
@@ -201,4 +212,96 @@ impl<'cx, 'cg, 'tcx> ConstraintGeneration<'cx, 'cg, 'tcx> {
                 self.liveness_constraints.add_element(vid, location);
             });
     }
+
+    /// When recording facts for Polonius, records the borrows on the specified place
+    /// as `killed`. For example, when assigning to a local, or on a call's return destination.
+    fn record_killed_borrows_for_place(&mut self, place: &Place<'tcx>, location: Location) {
+        if let Some(all_facts) = self.all_facts {
+            // Depending on the `Place` we're killing:
+            // - if it's a local, or a single deref of a local,
+            //   we kill all the borrows on the local.
+            // - if it's a deeper projection, we have to filter which
+            //   of the borrows are killed: the ones whose `borrowed_place`
+            //   conflicts with the `place`.
+            match place {
+                Place {
+                    base: PlaceBase::Local(local),
+                    projection: None,
+                } |
+                Place {
+                    base: PlaceBase::Local(local),
+                    projection: Some(box Projection {
+                        base: None,
+                        elem: ProjectionElem::Deref,
+                    }),
+                } => {
+                    debug!(
+                        "Recording `killed` facts for borrows of local={:?} at location={:?}",
+                        local, location
+                    );
+
+                    record_killed_borrows_for_local(
+                        all_facts,
+                        self.borrow_set,
+                        self.location_table,
+                        local,
+                        location,
+                    );
+                }
+
+                Place {
+                    base: PlaceBase::Static(_),
+                    ..
+                } => {
+                    // Ignore kills of static or static mut variables.
+                }
+
+                Place {
+                    base: PlaceBase::Local(local),
+                    projection: Some(_),
+                } => {
+                    // Kill conflicting borrows of the innermost local.
+                    debug!(
+                        "Recording `killed` facts for borrows of \
+                            innermost projected local={:?} at location={:?}",
+                        local, location
+                    );
+
+                    if let Some(borrow_indices) = self.borrow_set.local_map.get(local) {
+                        for &borrow_index in borrow_indices {
+                            let places_conflict = places_conflict::places_conflict(
+                                self.infcx.tcx,
+                                self.body,
+                                &self.borrow_set.borrows[borrow_index].borrowed_place,
+                                place,
+                                places_conflict::PlaceConflictBias::NoOverlap,
+                            );
+
+                            if places_conflict {
+                                let location_index = self.location_table.mid_index(location);
+                                all_facts.killed.push((borrow_index, location_index));
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
+/// When recording facts for Polonius, records the borrows on the specified local as `killed`.
+fn record_killed_borrows_for_local(
+    all_facts: &mut AllFacts,
+    borrow_set: &BorrowSet<'_>,
+    location_table: &LocationTable,
+    local: &Local,
+    location: Location,
+) {
+    if let Some(borrow_indices) = borrow_set.local_map.get(local) {
+        all_facts.killed.reserve(borrow_indices.len());
+        for &borrow_index in borrow_indices {
+            let location_index = location_table.mid_index(location);
+            all_facts.killed.push((borrow_index, location_index));
+        }
+    }
 }

src/test/ui/borrowck/borrowck-escaping-closure-error-2.polonius.stderr

@@ -0,0 +1,16 @@
+error[E0597]: `books` does not live long enough
+  --> $DIR/borrowck-escaping-closure-error-2.rs:11:17
+   |
+LL |     Box::new(|| books.push(4))
+   |     ------------^^^^^---------
+   |     |        |  |
+   |     |        |  borrowed value does not live long enough
+   |     |        value captured here
+   |     borrow later used here
+LL |
+LL | }
+   | - `books` dropped here while still borrowed
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.

src/test/ui/borrowck/borrowck-migrate-to-nll.edition.stderr

@@ -1,5 +1,5 @@
 warning[E0507]: cannot move out of `foo` in pattern guard
-  --> $DIR/borrowck-migrate-to-nll.rs:25:18
+  --> $DIR/borrowck-migrate-to-nll.rs:26:18
    |
 LL |                 (|| { let bar = foo; bar.take() })();
    |                  ^^             ---

src/test/ui/borrowck/borrowck-migrate-to-nll.rs

@@ -10,6 +10,7 @@
 // just ignore it instead:
 
 // ignore-compare-mode-nll
+// ignore-compare-mode-polonius
 
 // revisions: zflag edition
 //[zflag]compile-flags: -Z borrowck=migrate

src/test/ui/borrowck/borrowck-migrate-to-nll.zflag.stderr

@@ -1,5 +1,5 @@
 warning[E0507]: cannot move out of `foo` in pattern guard
-  --> $DIR/borrowck-migrate-to-nll.rs:25:18
+  --> $DIR/borrowck-migrate-to-nll.rs:26:18
    |
 LL |                 (|| { let bar = foo; bar.take() })();
    |                  ^^             ---

src/test/ui/borrowck/issue-45983.migrate.stderr

@@ -1,5 +1,5 @@
 error: borrowed data cannot be stored outside of its closure
-  --> $DIR/issue-45983.rs:19:27
+  --> $DIR/issue-45983.rs:20:27
    |
 LL |     let x = None;
    |         - borrowed data cannot be stored into here...

src/test/ui/borrowck/issue-45983.nll.stderr

@@ -1,5 +1,5 @@
 error[E0521]: borrowed data escapes outside of closure
-  --> $DIR/issue-45983.rs:19:18
+  --> $DIR/issue-45983.rs:20:18
    |
 LL |     let x = None;
    |         - `x` is declared here, outside of the closure body
@@ -9,7 +9,7 @@ LL |     give_any(|y| x = Some(y));
    |               `y` is a reference that is only valid in the closure body
 
 error[E0594]: cannot assign to `x`, as it is not declared as mutable
-  --> $DIR/issue-45983.rs:19:18
+  --> $DIR/issue-45983.rs:20:18
    |
 LL |     let x = None;
    |         - help: consider changing this to be mutable: `mut x`

src/test/ui/borrowck/issue-45983.rs

@@ -7,6 +7,7 @@
 // revisions, don't worry about the --compare-mode=nll on this test.
 
 // ignore-compare-mode-nll
+// ignore-compare-mode-polonius
 
 //[nll]compile-flags: -Z borrowck=mir
 

src/test/ui/borrowck/promote-ref-mut-in-let-issue-46557.polonius.stderr

@@ -0,0 +1,59 @@
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/promote-ref-mut-in-let-issue-46557.rs:5:21
+   |
+LL |     let ref mut x = 1234543;
+   |                     ^^^^^^^ creates a temporary which is freed while still in use
+LL |     x
+   |     - borrow later used here
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: consider using a `let` binding to create a longer lived value
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/promote-ref-mut-in-let-issue-46557.rs:10:25
+   |
+LL |     let (ref mut x, ) = (1234543, );
+   |                         ^^^^^^^^^^^ creates a temporary which is freed while still in use
+LL |     x
+   |     - borrow later used here
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: consider using a `let` binding to create a longer lived value
+
+error[E0515]: cannot return value referencing temporary value
+  --> $DIR/promote-ref-mut-in-let-issue-46557.rs:15:5
+   |
+LL |       match 1234543 {
+   |       ^     ------- temporary value created here
+   |  _____|
+   | |
+LL | |         ref mut x => x
+LL | |     }
+   | |_____^ returns a value referencing data owned by the current function
+
+error[E0515]: cannot return value referencing temporary value
+  --> $DIR/promote-ref-mut-in-let-issue-46557.rs:21:5
+   |
+LL |       match (123443,) {
+   |       ^     --------- temporary value created here
+   |  _____|
+   | |
+LL | |         (ref mut x,) => x,
+LL | |     }
+   | |_____^ returns a value referencing data owned by the current function
+
+error[E0515]: cannot return reference to temporary value
+  --> $DIR/promote-ref-mut-in-let-issue-46557.rs:27:5
+   |
+LL |     &mut 1234543
+   |     ^^^^^-------
+   |     |    |
+   |     |    temporary value created here
+   |     returns a reference to data owned by the current function
+
+error: aborting due to 5 previous errors
+
+Some errors have detailed explanations: E0515, E0716.
+For more information about an error, try `rustc --explain E0515`.

src/test/ui/borrowck/two-phase-reservation-sharing-interference-2.migrate2015.stderr

@@ -1,5 +1,5 @@
 error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
-  --> $DIR/two-phase-reservation-sharing-interference-2.rs:18:5
+  --> $DIR/two-phase-reservation-sharing-interference-2.rs:19:5
    |
 LL |     let shared = &v;
    |                  -- immutable borrow occurs here
@@ -11,7 +11,7 @@ LL |     v.extend(shared);
    |     mutable borrow occurs here
 
 error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
-  --> $DIR/two-phase-reservation-sharing-interference-2.rs:28:5
+  --> $DIR/two-phase-reservation-sharing-interference-2.rs:29:5
    |
 LL |     v.extend(&v);
    |     ^^------^--^
@@ -21,7 +21,7 @@ LL |     v.extend(&v);
    |     mutable borrow occurs here
 
 warning: cannot borrow `v` as mutable because it is also borrowed as immutable
-  --> $DIR/two-phase-reservation-sharing-interference-2.rs:39:5
+  --> $DIR/two-phase-reservation-sharing-interference-2.rs:40:5
    |
 LL |     let shared = &v;
    |                  -- immutable borrow occurs here

src/test/ui/borrowck/two-phase-reservation-sharing-interference-2.migrate2018.stderr

@@ -1,5 +1,5 @@
 error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
-  --> $DIR/two-phase-reservation-sharing-interference-2.rs:18:5
+  --> $DIR/two-phase-reservation-sharing-interference-2.rs:19:5
    |
 LL |     let shared = &v;
    |                  -- immutable borrow occurs here
@@ -11,7 +11,7 @@ LL |     v.extend(shared);
    |     mutable borrow occurs here
 
 error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
-  --> $DIR/two-phase-reservation-sharing-interference-2.rs:28:5
+  --> $DIR/two-phase-reservation-sharing-interference-2.rs:29:5
    |
 LL |     v.extend(&v);
    |     ^^------^--^
@@ -21,7 +21,7 @@ LL |     v.extend(&v);
    |     mutable borrow occurs here
 
 warning: cannot borrow `v` as mutable because it is also borrowed as immutable
-  --> $DIR/two-phase-reservation-sharing-interference-2.rs:39:5
+  --> $DIR/two-phase-reservation-sharing-interference-2.rs:40:5
    |
 LL |     let shared = &v;
    |                  -- immutable borrow occurs here

src/test/ui/borrowck/two-phase-reservation-sharing-interference-2.nll2015.stderr

@@ -1,5 +1,5 @@
 error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
-  --> $DIR/two-phase-reservation-sharing-interference-2.rs:18:5
+  --> $DIR/two-phase-reservation-sharing-interference-2.rs:19:5
    |
 LL |     let shared = &v;
    |                  -- immutable borrow occurs here
@@ -10,7 +10,7 @@ LL |     v.extend(shared);
    |     mutable borrow occurs here
 
 error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
-  --> $DIR/two-phase-reservation-sharing-interference-2.rs:28:5
+  --> $DIR/two-phase-reservation-sharing-interference-2.rs:29:5
    |
 LL |     v.extend(&v);
    |     ^^------^--^
@@ -20,7 +20,7 @@ LL |     v.extend(&v);
    |     mutable borrow occurs here
 
 error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
-  --> $DIR/two-phase-reservation-sharing-interference-2.rs:39:5
+  --> $DIR/two-phase-reservation-sharing-interference-2.rs:40:5
    |
 LL |     let shared = &v;
    |                  -- immutable borrow occurs here