Add a couple polonius tests and move universal region liveness generation

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -347,7 +347,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             outlives_constraints,
             scc_annotations,
             type_tests,
-            mut liveness_constraints,
+            liveness_constraints,
             universe_causes,
             placeholder_indices,
         } = lowered_constraints;
@@ -376,9 +376,6 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             match definition.origin {
                 // For each free, universally quantified region X:
                 NllRegionVariableOrigin::FreeRegion => {
-                    // Add all nodes in the CFG to liveness constraints
-                    liveness_constraints.add_all_points(region);
-
                     // Add `end(X)` into the set for X.
                     scc_values.add_free_region(scc, region);
                 }
@@ -396,8 +393,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             // has them, setting `scc_values[scc(region)] |= liveness_constraints[region]`.
             //
             // These values will later be propagated during [`Self::propagate_constraints()`].
-            // The values include any live-at-all-points constraints added above
-            // for free regions.
+            // The values include any live-at-all-points constraints added previously in `liveness::generate`.
             if let Some(liveness) = liveness_constraints.point_liveness(region) {
                 scc_values.merge_liveness(scc, liveness)
             }

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

@@ -34,6 +34,11 @@ pub(super) fn generate<'tcx>(
     debug!("liveness::generate");
     let _timer = typeck.tcx().prof.generic_activity("borrowck_liveness");
 
+    // Universal regions are live at every point.
+    for region in typeck.universal_regions.universal_regions_iter() {
+        typeck.constraints.liveness_constraints.add_all_points(region);
+    }
+
     let mut free_regions = regions_that_outlive_free_regions(
         typeck.infcx.num_region_vars(),
         &typeck.universal_regions,

tests/ui/nll/polonius/boring-local-drop-liveness.nll.stderr

@@ -0,0 +1,32 @@
+error[E0506]: cannot assign to `*x` because it is borrowed
+  --> $DIR/boring-local-drop-liveness.rs:28:5
+   |
+LL | fn assigning_under_a_live_destructor<'a>(
+   |                                      -- lifetime `'a` defined here
+...
+LL |     *slot = Some(d.0);
+   |     ----------------- assignment requires that `*x` is borrowed for `'a`
+LL |     d = D(&*x);
+   |           --- `*x` is borrowed here
+LL |     *x = String::new();
+   |     ^^ `*x` is assigned to here but it was already borrowed
+
+error[E0597]: `local` does not live long enough
+  --> $DIR/boring-local-drop-liveness.rs:35:11
+   |
+LL | fn dropping_under_a_live_destructor<'a>(x: &'a String, slot: &mut Option<&'a String>) {
+   |                                     -- lifetime `'a` defined here
+LL |     let mut d = D(x);
+LL |     *slot = Some(d.0);
+   |     ----------------- assignment requires that `local` is borrowed for `'a`
+LL |     let local = String::from("gone");
+   |         ----- binding `local` declared here
+LL |     d = D(&local);
+   |           ^^^^^^ borrowed value does not live long enough
+LL | }
+   | - `local` dropped here while still borrowed
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0506, E0597.
+For more information about an error, try `rustc --explain E0506`.

tests/ui/nll/polonius/boring-local-drop-liveness.polonius_next.stderr

@@ -0,0 +1,32 @@
+error[E0506]: cannot assign to `*x` because it is borrowed
+  --> $DIR/boring-local-drop-liveness.rs:28:5
+   |
+LL | fn assigning_under_a_live_destructor<'a>(
+   |                                      -- lifetime `'a` defined here
+...
+LL |     *slot = Some(d.0);
+   |     ----------------- assignment requires that `*x` is borrowed for `'a`
+LL |     d = D(&*x);
+   |           --- `*x` is borrowed here
+LL |     *x = String::new();
+   |     ^^ `*x` is assigned to here but it was already borrowed
+
+error[E0597]: `local` does not live long enough
+  --> $DIR/boring-local-drop-liveness.rs:35:11
+   |
+LL | fn dropping_under_a_live_destructor<'a>(x: &'a String, slot: &mut Option<&'a String>) {
+   |                                     -- lifetime `'a` defined here
+LL |     let mut d = D(x);
+LL |     *slot = Some(d.0);
+   |     ----------------- assignment requires that `local` is borrowed for `'a`
+LL |     let local = String::from("gone");
+   |         ----- binding `local` declared here
+LL |     d = D(&local);
+   |           ^^^^^^ borrowed value does not live long enough
+LL | }
+   | - `local` dropped here while still borrowed
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0506, E0597.
+For more information about an error, try `rustc --explain E0506`.

tests/ui/nll/polonius/boring-local-drop-liveness.rs

@@ -0,0 +1,38 @@
+// Polonius requires liveness for some locals that NLL leaves "boring": locals
+// containing some region that outlives a universal region but is not universal itself.
+//
+// This test checks that we correctly compute liveness for NLL-boring locals that
+// are live only because of their drop.
+
+//@ ignore-compare-mode-polonius (explicit revisions)
+//@ revisions: nll polonius_next
+//@ [nll] compile-flags: -Zpolonius=off
+//@ [polonius_next] compile-flags: -Zpolonius=next
+
+struct D<'a>(&'a String);
+
+impl<'a> Drop for D<'a> {
+    fn drop(&mut self) {
+        println!("{}", self.0);
+    }
+}
+
+fn assigning_under_a_live_destructor<'a>(
+    x: &'a mut String,
+    slot: &mut Option<&'a String>,
+    y: &'a String,
+) {
+    let mut d = D(y);
+    *slot = Some(d.0);
+    d = D(&*x);
+    *x = String::new(); //~ ERROR cannot assign to `*x` because it is borrowed
+}
+
+fn dropping_under_a_live_destructor<'a>(x: &'a String, slot: &mut Option<&'a String>) {
+    let mut d = D(x);
+    *slot = Some(d.0);
+    let local = String::from("gone");
+    d = D(&local); //~ ERROR `local` does not live long enough
+}
+
+fn main() {}

tests/ui/nll/polonius/boring-local-liveness.nll.stderr

@@ -0,0 +1,30 @@
+error[E0506]: cannot assign to `*x` because it is borrowed
+  --> $DIR/boring-local-liveness.rs:22:5
+   |
+LL | fn assigning_into_a_slot<'a>(x: &'a mut u32, slot: &mut Option<D<'a>>) {
+   |                          -- lifetime `'a` defined here
+LL |     let r: &'a u32 = &*x;
+   |            -------   --- `*x` is borrowed here
+   |            |
+   |            type annotation requires that `*x` is borrowed for `'a`
+LL |     *slot = Some(D(r));
+LL |     *x = 1;
+   |     ^^^^^^ `*x` is assigned to here but it was already borrowed
+
+error[E0506]: cannot assign to `*x` because it is borrowed
+  --> $DIR/boring-local-liveness.rs:32:5
+   |
+LL | fn clearing_the_slot_does_not_release_it<'a>(x: &'a mut u32, slot: &mut Option<D<'a>>) {
+   |                                          -- lifetime `'a` defined here
+LL |     {
+LL |         let r: &'a u32 = &*x;
+   |                -------   --- `*x` is borrowed here
+   |                |
+   |                type annotation requires that `*x` is borrowed for `'a`
+...
+LL |     *x = 1;
+   |     ^^^^^^ `*x` is assigned to here but it was already borrowed
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0506`.

tests/ui/nll/polonius/boring-local-liveness.polonius_next.stderr

@@ -0,0 +1,30 @@
+error[E0506]: cannot assign to `*x` because it is borrowed
+  --> $DIR/boring-local-liveness.rs:22:5
+   |
+LL | fn assigning_into_a_slot<'a>(x: &'a mut u32, slot: &mut Option<D<'a>>) {
+   |                          -- lifetime `'a` defined here
+LL |     let r: &'a u32 = &*x;
+   |            -------   --- `*x` is borrowed here
+   |            |
+   |            type annotation requires that `*x` is borrowed for `'a`
+LL |     *slot = Some(D(r));
+LL |     *x = 1;
+   |     ^^^^^^ `*x` is assigned to here but it was already borrowed
+
+error[E0506]: cannot assign to `*x` because it is borrowed
+  --> $DIR/boring-local-liveness.rs:32:5
+   |
+LL | fn clearing_the_slot_does_not_release_it<'a>(x: &'a mut u32, slot: &mut Option<D<'a>>) {
+   |                                          -- lifetime `'a` defined here
+LL |     {
+LL |         let r: &'a u32 = &*x;
+   |                -------   --- `*x` is borrowed here
+   |                |
+   |                type annotation requires that `*x` is borrowed for `'a`
+...
+LL |     *x = 1;
+   |     ^^^^^^ `*x` is assigned to here but it was already borrowed
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0506`.

tests/ui/nll/polonius/boring-local-liveness.rs

@@ -0,0 +1,35 @@
+// Polonius requires liveness for some locals that NLL leaves "boring": locals
+// containing some region that outlives a universal region but is not universal itself.
+//
+// This test checks that we correctly compute liveness for NLL-boring locals that
+// are live because of a use.
+
+//@ ignore-compare-mode-polonius (explicit revisions)
+//@ revisions: nll polonius_next
+//@ [nll] compile-flags: -Zpolonius=off
+//@ [polonius_next] compile-flags: -Zpolonius=next
+
+struct D<'a>(&'a u32);
+
+impl<'a> Drop for D<'a> {
+    fn drop(&mut self) {}
+}
+
+// The loan of `*x` flows into `slot`'s region, which outlives `'a` and so is boring to NLLs.
+fn assigning_into_a_slot<'a>(x: &'a mut u32, slot: &mut Option<D<'a>>) {
+    let r: &'a u32 = &*x;
+    *slot = Some(D(r));
+    *x = 1; //~ ERROR cannot assign to `*x` because it is borrowed
+}
+
+// The same, with the borrow confined to an inner scope and the slot cleared afterwards.
+fn clearing_the_slot_does_not_release_it<'a>(x: &'a mut u32, slot: &mut Option<D<'a>>) {
+    {
+        let r: &'a u32 = &*x;
+        *slot = Some(D(r));
+    }
+    *slot = None;
+    *x = 1; //~ ERROR cannot assign to `*x` because it is borrowed
+}
+
+fn main() {}