Auto merge of #88698 - Noble-Mushtak:master, r=nikomatsakis,oli-obk

Add check that live_region is live in sanitize_promoted

This pull request fixes #88434 by adding a check in `sanitize_promoted` to ensure that only regions which are actually live are added to the `liveness_constraints` of the `BorrowCheckContext`.

To implement this change, I needed to add a method to `LivenessValues` which gets the elements contained by a region:

    /// Returns an iterator of all the elements contained by the region `r`
    crate fn get_elements(&self, row: N) -> impl Iterator<Item = Location> + '_

Then, inside `sanitize_promoted`, we check whether the iterator returned by this method is non-empty to ensure that the region is actually live at at least one location before adding that region to the `liveness_constraints` of the `BorrowCheckContext`.

This is my first pull request to the Rust repo, so any feedback on how I can improve this pull request or if there is a better way to fix this issue would be very appreciated.

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -2154,7 +2154,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         // appears to be the most interesting point to report to the
         // user via an even more ad-hoc guess.
         categorized_path.sort_by(|p0, p1| p0.category.cmp(&p1.category));
-        debug!("`: sorted_path={:#?}", categorized_path);
+        debug!("best_blame_constraint: sorted_path={:#?}", categorized_path);
 
         categorized_path.remove(0)
     }

compiler/rustc_borrowck/src/region_infer/values.rs

@@ -174,17 +174,19 @@ impl<N: Idx> LivenessValues<N> {
         self.points.contains(row, index)
     }
 
+    /// Returns an iterator of all the elements contained by the region `r`
+    crate fn get_elements(&self, row: N) -> impl Iterator<Item = Location> + '_ {
+        self.points
+            .row(row)
+            .into_iter()
+            .flat_map(|set| set.iter())
+            .take_while(move |&p| self.elements.point_in_range(p))
+            .map(move |p| self.elements.to_location(p))
+    }
+
     /// Returns a "pretty" string value of the region. Meant for debugging.
     crate fn region_value_str(&self, r: N) -> String {
-        region_value_str(
-            self.points
-                .row(r)
-                .into_iter()
-                .flat_map(|set| set.iter())
-                .take_while(|&p| self.elements.point_in_range(p))
-                .map(|p| self.elements.to_location(p))
-                .map(RegionElement::Location),
-        )
+        region_value_str(self.get_elements(r).map(RegionElement::Location))
     }
 }
 

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -663,12 +663,17 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
             }
             self.cx.borrowck_context.constraints.outlives_constraints.push(constraint)
         }
-        for live_region in liveness_constraints.rows() {
-            self.cx
-                .borrowck_context
-                .constraints
-                .liveness_constraints
-                .add_element(live_region, location);
+        for region in liveness_constraints.rows() {
+            // If the region is live at at least one location in the promoted MIR,
+            // then add a liveness constraint to the main MIR for this region
+            // at the location provided as an argument to this method
+            if let Some(_) = liveness_constraints.get_elements(region).next() {
+                self.cx
+                    .borrowck_context
+                    .constraints
+                    .liveness_constraints
+                    .add_element(region, location);
+            }
         }
 
         if !closure_bounds.is_empty() {

src/test/ui/borrowck/issue-88434-minimal-example.rs

@@ -0,0 +1,13 @@
+#![feature(const_fn_trait_bound)]
+// Regression test related to issue 88434
+
+const _CONST: &() = &f(&|_| {});
+
+const fn f<F>(_: &F)
+where
+    F: FnMut(&u8),
+{
+    panic!() //~ ERROR evaluation of constant value failed
+}
+
+fn main() { }

src/test/ui/borrowck/issue-88434-minimal-example.stderr

@@ -0,0 +1,17 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/issue-88434-minimal-example.rs:10:5
+   |
+LL | const _CONST: &() = &f(&|_| {});
+   |                      ---------- inside `_CONST` at $DIR/issue-88434-minimal-example.rs:4:22
+...
+LL |     panic!()
+   |     ^^^^^^^^
+   |     |
+   |     the evaluated program panicked at 'explicit panic', $DIR/issue-88434-minimal-example.rs:10:5
+   |     inside `f::<[closure@$DIR/issue-88434-minimal-example.rs:4:25: 4:31]>` at $SRC_DIR/std/src/panic.rs:LL:COL
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` (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 E0080`.

src/test/ui/borrowck/issue-88434-removal-index-should-be-less.rs

@@ -0,0 +1,13 @@
+#![feature(const_fn_trait_bound)]
+// Regression test for issue 88434
+
+const _CONST: &[u8] = &f(&[], |_| {});
+
+const fn f<F>(_: &[u8], _: F) -> &[u8]
+where
+    F: FnMut(&u8),
+{
+    panic!() //~ ERROR evaluation of constant value failed
+}
+
+fn main() { }

src/test/ui/borrowck/issue-88434-removal-index-should-be-less.stderr

@@ -0,0 +1,17 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/issue-88434-removal-index-should-be-less.rs:10:5
+   |
+LL | const _CONST: &[u8] = &f(&[], |_| {});
+   |                        -------------- inside `_CONST` at $DIR/issue-88434-removal-index-should-be-less.rs:4:24
+...
+LL |     panic!()
+   |     ^^^^^^^^
+   |     |
+   |     the evaluated program panicked at 'explicit panic', $DIR/issue-88434-removal-index-should-be-less.rs:10:5
+   |     inside `f::<[closure@$DIR/issue-88434-removal-index-should-be-less.rs:4:31: 4:37]>` at $SRC_DIR/std/src/panic.rs:LL:COL
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` (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 E0080`.