Exhaustiveness: track overlapping ranges precisely

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -712,6 +712,7 @@
 //! I (Nadrieril) prefer to put new tests in `ui/pattern/usefulness` unless there's a specific
 //! reason not to, for example if they crucially depend on a particular feature like `or_patterns`.
 
+use rustc_index::bit_set::BitSet;
 use smallvec::{smallvec, SmallVec};
 use std::fmt;
 
@@ -911,6 +912,11 @@ struct MatrixRow<'p, Cx: TypeCx> {
     /// [`compute_exhaustiveness_and_usefulness`] if the arm is found to be useful.
     /// This is reset to `false` when specializing.
     useful: bool,
+    /// Tracks which rows above this one have an intersection with this one, i.e. such that there is
+    /// a value that matches both rows.
+    /// Note: Because of relevancy we may miss some intersections. The intersections we do find are
+    /// correct.
+    intersects: BitSet<usize>,
 }
 
 impl<'p, Cx: TypeCx> MatrixRow<'p, Cx> {
@@ -938,6 +944,7 @@ impl<'p, Cx: TypeCx> MatrixRow<'p, Cx> {
             parent_row: self.parent_row,
             is_under_guard: self.is_under_guard,
             useful: false,
+            intersects: BitSet::new_empty(0), // Initialized in `Matrix::expand_and_push`.
         })
     }
 
@@ -955,6 +962,7 @@ impl<'p, Cx: TypeCx> MatrixRow<'p, Cx> {
             parent_row,
             is_under_guard: self.is_under_guard,
             useful: false,
+            intersects: BitSet::new_empty(0), // Initialized in `Matrix::expand_and_push`.
         }
     }
 }
@@ -993,13 +1001,15 @@ struct Matrix<'p, Cx: TypeCx> {
 impl<'p, Cx: TypeCx> Matrix<'p, Cx> {
     /// Pushes a new row to the matrix. If the row starts with an or-pattern, this recursively
     /// expands it. Internal method, prefer [`Matrix::new`].
-    fn expand_and_push(&mut self, row: MatrixRow<'p, Cx>) {
+    fn expand_and_push(&mut self, mut row: MatrixRow<'p, Cx>) {
         if !row.is_empty() && row.head().is_or_pat() {
             // Expand nested or-patterns.
-            for new_row in row.expand_or_pat() {
+            for mut new_row in row.expand_or_pat() {
+                new_row.intersects = BitSet::new_empty(self.rows.len());
                 self.rows.push(new_row);
             }
         } else {
+            row.intersects = BitSet::new_empty(self.rows.len());
             self.rows.push(row);
         }
     }
@@ -1019,9 +1029,10 @@ impl<'p, Cx: TypeCx> Matrix<'p, Cx> {
         for (row_id, arm) in arms.iter().enumerate() {
             let v = MatrixRow {
                 pats: PatStack::from_pattern(arm.pat),
-                parent_row: row_id, // dummy, we won't read it
+                parent_row: row_id, // dummy, we don't read it
                 is_under_guard: arm.has_guard,
                 useful: false,
+                intersects: BitSet::new_empty(0), // Initialized in `Matrix::expand_and_push`.
             };
             matrix.expand_and_push(v);
         }
@@ -1349,21 +1360,19 @@ fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>(
     let Some(ty) = matrix.head_ty() else {
         // The base case: there are no columns in the matrix. We are morally pattern-matching on ().
         // A row is useful iff it has no (unguarded) rows above it.
-        for row in matrix.rows_mut() {
-            // All rows are useful until they're not.
-            row.useful = true;
-            // When there's an unguarded row, the match is exhaustive and any subsequent row is not
-            // useful.
-            if !row.is_under_guard {
-                return Ok(WitnessMatrix::empty());
-            }
+        let mut useful = true; // Whether the next row is useful.
+        for (i, row) in matrix.rows_mut().enumerate() {
+            row.useful = useful;
+            row.intersects.insert_range(0..i);
+            // The next rows stays useful if this one is under a guard.
+            useful &= row.is_under_guard;
         }
-        // No (unguarded) rows, so the match is not exhaustive. We return a new witness unless
-        // irrelevant.
-        return if matrix.wildcard_row_is_relevant {
+        return if useful && matrix.wildcard_row_is_relevant {
+            // The wildcard row is useful; the match is non-exhaustive.
             Ok(WitnessMatrix::unit_witness())
         } else {
-            // We choose to not report anything here; see at the top for details.
+            // Either the match is exhaustive, or we choose not to report anything because of
+            // relevancy. See at the top for details.
             Ok(WitnessMatrix::empty())
         };
     };
@@ -1424,10 +1433,19 @@ fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>(
         // Accumulate the found witnesses.
         ret.extend(witnesses);
 
-        // A parent row is useful if any of its children is.
         for child_row in spec_matrix.rows() {
-            let parent_row = &mut matrix.rows[child_row.parent_row];
-            parent_row.useful = parent_row.useful || child_row.useful;
+            let parent_row_id = child_row.parent_row;
+            let parent_row = &mut matrix.rows[parent_row_id];
+            // A parent row is useful if any of its children is.
+            parent_row.useful |= child_row.useful;
+            for child_intersection in child_row.intersects.iter() {
+                // Convert the intersecting ids into ids for the parent matrix.
+                let parent_intersection = spec_matrix.rows[child_intersection].parent_row;
+                // Note: self-intersection can happen with or-patterns.
+                if parent_intersection != parent_row_id {
+                    parent_row.intersects.insert(parent_intersection);
+                }
+            }
         }
     }