Rollup merge of #153759 - Zalathar:chunk-tests, r=wesleywiser

Add a suite of ChunkedBitSet union/subtract/intersect test scenarios

While working on https://github.com/rust-lang/rust/pull/153754, I found that these code paths are under-tested.

compiler/rustc_index/src/bit_set/tests.rs

@@ -362,6 +362,116 @@ fn chunked_bitset() {
     );
 }
 
+/// Additional helper methods for testing.
+impl ChunkedBitSet<usize> {
+    /// Creates a new `ChunkedBitSet` containing all `i` for which `fill_fn(i)` is true.
+    fn fill_with(domain_size: usize, fill_fn: impl Fn(usize) -> bool) -> Self {
+        let mut this = ChunkedBitSet::new_empty(domain_size);
+        for i in 0..domain_size {
+            if fill_fn(i) {
+                this.insert(i);
+            }
+        }
+        this
+    }
+
+    /// Asserts that for each `i` in `0..self.domain_size()`, `self.contains(i) == expected_fn(i)`.
+    #[track_caller]
+    fn assert_filled_with(&self, expected_fn: impl Fn(usize) -> bool) {
+        for i in 0..self.domain_size() {
+            let expected = expected_fn(i);
+            assert_eq!(self.contains(i), expected, "i = {i}");
+        }
+    }
+}
+
+#[test]
+fn chunked_bulk_ops() {
+    struct ChunkedBulkOp {
+        name: &'static str,
+        op_fn: fn(&mut ChunkedBitSet<usize>, &ChunkedBitSet<usize>) -> bool,
+        spec_fn: fn(fn(usize) -> bool, fn(usize) -> bool, usize) -> bool,
+    }
+    let ops = &[
+        ChunkedBulkOp {
+            name: "union",
+            op_fn: ChunkedBitSet::union,
+            spec_fn: |fizz, buzz, i| fizz(i) || buzz(i),
+        },
+        ChunkedBulkOp {
+            name: "subtract",
+            op_fn: ChunkedBitSet::subtract,
+            spec_fn: |fizz, buzz, i| fizz(i) && !buzz(i),
+        },
+        ChunkedBulkOp {
+            name: "intersect",
+            op_fn: ChunkedBitSet::intersect,
+            spec_fn: |fizz, buzz, i| fizz(i) && buzz(i),
+        },
+    ];
+
+    let domain_sizes = [
+        CHUNK_BITS / 7, // Smaller than a full chunk.
+        CHUNK_BITS,
+        (CHUNK_BITS + CHUNK_BITS / 7), // Larger than a full chunk.
+    ];
+
+    for ChunkedBulkOp { name, op_fn, spec_fn } in ops {
+        for domain_size in domain_sizes {
+            // If false, use different values for LHS and RHS, to test "fizz op buzz".
+            // If true, use identical values, to test "fizz op fizz".
+            for identical in [false, true] {
+                // If false, make a clone of LHS before doing the op.
+                // This covers optimizations that depend on whether chunk words are shared or not.
+                for unique in [false, true] {
+                    // Print the current test case, so that we can see which one failed.
+                    println!(
+                        "Testing op={name}, domain_size={domain_size}, identical={identical}, unique={unique} ..."
+                    );
+
+                    let fizz_fn = |i| i % 3 == 0;
+                    let buzz_fn = if identical { fizz_fn } else { |i| i % 5 == 0 };
+
+                    // Check that `fizz op buzz` gives the expected results.
+                    chunked_bulk_ops_test_inner(
+                        domain_size,
+                        unique,
+                        fizz_fn,
+                        buzz_fn,
+                        op_fn,
+                        |i| spec_fn(fizz_fn, buzz_fn, i),
+                    );
+                }
+            }
+        }
+    }
+}
+
+fn chunked_bulk_ops_test_inner(
+    domain_size: usize,
+    unique: bool,
+    fizz_fn: impl Fn(usize) -> bool + Copy,
+    buzz_fn: impl Fn(usize) -> bool + Copy,
+    op_fn: impl Fn(&mut ChunkedBitSet<usize>, &ChunkedBitSet<usize>) -> bool,
+    expected_fn: impl Fn(usize) -> bool + Copy,
+) {
+    // Create two bitsets, "fizz" (LHS) and "buzz" (RHS).
+    let mut fizz = ChunkedBitSet::fill_with(domain_size, fizz_fn);
+    let buzz = ChunkedBitSet::fill_with(domain_size, buzz_fn);
+
+    // If requested, clone `fizz` so that its word Rcs are not uniquely-owned.
+    let _cloned = (!unique).then(|| fizz.clone());
+
+    // Perform the op (e.g. union/subtract/intersect), and verify that the
+    // mutated LHS contains exactly the expected values.
+    let changed = op_fn(&mut fizz, &buzz);
+    fizz.assert_filled_with(expected_fn);
+
+    // Verify that the "changed" return value is correct.
+    let should_change = (0..domain_size).any(|i| fizz_fn(i) != expected_fn(i));
+    assert_eq!(changed, should_change);
+}
+
 fn with_elements_chunked(elements: &[usize], domain_size: usize) -> ChunkedBitSet<usize> {
     let mut s = ChunkedBitSet::new_empty(domain_size);
     for &e in elements {