coverage: Check for some other edge cases that might confuse llvm-cov

Author: Zalathar

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -294,6 +294,26 @@ fn make_code_region(
     let end_line = end_line as u32;
     let end_col = end_col as u32;
 
+    // Line/column coordinates are supposed to be 1-based. If we ever emit
+    // coordinates of 0, `llvm-cov` might misinterpret them.
+    let all_nonzero = [start_line, start_col, end_line, end_col].iter().all(|&n| n != 0);
+    debug_assert!(all_nonzero, "{start_line}:{start_col} to {end_line}:{end_col}");
+    if !all_nonzero {
+        debug!(
+            "Skipping region with zero coordinates: {start_line}:{start_col} to {end_line}:{end_col}"
+        );
+        return None;
+    }
+
+    // Coverage mappings use the high bit of `end_col` to indicate that a
+    // region is actually a "gap" region, so make sure it's unset.
+    let end_col_has_high_bit_unset = (end_col & (1 << 31)) == 0;
+    debug_assert!(end_col_has_high_bit_unset, "{end_col}");
+    if !end_col_has_high_bit_unset {
+        debug!("Skipping region with high bit set in end column: {end_col}");
+        return None;
+    }
+
     // If we ever emit a region that is improperly ordered (end < start),
     // `llvm-cov` will fail with `coveragemap_error::malformed`, which is
     // inconvenient for users and also very hard to debug.