coverage: Eagerly deduplicate covspans with the same span

Author: Zalathar

compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs

@@ -52,14 +52,19 @@ pub(super) fn mir_to_initial_sorted_coverage_spans(
             // - Span A extends further left, or
             // - Both have the same start and span A extends further right
             .then_with(|| Ord::cmp(&a.span.hi(), &b.span.hi()).reverse())
-            // If both spans are equal, sort the BCBs in dominator order,
-            // so that dominating BCBs come before other BCBs they dominate.
-            .then_with(|| basic_coverage_blocks.cmp_in_dominator_order(a.bcb, b.bcb))
-            // If two spans are otherwise identical, put closure spans first,
-            // as this seems to be what the refinement step expects.
+            // If two spans have the same lo & hi, put closure spans first,
+            // as they take precedence over non-closure spans.
             .then_with(|| Ord::cmp(&a.is_closure, &b.is_closure).reverse())
+            // After deduplication, we want to keep only the most-dominated BCB.
+            .then_with(|| basic_coverage_blocks.cmp_in_dominator_order(a.bcb, b.bcb).reverse())
     });
 
+    // Among covspans with the same span, keep only one. Closure spans take
+    // precedence, otherwise keep the one with the most-dominated BCB.
+    // (Ideally we should try to preserve _all_ non-dominating BCBs, but that
+    // requires a lot more complexity in the span refiner, for little benefit.)
+    initial_spans.dedup_by(|b, a| a.span.source_equal(b.span));
+
     initial_spans
 }
 

tests/coverage/closure_macro.cov-map

@@ -7,18 +7,17 @@ Number of file 0 mappings: 1
 - Code(Counter(0)) at (prev + 29, 1) to (start + 2, 2)
 
 Function name: closure_macro::main
-Raw bytes (43): 0x[01, 01, 02, 01, 05, 05, 02, 07, 01, 21, 01, 01, 21, 02, 02, 09, 00, 0f, 05, 00, 12, 00, 13, 02, 00, 12, 00, 13, 05, 00, 54, 00, 55, 02, 02, 09, 02, 0b, 07, 03, 01, 00, 02]
+Raw bytes (38): 0x[01, 01, 02, 01, 05, 05, 02, 06, 01, 21, 01, 01, 21, 02, 02, 09, 00, 12, 02, 00, 0f, 00, 54, 05, 00, 54, 00, 55, 02, 02, 09, 02, 0b, 07, 03, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 2
 - expression 0 operands: lhs = Counter(0), rhs = Counter(1)
 - expression 1 operands: lhs = Counter(1), rhs = Expression(0, Sub)
-Number of file 0 mappings: 7
+Number of file 0 mappings: 6
 - Code(Counter(0)) at (prev + 33, 1) to (start + 1, 33)
-- Code(Expression(0, Sub)) at (prev + 2, 9) to (start + 0, 15)
+- Code(Expression(0, Sub)) at (prev + 2, 9) to (start + 0, 18)
     = (c0 - c1)
-- Code(Counter(1)) at (prev + 0, 18) to (start + 0, 19)
-- Code(Expression(0, Sub)) at (prev + 0, 18) to (start + 0, 19)
+- Code(Expression(0, Sub)) at (prev + 0, 15) to (start + 0, 84)
     = (c0 - c1)
 - Code(Counter(1)) at (prev + 0, 84) to (start + 0, 85)
 - Code(Expression(0, Sub)) at (prev + 2, 9) to (start + 2, 11)

tests/coverage/closure_macro_async.cov-map

@@ -15,18 +15,17 @@ Number of file 0 mappings: 1
 - Code(Counter(0)) at (prev + 35, 1) to (start + 0, 43)
 
 Function name: closure_macro_async::test::{closure#0}
-Raw bytes (43): 0x[01, 01, 02, 01, 05, 05, 02, 07, 01, 23, 2b, 01, 21, 02, 02, 09, 00, 0f, 05, 00, 12, 00, 13, 02, 00, 12, 00, 13, 05, 00, 54, 00, 55, 02, 02, 09, 02, 0b, 07, 03, 01, 00, 02]
+Raw bytes (38): 0x[01, 01, 02, 01, 05, 05, 02, 06, 01, 23, 2b, 01, 21, 02, 02, 09, 00, 12, 02, 00, 0f, 00, 54, 05, 00, 54, 00, 55, 02, 02, 09, 02, 0b, 07, 03, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 2
 - expression 0 operands: lhs = Counter(0), rhs = Counter(1)
 - expression 1 operands: lhs = Counter(1), rhs = Expression(0, Sub)
-Number of file 0 mappings: 7
+Number of file 0 mappings: 6
 - Code(Counter(0)) at (prev + 35, 43) to (start + 1, 33)
-- Code(Expression(0, Sub)) at (prev + 2, 9) to (start + 0, 15)
+- Code(Expression(0, Sub)) at (prev + 2, 9) to (start + 0, 18)
     = (c0 - c1)
-- Code(Counter(1)) at (prev + 0, 18) to (start + 0, 19)
-- Code(Expression(0, Sub)) at (prev + 0, 18) to (start + 0, 19)
+- Code(Expression(0, Sub)) at (prev + 0, 15) to (start + 0, 84)
     = (c0 - c1)
 - Code(Counter(1)) at (prev + 0, 84) to (start + 0, 85)
 - Code(Expression(0, Sub)) at (prev + 2, 9) to (start + 2, 11)