Added GapRegions to support non-coverage counters

Author: Rich Kadel

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs

@@ -127,14 +127,32 @@ impl CoverageMapGenerator {
                 virtual_file_mapping.push(filenames_index as u32);
             }
             debug!("Adding counter {:?} to map for {:?}", counter, region,);
-            mapping_regions.push(CounterMappingRegion::code_region(
-                counter,
-                current_file_id,
-                start_line,
-                start_col,
-                end_line,
-                end_col,
-            ));
+            if start_line == end_line && start_col == end_col {
+                // The MIR `InstrumentCoverage` pass generates empty spans _ONLY_ for `BasicBlocks`
+                // that contribute to the counts of `CoverageKind::Expression`s, but don't represent
+                // specific source code to count.
+                //
+                // Make empty spans `GapRegion`s so their region executions counts are still
+                // available to any expressions that might reference them, but they don't affect
+                // the line execution count (as long as the line has at least one other counter).
+                mapping_regions.push(CounterMappingRegion::gap_region(
+                    counter,
+                    current_file_id,
+                    start_line,
+                    start_col,
+                    end_line,
+                    end_col,
+                ));
+            } else {
+                mapping_regions.push(CounterMappingRegion::code_region(
+                    counter,
+                    current_file_id,
+                    start_line,
+                    start_col,
+                    end_line,
+                    end_col,
+                ));
+            }
         }
 
         // Encode and append the current function's coverage mapping data