coverage: Simplify grouping of mappings by file

Author: Zalathar

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs

@@ -174,36 +174,40 @@ fn write_coverage_mapping<'a>(
 
     let mut virtual_file_mapping = Vec::new();
     let mut mapping_regions = Vec::new();
-    let mut current_file_name = None;
-    let mut current_file_id = 0;
 
     // Convert the list of (Counter, CodeRegion) pairs to an array of `CounterMappingRegion`, sorted
     // by filename and position. Capture any new files to compute the `CounterMappingRegion`s
     // `file_id` (indexing files referenced by the current function), and construct the
     // function-specific `virtual_file_mapping` from `file_id` to its index in the module's
     // `filenames` array.
-    counter_regions.sort_by_key(|(_counter, region)| *region);
-    for (counter, region) in counter_regions {
-        let CodeRegion { file_name, start_line, start_col, end_line, end_col } = *region;
-        let same_file = current_file_name.is_some_and(|p| p == file_name);
-        if !same_file {
-            if current_file_name.is_some() {
-                current_file_id += 1;
-            }
-            current_file_name = Some(file_name);
-            debug!("  file_id: {} = '{:?}'", current_file_id, file_name);
-            let global_file_id = global_file_table.global_file_id_for_file_name(file_name);
-            virtual_file_mapping.push(global_file_id);
+    counter_regions.sort_by_key(|(_counter, region)| region.file_name);
+    for (local_file_id, counter_regions_for_file) in
+        (0u32..).zip(counter_regions.group_by(|(_, a), (_, b)| a.file_name == b.file_name))
+    {
+        // Look up (or allocate) the global file ID for this file name.
+        let file_name = counter_regions_for_file[0].1.file_name;
+        let global_file_id = global_file_table.global_file_id_for_file_name(file_name);
+
+        // Associate that global file ID with a local file ID for this function.
+        assert_eq!(local_file_id as usize, virtual_file_mapping.len());
+        virtual_file_mapping.push(global_file_id);
+        debug!("  file_id: local {local_file_id} => global {global_file_id} = '{file_name:?}'");
+
+        // For each counter/region pair in this function+file, convert it
+        // to a form suitable for FFI.
+        for &(counter, region) in counter_regions_for_file {
+            let CodeRegion { file_name: _, start_line, start_col, end_line, end_col } = *region;
+
+            debug!("Adding counter {counter:?} to map for {region:?}");
+            mapping_regions.push(CounterMappingRegion::code_region(
+                counter,
+                local_file_id,
+                start_line,
+                start_col,
+                end_line,
+                end_col,
+            ));
         }
-        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,
-        ));
     }
 
     // Encode and append the current function's coverage mapping data

compiler/rustc_codegen_llvm/src/lib.rs

@@ -10,6 +10,7 @@
 #![feature(iter_intersperse)]
 #![feature(let_chains)]
 #![feature(never_type)]
+#![feature(slice_group_by)]
 #![feature(impl_trait_in_assoc_type)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]