Encode index of SourceFile along with span.

compiler/rustc_span/src/lib.rs

@@ -1098,6 +1098,8 @@ pub enum ExternalSource {
         original_start_pos: BytePos,
         /// The end of this SourceFile within the source_map of its original crate.
         original_end_pos: BytePos,
+        /// Index of the file inside metadata.
+        metadata_index: u32,
     },
 }
 

compiler/rustc_span/src/source_map.rs

@@ -337,6 +337,7 @@ impl SourceMap {
         mut file_local_normalized_pos: Vec<NormalizedPos>,
         original_start_pos: BytePos,
         original_end_pos: BytePos,
+        metadata_index: u32,
     ) -> Lrc<SourceFile> {
         let start_pos = self
             .allocate_address_space(source_len)
@@ -383,6 +384,7 @@ impl SourceMap {
                 kind: ExternalSourceKind::AbsentOk,
                 original_start_pos,
                 original_end_pos,
+                metadata_index,
             }),
             start_pos,
             end_pos,

compiler/rustc_span/src/source_map/tests.rs

@@ -252,6 +252,7 @@ fn t10() {
         normalized_pos,
         start_pos,
         end_pos,
+        0,
     );
 
     assert!(