add a cross-reference index

it still does not *do* anything

src/librustc/metadata/common.rs

@@ -43,9 +43,11 @@ pub const tag_items_data_parent_item: usize = 0x28;
 
 pub const tag_items_data_item_is_tuple_struct_ctor: usize = 0x29;
 
-pub const tag_index: usize = 0x2a;
+pub const tag_index: usize = 0x110; // top-level only
+pub const tag_xref_index: usize = 0x111; // top-level only
+pub const tag_xref_data: usize = 0x112; // top-level only
 
-// GAP 0x2b, 0x2c, 0x2d, 0x2e
+// GAP 0x2a, 0x2b, 0x2c, 0x2d, 0x2e
 
 pub const tag_meta_item_name_value: usize = 0x2f;
 

src/librustc/metadata/creader.rs

@@ -326,6 +326,7 @@ impl<'a> CrateReader<'a> {
             name: name.to_string(),
             local_path: RefCell::new(SmallVector::zero()),
             index: decoder::load_index(metadata.as_slice()),
+            xref_index: decoder::load_xrefs(metadata.as_slice()),
             data: metadata,
             cnum_map: RefCell::new(cnum_map),
             cnum: cnum,

src/librustc/metadata/cstore.rs

@@ -65,7 +65,9 @@ pub struct crate_metadata {
     pub codemap_import_info: RefCell<Vec<ImportedFileMap>>,
     pub span: codemap::Span,
     pub staged_api: bool,
+
     pub index: index::Index,
+    pub xref_index: index::DenseIndex,
 
     /// Flag if this crate is required by an rlib version of this crate, or in
     /// other words whether it was explicitly linked to. An example of a crate

src/librustc/metadata/decoder.rs

@@ -82,6 +82,11 @@ pub fn crate_rustc_version(data: &[u8]) -> Option<String> {
     reader::maybe_get_doc(doc, tag_rustc_version).map(|s| s.as_str())
 }
 
+pub fn load_xrefs(data: &[u8]) -> index::DenseIndex {
+    let index = reader::get_doc(rbml::Doc::new(data), tag_xref_index);
+    index::DenseIndex::from_buf(index.data, index.start, index.end)
+}
+
 #[derive(Debug, PartialEq)]
 enum Family {
     ImmStatic,             // c