rustc_metadata: Remove Span from ModChild

It can be decoded on demand from regular `def_span` tables.

Partially mitigates perf regressions from #109500.

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -998,9 +998,8 @@ fn get_mod_child(self, id: DefIndex, sess: &Session) -> ModChild {
         let ident = self.item_ident(id, sess);
         let res = Res::Def(self.def_kind(id), self.local_def_id(id));
         let vis = self.get_visibility(id);
-        let span = self.get_span(id, sess);
 
-        ModChild { ident, res, vis, span, reexport_chain: Default::default() }
+        ModChild { ident, res, vis, reexport_chain: Default::default() }
     }
 
     /// Iterates over all named children of the given module,

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -831,6 +831,8 @@ fn should_encode_span(def_kind: DefKind) -> bool {
         | DefKind::AssocFn
         | DefKind::AssocConst
         | DefKind::Macro(_)
+        | DefKind::ExternCrate
+        | DefKind::Use
         | DefKind::AnonConst
         | DefKind::InlineConst
         | DefKind::OpaqueTy
@@ -838,9 +840,7 @@ fn should_encode_span(def_kind: DefKind) -> bool {
         | DefKind::Impl { .. }
         | DefKind::Closure
         | DefKind::Generator => true,
-        DefKind::ExternCrate
-        | DefKind::Use
-        | DefKind::ForeignMod
+        DefKind::ForeignMod
         | DefKind::ImplTraitPlaceholder
         | DefKind::LifetimeParam
         | DefKind::GlobalAsm => false,

compiler/rustc_middle/src/metadata.rs

@@ -4,7 +4,6 @@
 use rustc_macros::HashStable;
 use rustc_span::def_id::DefId;
 use rustc_span::symbol::Ident;
-use rustc_span::Span;
 use smallvec::SmallVec;
 
 /// A simplified version of `ImportKind` from resolve.
@@ -41,8 +40,6 @@ pub struct ModChild {
     pub res: Res<!>,
     /// Visibility of the item.
     pub vis: ty::Visibility<DefId>,
-    /// Span of the item.
-    pub span: Span,
     /// Reexport chain linking this module child to its original reexported item.
     /// Empty if the module child is a proper item.
     pub reexport_chain: SmallVec<[Reexport; 2]>,

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -931,7 +931,13 @@ fn build_reduced_graph_for_block(&mut self, block: &Block) {
     /// Builds the reduced graph for a single item in an external crate.
     fn build_reduced_graph_for_external_crate_res(&mut self, child: ModChild) {
         let parent = self.parent_scope.module;
-        let ModChild { ident, res, vis, span, .. } = child;
+        let ModChild { ident, res, vis, reexport_chain } = child;
+        let span = self.r.def_span(
+            reexport_chain
+                .first()
+                .and_then(|reexport| reexport.id())
+                .unwrap_or_else(|| res.def_id()),
+        );
         let res = res.expect_non_local();
         let expansion = self.parent_scope.expansion;
         // Record primary definitions.

compiler/rustc_resolve/src/imports.rs

@@ -1276,13 +1276,7 @@ fn finalize_resolutions_in(&mut self, module: Module<'a>) {
                         next_binding = binding;
                     }
 
-                    reexports.push(ModChild {
-                        ident,
-                        res,
-                        vis: binding.vis,
-                        span: binding.span,
-                        reexport_chain,
-                    });
+                    reexports.push(ModChild { ident, res, vis: binding.vis, reexport_chain });
                 }
             });