resolve: Preserve reexport chains in ModChildren

This may be potentially useful for
- avoiding uses of `hir::ItemKind::Use`
- preserving documentation comments on all reexports
- preserving and checking stability/deprecation info on reexports
- all kinds of diagnostics

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -931,7 +931,7 @@ impl<'a, 'b, 'tcx> BuildReducedGraphVisitor<'a, 'b, 'tcx> {
     /// 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, macro_rules } = child;
+        let ModChild { ident, res, vis, span, macro_rules, .. } = child;
         let res = res.expect_non_local();
         let expansion = self.parent_scope.expansion;
         // Record primary definitions.

compiler/rustc_resolve/src/imports.rs

@@ -17,6 +17,7 @@ use rustc_data_structures::intern::Interned;
 use rustc_errors::{pluralize, struct_span_err, Applicability, MultiSpan};
 use rustc_hir::def::{self, DefKind, PartialRes};
 use rustc_middle::metadata::ModChild;
+use rustc_middle::metadata::Reexport;
 use rustc_middle::span_bug;
 use rustc_middle::ty;
 use rustc_session::lint::builtin::{
@@ -27,6 +28,7 @@ use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::hygiene::LocalExpnId;
 use rustc_span::symbol::{kw, Ident, Symbol};
 use rustc_span::Span;
+use smallvec::SmallVec;
 
 use std::cell::Cell;
 use std::{mem, ptr};
@@ -190,6 +192,17 @@ impl<'a> Import<'a> {
             ImportKind::MacroUse | ImportKind::MacroExport => None,
         }
     }
+
+    fn simplify(&self, r: &Resolver<'_, '_>) -> Reexport {
+        let to_def_id = |id| r.local_def_id(id).to_def_id();
+        match self.kind {
+            ImportKind::Single { id, .. } => Reexport::Single(to_def_id(id)),
+            ImportKind::Glob { id, .. } => Reexport::Glob(to_def_id(id)),
+            ImportKind::ExternCrate { id, .. } => Reexport::ExternCrate(to_def_id(id)),
+            ImportKind::MacroUse => Reexport::MacroUse,
+            ImportKind::MacroExport => Reexport::MacroExport,
+        }
+    }
 }
 
 /// Records information about the resolution of a name in a namespace of a module.
@@ -1252,12 +1265,20 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
 
             module.for_each_child(self, |this, ident, _, binding| {
                 if let Some(res) = this.is_reexport(binding) {
+                    let mut reexport_chain = SmallVec::new();
+                    let mut next_binding = binding;
+                    while let NameBindingKind::Import { binding, import, .. } = next_binding.kind {
+                        reexport_chain.push(import.simplify(this));
+                        next_binding = binding;
+                    }
+
                     reexports.push(ModChild {
                         ident,
                         res,
                         vis: binding.vis,
                         span: binding.span,
                         macro_rules: false,
+                        reexport_chain,
                     });
                 }
             });