Prefer visibility paths where items are not named _

Author: compiler-errors

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -17,7 +17,7 @@ use rustc_session::utils::NativeLibKind;
 use rustc_session::{Session, StableCrateId};
 use rustc_span::hygiene::{ExpnHash, ExpnId};
 use rustc_span::source_map::{Span, Spanned};
-use rustc_span::symbol::Symbol;
+use rustc_span::symbol::{kw, Symbol};
 
 use rustc_data_structures::sync::Lrc;
 use smallvec::SmallVec;
@@ -295,6 +295,10 @@ pub fn provide(providers: &mut Providers) {
             use std::collections::vec_deque::VecDeque;
 
             let mut visible_parent_map: DefIdMap<DefId> = Default::default();
+            // This is a secondary visible_parent_map, storing the DefId of parents that re-export
+            // the child as `_`. Since we prefer parents that don't do this, merge this map at the
+            // end, only if we're missing any keys from the former.
+            let mut fallback_map: DefIdMap<DefId> = Default::default();
 
             // Issue 46112: We want the map to prefer the shortest
             // paths when reporting the path to an item. Therefore we
@@ -317,12 +321,17 @@ pub fn provide(providers: &mut Providers) {
                 bfs_queue.push_back(DefId { krate: cnum, index: CRATE_DEF_INDEX });
             }
 
-            let mut add_child = |bfs_queue: &mut VecDeque<_>, child: &Export, parent: DefId| {
-                if !child.vis.is_public() {
+            let mut add_child = |bfs_queue: &mut VecDeque<_>, export: &Export, parent: DefId| {
+                if !export.vis.is_public() {
                     return;
                 }
 
-                if let Some(child) = child.res.opt_def_id() {
+                if let Some(child) = export.res.opt_def_id() {
+                    if export.ident.name == kw::Underscore {
+                        fallback_map.insert(child, parent);
+                        return;
+                    }
+
                     match visible_parent_map.entry(child) {
                         Entry::Occupied(mut entry) => {
                             // If `child` is defined in crate `cnum`, ensure
@@ -345,6 +354,12 @@ pub fn provide(providers: &mut Providers) {
                 }
             }
 
+            // Fill in any missing entries with the (less preferable) path ending in `::_`.
+            // We still use this path in a diagnostic that suggests importing `::*`.
+            for (child, parent) in fallback_map {
+                visible_parent_map.entry(child).or_insert(parent);
+            }
+
             visible_parent_map
         },