resolve: re-export ambiguity as warning

compiler/rustc_errors/src/lib.rs

@@ -1695,6 +1695,9 @@ pub fn report_ambiguity_error<'a, G: EmissionGuarantee>(
     for help_msg in ambiguity.b1_help_msgs {
         db.help(help_msg);
     }
+    if ambiguity.extern_crate {
+        return;
+    }
     db.span_note(ambiguity.b2_span, ambiguity.b2_note_msg);
     for help_msg in ambiguity.b2_help_msgs {
         db.help(help_msg);

compiler/rustc_lint_defs/src/lib.rs

@@ -553,6 +553,8 @@ impl StableCompare for LintId {
 
 #[derive(Debug)]
 pub struct AmbiguityErrorDiag {
+    /// Does this ambiguity binding come from a different crate?
+    pub extern_crate: bool,
     pub msg: String,
     pub span: Span,
     pub label_span: Span,

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1250,6 +1250,21 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         })
     }
 
+    fn get_ambiguity_module_children(
+        self,
+        id: DefIndex,
+        sess: &'a Session,
+    ) -> impl Iterator<Item = AmbiguityModChild> + 'a {
+        iter::from_coroutine(move || {
+            let children = self.root.tables.ambiguity_module_children.get(self, id);
+            if !children.is_default() {
+                for child in children.decode((self, sess)) {
+                    yield child;
+                }
+            }
+        })
+    }
+
     fn is_ctfe_mir_available(self, id: DefIndex) -> bool {
         self.root.tables.mir_for_ctfe.get(self, id).is_some()
     }

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

@@ -11,7 +11,7 @@ use rustc_hir::def::{CtorKind, DefKind, Res};
 use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, LOCAL_CRATE};
 use rustc_hir::definitions::{DefKey, DefPath, DefPathHash};
 use rustc_middle::arena::ArenaAllocatable;
-use rustc_middle::metadata::ModChild;
+use rustc_middle::metadata::{AmbiguityModChild, ModChild};
 use rustc_middle::middle::exported_symbols::ExportedSymbol;
 use rustc_middle::middle::stability::DeprecationEntry;
 use rustc_middle::query::ExternProviders;
@@ -581,6 +581,16 @@ impl CStore {
             self.get_crate_data_mut(cnum).dependencies = dependencies;
         }
     }
+
+    pub fn ambiguity_module_children_untracked(
+        &self,
+        def_id: DefId,
+        sess: &Session,
+    ) -> Vec<AmbiguityModChild> {
+        self.get_crate_data(def_id.krate)
+            .get_ambiguity_module_children(def_id.index, sess)
+            .collect()
+    }
 }
 
 impl CrateStore for CStore {

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1568,6 +1568,8 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
 
             record_defaulted_array!(self.tables.module_children_reexports[def_id] <-
                 module_children.iter().filter(|child| !child.reexport_chain.is_empty()));
+
+            record_defaulted_array!(self.tables.ambiguity_module_children[def_id] <- tcx.ambiguity_module_children_local(local_def_id));
         }
     }
 

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -17,7 +17,7 @@ use rustc_hir::definitions::DefKey;
 use rustc_hir::lang_items::LangItem;
 use rustc_index::bit_set::BitSet;
 use rustc_index::IndexVec;
-use rustc_middle::metadata::ModChild;
+use rustc_middle::metadata::{AmbiguityModChild, ModChild};
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
 use rustc_middle::middle::exported_symbols::{ExportedSymbol, SymbolExportInfo};
 use rustc_middle::middle::resolve_bound_vars::ObjectLifetimeDefault;
@@ -398,6 +398,7 @@ define_tables! {
     // individually instead of `DefId`s.
     module_children_reexports: Table<DefIndex, LazyArray<ModChild>>,
     cross_crate_inlinable: Table<DefIndex, bool>,
+    ambiguity_module_children: Table<DefIndex, LazyArray<AmbiguityModChild>>,
 
 - optional:
     attributes: Table<DefIndex, LazyArray<ast::Attribute>>,

compiler/rustc_middle/src/metadata.rs

@@ -44,3 +44,6 @@ pub struct ModChild {
     /// Empty if the module child is a proper item.
     pub reexport_chain: SmallVec<[Reexport; 2]>,
 }
+
+/// Same as `ModChild`, however, it includes ambiguity error.
+pub type AmbiguityModChild = ModChild;

compiler/rustc_middle/src/ty/context.rs

@@ -8,7 +8,7 @@ use crate::arena::Arena;
 use crate::dep_graph::{DepGraph, DepKindStruct};
 use crate::infer::canonical::{CanonicalParamEnvCache, CanonicalVarInfo, CanonicalVarInfos};
 use crate::lint::struct_lint_level;
-use crate::metadata::ModChild;
+use crate::metadata::{AmbiguityModChild, ModChild};
 use crate::middle::codegen_fn_attrs::CodegenFnAttrs;
 use crate::middle::resolve_bound_vars;
 use crate::middle::stability;
@@ -2286,6 +2286,10 @@ impl<'tcx> TyCtxt<'tcx> {
     pub fn module_children_local(self, def_id: LocalDefId) -> &'tcx [ModChild] {
         self.resolutions(()).module_children.get(&def_id).map_or(&[], |v| &v[..])
     }
+
+    pub fn ambiguity_module_children_local(self, def_id: LocalDefId) -> &'tcx [AmbiguityModChild] {
+        self.resolutions(()).ambiguity_module_children.get(&def_id).map_or(&[], |v| &v[..])
+    }
 }
 
 /// Parameter attributes that can only be determined by examining the body of a function instead

compiler/rustc_middle/src/ty/mod.rs

@@ -18,7 +18,7 @@ pub use self::BorrowKind::*;
 pub use self::IntVarValue::*;
 pub use self::Variance::*;
 use crate::error::{OpaqueHiddenTypeMismatch, TypeMismatchReason};
-use crate::metadata::ModChild;
+use crate::metadata::{AmbiguityModChild, ModChild};
 use crate::middle::privacy::EffectiveVisibilities;
 use crate::mir::{Body, CoroutineLayout};
 use crate::query::Providers;
@@ -160,6 +160,7 @@ pub struct ResolverGlobalCtxt {
     pub extern_crate_map: FxHashMap<LocalDefId, CrateNum>,
     pub maybe_unused_trait_imports: FxIndexSet<LocalDefId>,
     pub module_children: LocalDefIdMap<Vec<ModChild>>,
+    pub ambiguity_module_children: LocalDefIdMap<Vec<AmbiguityModChild>>,
     pub glob_map: FxHashMap<LocalDefId, FxHashSet<Symbol>>,
     pub main_def: Option<MainDefinition>,
     pub trait_impls: FxIndexMap<DefId, Vec<LocalDefId>>,