resolve: Partially convert ambiguous_glob_imports lint into a hard error

compiler/rustc_codegen_gcc/build_system/src/test.rs

@@ -886,8 +886,6 @@ fn valid_ui_error_pattern_test(file: &str) -> bool {
         "type-alias-impl-trait/auxiliary/cross_crate_ice.rs",
         "type-alias-impl-trait/auxiliary/cross_crate_ice2.rs",
         "macros/rfc-2011-nicer-assert-messages/auxiliary/common.rs",
-        "imports/ambiguous-1.rs",
-        "imports/ambiguous-4-extern.rs",
         "entry-point/auxiliary/bad_main_functions.rs",
     ]
     .iter()

compiler/rustc_lint_defs/src/builtin.rs

@@ -4529,28 +4529,21 @@ declare_lint! {
     ///
     /// ### Example
     ///
-    /// ```rust,compile_fail
-    /// #![deny(ambiguous_glob_imports)]
-    /// pub fn foo() -> u32 {
-    ///     use sub::*;
-    ///     C
-    /// }
-    ///
-    /// mod sub {
-    ///     mod mod1 { pub const C: u32 = 1; }
-    ///     mod mod2 { pub const C: u32 = 2; }
+    /// ```rust,ignore (needs extern crate)
+    /// // library crate `my_library`
+    /// mod mod1 { pub const C: u32 = 1; }
+    /// mod mod2 { pub const C: u32 = 2; }
+    /// pub use mod1::*;
+    /// pub use mod2::*;
     ///
-    ///     pub use mod1::*;
-    ///     pub use mod2::*;
-    /// }
+    /// // another crate using `my_library`
+    /// let c = my_library::C; // `C` is ambiguous
     /// ```
     ///
-    /// {{produces}}
-    ///
     /// ### Explanation
     ///
-    /// Previous versions of Rust compile it successfully because it
-    /// had lost the ambiguity error when resolve `use sub::mod2::*`.
+    /// Previous versions of Rust compile it successfully because
+    /// ambiguous glob imports weren't preserved correctly over crate boundaries.
     ///
     /// This is a [future-incompatible] lint to transition this to a
     /// hard error in the future.

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -52,7 +52,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         decl: Decl<'ra>,
     ) {
         if let Err(old_decl) =
-            self.try_plant_decl_into_local_module(ident, orig_ident_span, ns, decl, false)
+            self.try_plant_decl_into_local_module(ident, orig_ident_span, ns, decl)
         {
             self.report_conflict(ident, ns, old_decl, decl);
         }
@@ -87,13 +87,11 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         vis: Visibility<DefId>,
         span: Span,
         expansion: LocalExpnId,
-        ambiguity: Option<Decl<'ra>>,
+        ambiguity: Option<(Decl<'ra>, bool)>,
     ) {
         let decl = self.arenas.alloc_decl(DeclData {
             kind: DeclKind::Def(res),
             ambiguity: CmCell::new(ambiguity),
-            // External ambiguities always report the `AMBIGUOUS_GLOB_IMPORTS` lint at the moment.
-            warn_ambiguity: CmCell::new(true),
             initial_vis: vis,
             ambiguity_vis_max: CmCell::new(None),
             ambiguity_vis_min: CmCell::new(None),
@@ -392,7 +390,8 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             let ModChild { ident: _, res, vis, ref reexport_chain } = *ambig_child;
             let span = child_span(self, reexport_chain, res);
             let res = res.expect_non_local();
-            self.arenas.new_def_decl(res, vis, span, expansion, Some(parent.to_module()))
+            // External ambiguities always report the `AMBIGUOUS_GLOB_IMPORTS` lint at the moment.
+            (self.arenas.new_def_decl(res, vis, span, expansion, Some(parent.to_module())), true)
         });
 
         // Record primary definitions.

compiler/rustc_resolve/src/diagnostics.rs

@@ -1188,7 +1188,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         .emit()
     }
 
-    fn def_path_str(&self, mut def_id: DefId) -> String {
+    pub(crate) fn def_path_str(&self, mut def_id: DefId) -> String {
         // We can't use `def_path_str` in resolve.
         let mut path = vec![def_id];
         while let Some(parent) = self.tcx.opt_parent(def_id) {