Do not encode gensymed imports in metadata

5 files changed, 20 insertions, 4 deletions

diff --git a/src/librustc_resolve/build_reduced_graph.rs b/src/librustc_resolve/build_reduced_graph.rs
index 6fad4b2db97..f312ef21682 100644
--- a/src/librustc_resolve/build_reduced_graph.rs
+++ b/src/librustc_resolve/build_reduced_graph.rs
@@ -303,7 +303,7 @@ impl<'a> Resolver<'a> {
                 }
 
                 // Empty groups `a::b::{}` are turned into synthetic `self` imports
-                // `a::b::c::{self as _}`, so that their prefixes are correctly
+                // `a::b::c::{self as __dummy}`, so that their prefixes are correctly
                 // resolved and checked for privacy/stability/etc.
                 if items.is_empty() && !empty_for_self(&prefix) {
                     let new_span = prefix[prefix.len() - 1].ident.span;
@@ -312,7 +312,7 @@ impl<'a> Resolver<'a> {
                             Ident::new(keywords::SelfLower.name(), new_span)
                         ),
                         kind: ast::UseTreeKind::Simple(
-                            Some(Ident::new(keywords::Underscore.name().gensymed(), new_span)),
+                            Some(Ident::new(Name::gensym("__dummy"), new_span)),
                             ast::DUMMY_NODE_ID,
                             ast::DUMMY_NODE_ID,
                         ),
diff --git a/src/librustc_resolve/resolve_imports.rs b/src/librustc_resolve/resolve_imports.rs
index 9daffd522bf..bda59c6c46c 100644
--- a/src/librustc_resolve/resolve_imports.rs
+++ b/src/librustc_resolve/resolve_imports.rs
@@ -1295,9 +1295,11 @@ impl<'a, 'b:'a> ImportResolver<'a, 'b> {
                 None => continue,
             };
 
-            // Filter away "empty import canaries" and ambiguous imports.
+            // Filter away ambiguous and gensymed imports. Gensymed imports
+            // (e.g. implicitly injected `std`) cannot be properly encoded in metadata,
+            // so they can cause name conflict errors downstream.
             let is_good_import = binding.is_import() && !binding.is_ambiguity() &&
-                                 binding.vis != ty::Visibility::Invisible;
+                                 !(ident.name.is_gensymed() && ident.name != "_");
             if is_good_import || binding.is_macro_def() {
                 let def = binding.def();
                 if def != Def::Err {
diff --git a/src/libsyntax_pos/symbol.rs b/src/libsyntax_pos/symbol.rs
index e8d215a562e..f61aa4284d2 100644
--- a/src/libsyntax_pos/symbol.rs
+++ b/src/libsyntax_pos/symbol.rs
@@ -179,6 +179,10 @@ impl Symbol {
         with_interner(|interner| interner.gensymed(self))
     }
 
+    pub fn is_gensymed(self) -> bool {
+        with_interner(|interner| interner.is_gensymed(self))
+    }
+
     pub fn as_str(self) -> LocalInternedString {
         with_interner(|interner| unsafe {
             LocalInternedString {
diff --git a/src/test/ui/imports/auxiliary/gensymed.rs b/src/test/ui/imports/auxiliary/gensymed.rs
new file mode 100644
index 00000000000..bbb19f5ec65
--- /dev/null
+++ b/src/test/ui/imports/auxiliary/gensymed.rs
@@ -0,0 +1,3 @@
+// edition:2018
+
+mod std {}
diff --git a/src/test/ui/imports/gensymed.rs b/src/test/ui/imports/gensymed.rs
new file mode 100644
index 00000000000..317441079ff
--- /dev/null
+++ b/src/test/ui/imports/gensymed.rs
@@ -0,0 +1,7 @@
+// compile-pass
+// edition:2018
+// aux-build:gensymed.rs
+
+extern crate gensymed;
+
+fn main() {}