Do not encode gensymed imports in metadata

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,
  ),

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 {

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 {

src/test/ui/imports/auxiliary/gensymed.rs

@@ -0,0 +1,3 @@
+// edition:2018
+
+mod std {}

src/test/ui/imports/gensymed.rs

@@ -0,0 +1,7 @@
+// compile-pass
+// edition:2018
+// aux-build:gensymed.rs
+
+extern crate gensymed;
+
+fn main() {}