Rollup merge of rust-lang#59296 - petrochenkov:stdup, r=estebank

Do not encode gensymed imports in metadata

(Unless they are underscore `_` imports which are re-gensymed on crate loading, see rust-lang#56392.)

We cannot encode gensymed imports properly in metadata and if we encode them improperly, we can get erroneous name conflicts downstream.
Gensymed imports are produced by the compiler, so we control their set, and can be sure that none of them needs being encoded for use from other crates.

A workaround that fixes rust-lang#59243.

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() {}