Delay gensym creation for "underscore items" until name resolution

Prohibit `static _`
Fis unused import warnings for `use foo as _`
Add more tests for `use foo as _`

1 files changed, 5 insertions, 1 deletions

diff --git a/src/libsyntax_pos/symbol.rs b/src/libsyntax_pos/symbol.rs
index 741877bb4c8..2721ab70e22 100644
--- a/src/libsyntax_pos/symbol.rs
+++ b/src/libsyntax_pos/symbol.rs
@@ -80,6 +80,10 @@ impl Ident {
         Ident::new(self.name.gensymed(), self.span)
     }
 
+    pub fn gensym_if_underscore(self) -> Ident {
+        if self.name == keywords::Underscore.name() { self.gensym() } else { self }
+    }
+
     pub fn as_str(self) -> LocalInternedString {
         self.name.as_str()
     }
@@ -465,7 +469,7 @@ impl Ident {
     // We see this identifier in a normal identifier position, like variable name or a type.
     // How was it written originally? Did it use the raw form? Let's try to guess.
     pub fn is_raw_guess(self) -> bool {
-        self.name != keywords::Invalid.name() &&
+        self.name != keywords::Invalid.name() && self.name != keywords::Underscore.name() &&
         self.is_reserved() && !self.is_path_segment_keyword()
     }
 }