Auto merge of #49051 - kennytm:rollup, r=kennytm

Rollup of 17 pull requests

- Successful merges: #48706, #48875, #48892, #48922, #48957, #48959, #48961, #48965, #49007, #49024, #49042, #49050, #48853, #48990, #49037, #49049, #48972
- Failed merges:

src/libsyntax/parse/parser.rs

@@ -7065,7 +7065,11 @@ impl<'a> Parser<'a> {
 
     fn parse_rename(&mut self) -> PResult<'a, Option<Ident>> {
         if self.eat_keyword(keywords::As) {
-            self.parse_ident().map(Some)
+            if self.eat(&token::Underscore) {
+                Ok(Some(Ident::with_empty_ctxt(Symbol::gensym("_"))))
+            } else {
+                self.parse_ident().map(Some)
+            }
         } else {
             Ok(None)
         }