Auto merge of #48842 - petrochenkov:under, r=nikomatsakis

syntax: Make `_` a reserved identifier

Why:
- Lexically `_` is an identifier.
- Internally it makes implementation of `use Trait as _;` (https://github.com/rust-lang/rust/issues/48216) and some other things cleaner.
- We prevent the externally observable effect of `_` being accepted by macros expecting `ident` by treating `_` specially in the `ident` matcher:
```rust
macro_rules! m {
    ($i: ident) => { let $i = 10; }
}

m!(_); // Still an error
```

1 files changed, 4 insertions, 10 deletions

diff --git a/src/libsyntax/parse/lexer/mod.rs b/src/libsyntax/parse/lexer/mod.rs
index 815ba49a60a..9d1bfba7b94 100644
--- a/src/libsyntax/parse/lexer/mod.rs
+++ b/src/libsyntax/parse/lexer/mod.rs
@@ -34,7 +34,7 @@ pub struct TokenAndSpan {
 
 impl Default for TokenAndSpan {
     fn default() -> Self {
-        TokenAndSpan { tok: token::Underscore, sp: syntax_pos::DUMMY_SP }
+        TokenAndSpan { tok: token::Whitespace, sp: syntax_pos::DUMMY_SP }
     }
 }
 
@@ -126,7 +126,7 @@ impl<'a> StringReader<'a> {
     pub fn try_next_token(&mut self) -> Result<TokenAndSpan, ()> {
         assert!(self.fatal_errs.is_empty());
         let ret_val = TokenAndSpan {
-            tok: replace(&mut self.peek_tok, token::Underscore),
+            tok: replace(&mut self.peek_tok, token::Whitespace),
             sp: self.peek_span,
         };
         self.advance_token()?;
@@ -1133,14 +1133,8 @@ impl<'a> StringReader<'a> {
                 self.bump();
             }
 
-            return Ok(self.with_str_from(start, |string| {
-                if string == "_" {
-                    token::Underscore
-                } else {
-                    // FIXME: perform NFKC normalization here. (Issue #2253)
-                    token::Ident(self.mk_ident(string))
-                }
-            }));
+            // FIXME: perform NFKC normalization here. (Issue #2253)
+            return Ok(self.with_str_from(start, |string| token::Ident(self.mk_ident(string))));
         }
 
         if is_dec_digit(c) {