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 _;` (#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
```

src/libproc_macro/lib.rs

@@ -680,7 +680,6 @@ impl TokenTree {
             Pound => op!('#'),
             Dollar => op!('$'),
             Question => op!('?'),
-            Underscore => op!('_'),
 
             Ident(ident) | Lifetime(ident) => TokenNode::Term(Term(ident.name)),
             Literal(..) | DocComment(..) => TokenNode::Literal(self::Literal(token)),
@@ -743,7 +742,6 @@ impl TokenTree {
             '#' => Pound,
             '$' => Dollar,
             '?' => Question,
-            '_' => Underscore,
             _ => panic!("unsupported character {}", op),
         };
 

src/librustc/hir/mod.rs

@@ -214,7 +214,7 @@ impl LifetimeName {
         use self::LifetimeName::*;
         match *self {
             Implicit => keywords::Invalid.name(),
-            Underscore => Symbol::intern("'_"),
+            Underscore => keywords::UnderscoreLifetime.name(),
             Static => keywords::StaticLifetime.name(),
             Name(name) => name,
         }

src/librustc/ich/impls_syntax.rs

@@ -287,7 +287,6 @@ fn hash_token<'a, 'gcx, W: StableHasherResult>(
         token::Token::Pound |
         token::Token::Dollar |
         token::Token::Question |
-        token::Token::Underscore |
         token::Token::Whitespace |
         token::Token::Comment |
         token::Token::Eof => {}

src/librustc_passes/ast_validation.rs

@@ -37,15 +37,17 @@ impl<'a> AstValidator<'a> {
     }
 
     fn check_lifetime(&self, lifetime: &Lifetime) {
-        let valid_names = [keywords::StaticLifetime.name(), keywords::Invalid.name()];
+        let valid_names = [keywords::UnderscoreLifetime.name(),
+                           keywords::StaticLifetime.name(),
+                           keywords::Invalid.name()];
         if !valid_names.contains(&lifetime.ident.name) &&
             token::Ident(lifetime.ident.without_first_quote()).is_reserved_ident() {
             self.err_handler().span_err(lifetime.span, "lifetimes cannot use keyword names");
         }
     }
 
     fn check_label(&self, label: Ident, span: Span) {
-        if token::Ident(label.without_first_quote()).is_reserved_ident() || label.name == "'_" {
+        if token::Ident(label.without_first_quote()).is_reserved_ident() {
             self.err_handler().span_err(span, &format!("invalid label name `{}`", label.name));
         }
     }

src/librustdoc/html/highlight.rs

@@ -352,7 +352,7 @@ impl<'a> Classifier<'a> {
 
             token::Lifetime(..) => Class::Lifetime,
 
-            token::Underscore | token::Eof | token::Interpolated(..) |
+            token::Eof | token::Interpolated(..) |
             token::Tilde | token::At | token::DotEq => Class::None,
         };
 

src/libsyntax/diagnostics/plugin.rs

@@ -19,7 +19,7 @@ use ext::base::{ExtCtxt, MacEager, MacResult};
 use ext::build::AstBuilder;
 use parse::token;
 use ptr::P;
-use symbol::Symbol;
+use symbol::{keywords, Symbol};
 use tokenstream::{TokenTree};
 use util::small_vector::SmallVector;
 
@@ -192,7 +192,7 @@ pub fn expand_build_diagnostic_array<'cx>(ecx: &'cx mut ExtCtxt,
             (descriptions.len(), ecx.expr_vec(span, descriptions))
         });
 
-    let static_ = ecx.lifetime(span, Ident::from_str("'static"));
+    let static_ = ecx.lifetime(span, keywords::StaticLifetime.ident());
     let ty_str = ecx.ty_rptr(
         span,
         ecx.ty_ident(span, ecx.ident_of("str")),

src/libsyntax/ext/quote.rs

@@ -709,7 +709,6 @@ fn expr_mk_token(cx: &ExtCtxt, sp: Span, tok: &token::Token) -> P<ast::Expr> {
         token::Pound        => "Pound",
         token::Dollar       => "Dollar",
         token::Question     => "Question",
-        token::Underscore   => "Underscore",
         token::Eof          => "Eof",
 
         token::Whitespace | token::Comment | token::Shebang(_) => {

src/libsyntax/ext/tt/macro_parser.rs

@@ -86,7 +86,7 @@ use self::TokenTreeOrTokenTreeVec::*;
 
 use ast::Ident;
 use syntax_pos::{self, BytePos, Span};
-use codemap::Spanned;
+use codemap::respan;
 use errors::FatalError;
 use ext::tt::quoted::{self, TokenTree};
 use parse::{Directory, ParseSess};
@@ -709,6 +709,15 @@ pub fn parse(
     }
 }
 
+/// The token is an identifier, but not `_`.
+/// We prohibit passing `_` to macros expecting `ident` for now.
+fn get_macro_ident(token: &Token) -> Option<Ident> {
+    match *token {
+        token::Ident(ident) if ident.name != keywords::Underscore.name() => Some(ident),
+        _ => None,
+    }
+}
+
 /// Checks whether a non-terminal may begin with a particular token.
 ///
 /// Returning `false` is a *stability guarantee* that such a matcher will *never* begin with that
@@ -725,7 +734,7 @@ fn may_begin_with(name: &str, token: &Token) -> bool {
     match name {
         "expr" => token.can_begin_expr(),
         "ty" => token.can_begin_type(),
-        "ident" => token.is_ident(),
+        "ident" => get_macro_ident(token).is_some(),
         "vis" => match *token {
             // The follow-set of :vis + "priv" keyword + interpolated
             Token::Comma | Token::Ident(_) | Token::Interpolated(_) => true,
@@ -765,8 +774,7 @@ fn may_begin_with(name: &str, token: &Token) -> bool {
             Token::DotDotDot |                  // range pattern (future compat)
             Token::ModSep |                     // path
             Token::Lt |                         // path (UFCS constant)
-            Token::BinOp(token::Shl) |          // path (double UFCS)
-            Token::Underscore => true,          // placeholder
+            Token::BinOp(token::Shl) => true,   // path (double UFCS)
             Token::Interpolated(ref nt) => may_be_ident(&nt.0),
             _ => false,
         },
@@ -815,21 +823,14 @@ fn parse_nt<'a>(p: &mut Parser<'a>, sp: Span, name: &str) -> Nonterminal {
         "expr" => token::NtExpr(panictry!(p.parse_expr())),
         "ty" => token::NtTy(panictry!(p.parse_ty())),
         // this could be handled like a token, since it is one
-        "ident" => match p.token {
-            token::Ident(sn) => {
-                p.bump();
-                token::NtIdent(Spanned::<Ident> {
-                    node: sn,
-                    span: p.prev_span,
-                })
-            }
-            _ => {
-                let token_str = pprust::token_to_string(&p.token);
-                p.fatal(&format!("expected ident, found {}", &token_str[..]))
-                    .emit();
-                FatalError.raise()
-            }
-        },
+        "ident" => if let Some(ident) = get_macro_ident(&p.token) {
+            p.bump();
+            token::NtIdent(respan(p.prev_span, ident))
+        } else {
+            let token_str = pprust::token_to_string(&p.token);
+            p.fatal(&format!("expected ident, found {}", &token_str)).emit();
+            FatalError.raise()
+        }
         "path" => token::NtPath(panictry!(p.parse_path_common(PathStyle::Type, false))),
         "meta" => token::NtMeta(panictry!(p.parse_meta_item())),
         "vis" => token::NtVis(panictry!(p.parse_visibility(true))),

src/libsyntax/feature_gate.rs

@@ -1790,7 +1790,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
     }
 
     fn visit_lifetime(&mut self, lt: &'a ast::Lifetime) {
-        if lt.ident.name == "'_" {
+        if lt.ident.name == keywords::UnderscoreLifetime.name() {
             gate_feature_post!(&self, underscore_lifetimes, lt.span,
                                "underscore lifetimes are unstable");
         }

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) {

src/libsyntax/parse/parser.rs

@@ -549,7 +549,7 @@ impl<'a> Parser<'a> {
                -> Self {
         let mut parser = Parser {
             sess,
-            token: token::Underscore,
+            token: token::Whitespace,
             span: syntax_pos::DUMMY_SP,
             prev_span: syntax_pos::DUMMY_SP,
             meta_var_span: None,
@@ -800,11 +800,7 @@ impl<'a> Parser<'a> {
                 Err(if self.prev_token_kind == PrevTokenKind::DocComment {
                         self.span_fatal_err(self.prev_span, Error::UselessDocComment)
                     } else {
-                        let mut err = self.expected_ident_found();
-                        if self.token == token::Underscore {
-                            err.note("`_` is a wildcard pattern, not an identifier");
-                        }
-                        err
+                        self.expected_ident_found()
                     })
             }
         }
@@ -1602,7 +1598,7 @@ impl<'a> Parser<'a> {
             let e = self.parse_expr()?;
             self.expect(&token::CloseDelim(token::Paren))?;
             TyKind::Typeof(e)
-        } else if self.eat(&token::Underscore) {
+        } else if self.eat_keyword(keywords::Underscore) {
             // A type to be inferred `_`
             TyKind::Infer
         } else if self.token_is_bare_fn_keyword() {
@@ -1796,7 +1792,7 @@ impl<'a> Parser<'a> {
             _ => 0,
         };
 
-        self.look_ahead(offset, |t| t.is_ident() || t == &token::Underscore) &&
+        self.look_ahead(offset, |t| t.is_ident()) &&
         self.look_ahead(offset + 1, |t| t == &token::Colon)
     }
 
@@ -2782,7 +2778,7 @@ impl<'a> Parser<'a> {
             },
             token::CloseDelim(_) | token::Eof => unreachable!(),
             _ => {
-                let (token, span) = (mem::replace(&mut self.token, token::Underscore), self.span);
+                let (token, span) = (mem::replace(&mut self.token, token::Whitespace), self.span);
                 self.bump();
                 TokenTree::Token(span, token)
             }
@@ -3815,11 +3811,6 @@ impl<'a> Parser<'a> {
         let lo = self.span;
         let pat;
         match self.token {
-            token::Underscore => {
-                // Parse _
-                self.bump();
-                pat = PatKind::Wild;
-            }
             token::BinOp(token::And) | token::AndAnd => {
                 // Parse &pat / &mut pat
                 self.expect_and()?;
@@ -3849,8 +3840,11 @@ impl<'a> Parser<'a> {
                 self.expect(&token::CloseDelim(token::Bracket))?;
                 pat = PatKind::Slice(before, slice, after);
             }
-            // At this point, token != _, &, &&, (, [
-            _ => if self.eat_keyword(keywords::Mut) {
+            // At this point, token != &, &&, (, [
+            _ => if self.eat_keyword(keywords::Underscore) {
+                // Parse _
+                pat = PatKind::Wild;
+            } else if self.eat_keyword(keywords::Mut) {
                 // Parse mut ident @ pat / mut ref ident @ pat
                 let mutref_span = self.prev_span.to(self.span);
                 let binding_mode = if self.eat_keyword(keywords::Ref) {
@@ -7065,10 +7059,12 @@ impl<'a> Parser<'a> {
 
     fn parse_rename(&mut self) -> PResult<'a, Option<Ident>> {
         if self.eat_keyword(keywords::As) {
-            if self.eat(&token::Underscore) {
-                Ok(Some(Ident::with_empty_ctxt(Symbol::gensym("_"))))
-            } else {
-                self.parse_ident().map(Some)
+            match self.token {
+                token::Ident(ident) if ident.name == keywords::Underscore.name() => {
+                    self.bump(); // `_`
+                    Ok(Some(Ident { name: ident.name.gensymed(), ..ident }))
+                }
+                _ => self.parse_ident().map(Some),
             }
         } else {
             Ok(None)

src/libsyntax/parse/token.rs

@@ -122,6 +122,7 @@ fn ident_can_begin_type(ident: ast::Ident) -> bool {
     !ident_token.is_reserved_ident() ||
     ident_token.is_path_segment_keyword() ||
     [
+        keywords::Underscore.name(),
         keywords::For.name(),
         keywords::Impl.name(),
         keywords::Fn.name(),
@@ -175,7 +176,6 @@ pub enum Token {
 
     /* Name components */
     Ident(ast::Ident),
-    Underscore,
     Lifetime(ast::Ident),
 
     // The `LazyTokenStream` is a pure function of the `Nonterminal`,
@@ -242,7 +242,6 @@ impl Token {
             Ident(ident)                => ident_can_begin_type(ident), // type name or keyword
             OpenDelim(Paren)            | // tuple
             OpenDelim(Bracket)          | // array
-            Underscore                  | // placeholder
             Not                         | // never
             BinOp(Star)                 | // raw pointer
             BinOp(And)                  | // reference
@@ -371,7 +370,7 @@ impl Token {
     // unnamed method parameters, crate root module, error recovery etc.
     pub fn is_special_ident(&self) -> bool {
         match self.ident() {
-            Some(id) => id.name <= keywords::DollarCrate.name(),
+            Some(id) => id.name <= keywords::Underscore.name(),
             _ => false,
         }
     }
@@ -441,7 +440,7 @@ impl Token {
 
             Le | EqEq | Ne | Ge | AndAnd | OrOr | Tilde | BinOpEq(..) | At | DotDotDot | DotEq |
             DotDotEq | Comma | Semi | ModSep | RArrow | LArrow | FatArrow | Pound | Dollar |
-            Question | OpenDelim(..) | CloseDelim(..) | Underscore => return None,
+            Question | OpenDelim(..) | CloseDelim(..) => return None,
 
             Literal(..) | Ident(..) | Lifetime(..) | Interpolated(..) | DocComment(..) |
             Whitespace | Comment | Shebang(..) | Eof => return None,
@@ -573,7 +572,7 @@ impl fmt::Debug for Nonterminal {
 pub fn is_op(tok: &Token) -> bool {
     match *tok {
         OpenDelim(..) | CloseDelim(..) | Literal(..) | DocComment(..) |
-        Ident(..) | Underscore | Lifetime(..) | Interpolated(..) |
+        Ident(..) | Lifetime(..) | Interpolated(..) |
         Whitespace | Comment | Shebang(..) | Eof => false,
         _ => true,
     }

src/libsyntax/print/pprust.rs

@@ -252,7 +252,6 @@ pub fn token_to_string(tok: &Token) -> String {
         /* Name components */
         token::Ident(s)             => s.to_string(),
         token::Lifetime(s)          => s.to_string(),
-        token::Underscore           => "_".to_string(),
 
         /* Other */
         token::DocComment(s)        => s.to_string(),

src/libsyntax_ext/env.rs

@@ -17,7 +17,7 @@ use syntax::ast::{self, Ident};
 use syntax::ext::base::*;
 use syntax::ext::base;
 use syntax::ext::build::AstBuilder;
-use syntax::symbol::Symbol;
+use syntax::symbol::{keywords, Symbol};
 use syntax_pos::Span;
 use syntax::tokenstream;
 
@@ -35,14 +35,14 @@ pub fn expand_option_env<'cx>(cx: &'cx mut ExtCtxt,
     let sp = sp.with_ctxt(sp.ctxt().apply_mark(cx.current_expansion.mark));
     let e = match env::var(&*var.as_str()) {
         Err(..) => {
+            let lt = cx.lifetime(sp, keywords::StaticLifetime.ident());
             cx.expr_path(cx.path_all(sp,
                                      true,
                                      cx.std_path(&["option", "Option", "None"]),
                                      Vec::new(),
                                      vec![cx.ty_rptr(sp,
                                                      cx.ty_ident(sp, Ident::from_str("str")),
-                                                     Some(cx.lifetime(sp,
-                                                                      Ident::from_str("'static"))),
+                                                     Some(lt),
                                                      ast::Mutability::Immutable)],
                                      Vec::new()))
         }