src/libsyntax/ext/tt/macro_rules.rs

   1 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use {ast, attr};
  12 use syntax_pos::{Span, DUMMY_SP};
  13 use edition::Edition;
  14 use errors::FatalError;
  15 use ext::base::{DummyResult, ExtCtxt, MacResult, SyntaxExtension};
  16 use ext::base::{NormalTT, TTMacroExpander};
  17 use ext::expand::{AstFragment, AstFragmentKind};
  18 use ext::tt::macro_parser::{Success, Error, Failure};
  19 use ext::tt::macro_parser::{MatchedSeq, MatchedNonterminal};
  20 use ext::tt::macro_parser::{parse, parse_failure_msg};
  21 use ext::tt::quoted;
  22 use ext::tt::transcribe::transcribe;
  23 use feature_gate::Features;
  24 use parse::{Directory, ParseSess};
  25 use parse::parser::Parser;
  26 use parse::token::{self, NtTT};
  27 use parse::token::Token::*;
  28 use symbol::Symbol;
  29 use tokenstream::{DelimSpan, TokenStream, TokenTree};
  30
  31 use rustc_data_structures::fx::FxHashMap;
  32 use std::borrow::Cow;
  33 use std::collections::hash_map::Entry;
  34
  35 use rustc_data_structures::sync::Lrc;
  36 use errors::Applicability;
  37
  38 const VALID_FRAGMENT_NAMES_MSG: &str = "valid fragment specifiers are \
  39     `ident`, `block`, `stmt`, `expr`, `pat`, `ty`, `lifetime`, `literal`, \
  40     `path`, `meta`, `tt`, `item` and `vis`";
  41
  42 pub struct ParserAnyMacro<'a> {
  43     parser: Parser<'a>,
  44
  45     /// Span of the expansion site of the macro this parser is for
  46     site_span: Span,
  47     /// The ident of the macro we're parsing
  48     macro_ident: ast::Ident,
  49     arm_span: Span,
  50 }
  51
  52 impl<'a> ParserAnyMacro<'a> {
  53     pub fn make(mut self: Box<ParserAnyMacro<'a>>, kind: AstFragmentKind) -> AstFragment {
  54         let ParserAnyMacro { site_span, macro_ident, ref mut parser, arm_span } = *self;
  55         let fragment = panictry!(parser.parse_ast_fragment(kind, true).map_err(|mut e| {
  56             if parser.token == token::Eof && e.message().ends_with(", found `<eof>`") {
  57                 if !e.span.is_dummy() {  // early end of macro arm (#52866)
  58                     e.replace_span_with(parser.sess.source_map().next_point(parser.span));
  59                 }
  60                 let msg = &e.message[0];
  61                 e.message[0] = (
  62                     format!(
  63                         "macro expansion ends with an incomplete expression: {}",
  64                         msg.0.replace(", found `<eof>`", ""),
  65                     ),
  66                     msg.1,
  67                 );
  68             }
  69             if e.span.is_dummy() {  // Get around lack of span in error (#30128)
  70                 e.replace_span_with(site_span);
  71                 if parser.sess.source_map().span_to_filename(arm_span).is_real() {
  72                     e.span_label(arm_span, "in this macro arm");
  73                 }
  74             } else if !parser.sess.source_map().span_to_filename(parser.span).is_real() {
  75                 e.span_label(site_span, "in this macro invocation");
  76             }
  77             e
  78         }));
  79
  80         // We allow semicolons at the end of expressions -- e.g. the semicolon in
  81         // `macro_rules! m { () => { panic!(); } }` isn't parsed by `.parse_expr()`,
  82         // but `m!()` is allowed in expression positions (c.f. issue #34706).
  83         if kind == AstFragmentKind::Expr && parser.token == token::Semi {
  84             parser.bump();
  85         }
  86
  87         // Make sure we don't have any tokens left to parse so we don't silently drop anything.
  88         let path = ast::Path::from_ident(macro_ident.with_span_pos(site_span));
  89         parser.ensure_complete_parse(&path, kind.name(), site_span);
  90         fragment
  91     }
  92 }
  93
  94 struct MacroRulesMacroExpander {
  95     name: ast::Ident,
  96     lhses: Vec<quoted::TokenTree>,
  97     rhses: Vec<quoted::TokenTree>,
  98     valid: bool,
  99 }
 100
 101 impl TTMacroExpander for MacroRulesMacroExpander {
 102     fn expand<'cx>(
 103         &self,
 104         cx: &'cx mut ExtCtxt,
 105         sp: Span,
 106         input: TokenStream,
 107         def_span: Option<Span>,
 108     ) -> Box<dyn MacResult+'cx> {
 109         if !self.valid {
 110             return DummyResult::any(sp);
 111         }
 112         generic_extension(cx,
 113                           sp,
 114                           def_span,
 115                           self.name,
 116                           input,
 117                           &self.lhses,
 118                           &self.rhses)
 119     }
 120 }
 121
 122 fn trace_macros_note(cx: &mut ExtCtxt, sp: Span, message: String) {
 123     let sp = sp.macro_backtrace().last().map(|trace| trace.call_site).unwrap_or(sp);
 124     cx.expansions.entry(sp).or_default().push(message);
 125 }
 126
 127 /// Given `lhses` and `rhses`, this is the new macro we create
 128 fn generic_extension<'cx>(cx: &'cx mut ExtCtxt,
 129                           sp: Span,
 130                           def_span: Option<Span>,
 131                           name: ast::Ident,
 132                           arg: TokenStream,
 133                           lhses: &[quoted::TokenTree],
 134                           rhses: &[quoted::TokenTree])
 135                           -> Box<dyn MacResult+'cx> {
 136     if cx.trace_macros() {
 137         trace_macros_note(cx, sp, format!("expanding `{}! {{ {} }}`", name, arg));
 138     }
 139
 140     // Which arm's failure should we report? (the one furthest along)
 141     let mut best_fail_spot = DUMMY_SP;
 142     let mut best_fail_tok = None;
 143     let mut best_fail_text = None;
 144
 145     for (i, lhs) in lhses.iter().enumerate() { // try each arm's matchers
 146         let lhs_tt = match *lhs {
 147             quoted::TokenTree::Delimited(_, ref delim) => &delim.tts[..],
 148             _ => cx.span_bug(sp, "malformed macro lhs")
 149         };
 150
 151         match TokenTree::parse(cx, lhs_tt, arg.clone()) {
 152             Success(named_matches) => {
 153                 let rhs = match rhses[i] {
 154                     // ignore delimiters
 155                     quoted::TokenTree::Delimited(_, ref delimed) => delimed.tts.clone(),
 156                     _ => cx.span_bug(sp, "malformed macro rhs"),
 157                 };
 158                 let arm_span = rhses[i].span();
 159
 160                 let rhs_spans = rhs.iter().map(|t| t.span()).collect::<Vec<_>>();
 161                 // rhs has holes ( `$id` and `$(...)` that need filled)
 162                 let mut tts = transcribe(cx, Some(named_matches), rhs);
 163
 164                 // Replace all the tokens for the corresponding positions in the macro, to maintain
 165                 // proper positions in error reporting, while maintaining the macro_backtrace.
 166                 if rhs_spans.len() == tts.len() {
 167                     tts = tts.map_enumerated(|i, mut tt| {
 168                         let mut sp = rhs_spans[i];
 169                         sp = sp.with_ctxt(tt.span().ctxt());
 170                         tt.set_span(sp);
 171                         tt
 172                     });
 173                 }
 174
 175                 if cx.trace_macros() {
 176                     trace_macros_note(cx, sp, format!("to `{}`", tts));
 177                 }
 178
 179                 let directory = Directory {
 180                     path: Cow::from(cx.current_expansion.module.directory.as_path()),
 181                     ownership: cx.current_expansion.directory_ownership,
 182                 };
 183                 let mut p = Parser::new(cx.parse_sess(), tts, Some(directory), true, false);
 184                 p.root_module_name = cx.current_expansion.module.mod_path.last()
 185                     .map(|id| id.as_str().to_string());
 186
 187                 p.process_potential_macro_variable();
 188                 // Let the context choose how to interpret the result.
 189                 // Weird, but useful for X-macros.
 190                 return Box::new(ParserAnyMacro {
 191                     parser: p,
 192
 193                     // Pass along the original expansion site and the name of the macro
 194                     // so we can print a useful error message if the parse of the expanded
 195                     // macro leaves unparsed tokens.
 196                     site_span: sp,
 197                     macro_ident: name,
 198                     arm_span,
 199                 })
 200             }
 201             Failure(sp, tok, t) => if sp.lo() >= best_fail_spot.lo() {
 202                 best_fail_spot = sp;
 203                 best_fail_tok = Some(tok);
 204                 best_fail_text = Some(t);
 205             },
 206             Error(err_sp, ref msg) => {
 207                 cx.span_fatal(err_sp.substitute_dummy(sp), &msg[..])
 208             }
 209         }
 210     }
 211
 212     let best_fail_msg = parse_failure_msg(best_fail_tok.expect("ran no matchers"));
 213     let span = best_fail_spot.substitute_dummy(sp);
 214     let mut err = cx.struct_span_err(span, &best_fail_msg);
 215     err.span_label(span, best_fail_text.unwrap_or(best_fail_msg));
 216     if let Some(sp) = def_span {
 217         if cx.source_map().span_to_filename(sp).is_real() && !sp.is_dummy() {
 218             err.span_label(cx.source_map().def_span(sp), "when calling this macro");
 219         }
 220     }
 221
 222     // Check whether there's a missing comma in this macro call, like `println!("{}" a);`
 223     if let Some((arg, comma_span)) = arg.add_comma() {
 224         for lhs in lhses { // try each arm's matchers
 225             let lhs_tt = match *lhs {
 226                 quoted::TokenTree::Delimited(_, ref delim) => &delim.tts[..],
 227                 _ => continue,
 228             };
 229             match TokenTree::parse(cx, lhs_tt, arg.clone()) {
 230                 Success(_) => {
 231                     if comma_span.is_dummy() {
 232                         err.note("you might be missing a comma");
 233                     } else {
 234                         err.span_suggestion_short_with_applicability(
 235                             comma_span,
 236                             "missing comma here",
 237                             ", ".to_string(),
 238                             Applicability::MachineApplicable,
 239                         );
 240                     }
 241                 }
 242                 _ => {}
 243             }
 244         }
 245     }
 246     err.emit();
 247     cx.trace_macros_diag();
 248     DummyResult::any(sp)
 249 }
 250
 251 // Note that macro-by-example's input is also matched against a token tree:
 252 //                   $( $lhs:tt => $rhs:tt );+
 253 //
 254 // Holy self-referential!
 255
 256 /// Converts a `macro_rules!` invocation into a syntax extension.
 257 pub fn compile(sess: &ParseSess, features: &Features, def: &ast::Item, edition: Edition)
 258                -> SyntaxExtension {
 259     let lhs_nm = ast::Ident::with_empty_ctxt(Symbol::gensym("lhs"));
 260     let rhs_nm = ast::Ident::with_empty_ctxt(Symbol::gensym("rhs"));
 261
 262     // Parse the macro_rules! invocation
 263     let body = match def.node {
 264         ast::ItemKind::MacroDef(ref body) => body,
 265         _ => unreachable!(),
 266     };
 267
 268     // The pattern that macro_rules matches.
 269     // The grammar for macro_rules! is:
 270     // $( $lhs:tt => $rhs:tt );+
 271     // ...quasiquoting this would be nice.
 272     // These spans won't matter, anyways
 273     let argument_gram = vec![
 274         quoted::TokenTree::Sequence(DelimSpan::dummy(), Lrc::new(quoted::SequenceRepetition {
 275             tts: vec![
 276                 quoted::TokenTree::MetaVarDecl(DUMMY_SP, lhs_nm, ast::Ident::from_str("tt")),
 277                 quoted::TokenTree::Token(DUMMY_SP, token::FatArrow),
 278                 quoted::TokenTree::MetaVarDecl(DUMMY_SP, rhs_nm, ast::Ident::from_str("tt")),
 279             ],
 280             separator: Some(if body.legacy { token::Semi } else { token::Comma }),
 281             op: quoted::KleeneOp::OneOrMore,
 282             num_captures: 2,
 283         })),
 284         // to phase into semicolon-termination instead of semicolon-separation
 285         quoted::TokenTree::Sequence(DelimSpan::dummy(), Lrc::new(quoted::SequenceRepetition {
 286             tts: vec![quoted::TokenTree::Token(DUMMY_SP, token::Semi)],
 287             separator: None,
 288             op: quoted::KleeneOp::ZeroOrMore,
 289             num_captures: 0
 290         })),
 291     ];
 292
 293     let argument_map = match parse(sess, body.stream(), &argument_gram, None, true) {
 294         Success(m) => m,
 295         Failure(sp, tok, t) => {
 296             let s = parse_failure_msg(tok);
 297             let sp = sp.substitute_dummy(def.span);
 298             let mut err = sess.span_diagnostic.struct_span_fatal(sp, &s);
 299             err.span_label(sp, t);
 300             err.emit();
 301             FatalError.raise();
 302         }
 303         Error(sp, s) => {
 304             sess.span_diagnostic.span_fatal(sp.substitute_dummy(def.span), &s).raise();
 305         }
 306     };
 307
 308     let mut valid = true;
 309
 310     // Extract the arguments:
 311     let lhses = match *argument_map[&lhs_nm] {
 312         MatchedSeq(ref s, _) => {
 313             s.iter().map(|m| {
 314                 if let MatchedNonterminal(ref nt) = *m {
 315                     if let NtTT(ref tt) = **nt {
 316                         let tt = quoted::parse(
 317                             tt.clone().into(),
 318                             true,
 319                             sess,
 320                             features,
 321                             &def.attrs,
 322                             edition,
 323                             def.id,
 324                         )
 325                         .pop()
 326                         .unwrap();
 327                         valid &= check_lhs_nt_follows(sess, features, &def.attrs, &tt);
 328                         return tt;
 329                     }
 330                 }
 331                 sess.span_diagnostic.span_bug(def.span, "wrong-structured lhs")
 332             }).collect::<Vec<quoted::TokenTree>>()
 333         }
 334         _ => sess.span_diagnostic.span_bug(def.span, "wrong-structured lhs")
 335     };
 336
 337     let rhses = match *argument_map[&rhs_nm] {
 338         MatchedSeq(ref s, _) => {
 339             s.iter().map(|m| {
 340                 if let MatchedNonterminal(ref nt) = *m {
 341                     if let NtTT(ref tt) = **nt {
 342                         return quoted::parse(
 343                             tt.clone().into(),
 344                             false,
 345                             sess,
 346                             features,
 347                             &def.attrs,
 348                             edition,
 349                             def.id,
 350                         ).pop()
 351                          .unwrap();
 352                     }
 353                 }
 354                 sess.span_diagnostic.span_bug(def.span, "wrong-structured lhs")
 355             }).collect::<Vec<quoted::TokenTree>>()
 356         }
 357         _ => sess.span_diagnostic.span_bug(def.span, "wrong-structured rhs")
 358     };
 359
 360     for rhs in &rhses {
 361         valid &= check_rhs(sess, rhs);
 362     }
 363
 364     // don't abort iteration early, so that errors for multiple lhses can be reported
 365     for lhs in &lhses {
 366         valid &= check_lhs_no_empty_seq(sess, &[lhs.clone()])
 367     }
 368
 369     let expander: Box<_> = Box::new(MacroRulesMacroExpander {
 370         name: def.ident,
 371         lhses,
 372         rhses,
 373         valid,
 374     });
 375
 376     if body.legacy {
 377         let allow_internal_unstable = attr::contains_name(&def.attrs, "allow_internal_unstable");
 378         let allow_internal_unsafe = attr::contains_name(&def.attrs, "allow_internal_unsafe");
 379         let mut local_inner_macros = false;
 380         if let Some(macro_export) = attr::find_by_name(&def.attrs, "macro_export") {
 381             if let Some(l) = macro_export.meta_item_list() {
 382                 local_inner_macros = attr::list_contains_name(&l, "local_inner_macros");
 383             }
 384         }
 385
 386         let unstable_feature = attr::find_stability(&sess,
 387                                                     &def.attrs, def.span).and_then(|stability| {
 388             if let attr::StabilityLevel::Unstable { issue, .. } = stability.level {
 389                 Some((stability.feature, issue))
 390             } else {
 391                 None
 392             }
 393         });
 394
 395         NormalTT {
 396             expander,
 397             def_info: Some((def.id, def.span)),
 398             allow_internal_unstable,
 399             allow_internal_unsafe,
 400             local_inner_macros,
 401             unstable_feature,
 402             edition,
 403         }
 404     } else {
 405         let is_transparent = attr::contains_name(&def.attrs, "rustc_transparent_macro");
 406
 407         SyntaxExtension::DeclMacro {
 408             expander,
 409             def_info: Some((def.id, def.span)),
 410             is_transparent,
 411             edition,
 412         }
 413     }
 414 }
 415
 416 fn check_lhs_nt_follows(sess: &ParseSess,
 417                         features: &Features,
 418                         attrs: &[ast::Attribute],
 419                         lhs: &quoted::TokenTree) -> bool {
 420     // lhs is going to be like TokenTree::Delimited(...), where the
 421     // entire lhs is those tts. Or, it can be a "bare sequence", not wrapped in parens.
 422     if let quoted::TokenTree::Delimited(_, ref tts) = *lhs {
 423         check_matcher(sess, features, attrs, &tts.tts)
 424     } else {
 425         let msg = "invalid macro matcher; matchers must be contained in balanced delimiters";
 426         sess.span_diagnostic.span_err(lhs.span(), msg);
 427         false
 428     }
 429     // we don't abort on errors on rejection, the driver will do that for us
 430     // after parsing/expansion. we can report every error in every macro this way.
 431 }
 432
 433 /// Check that the lhs contains no repetition which could match an empty token
 434 /// tree, because then the matcher would hang indefinitely.
 435 fn check_lhs_no_empty_seq(sess: &ParseSess, tts: &[quoted::TokenTree]) -> bool {
 436     use self::quoted::TokenTree;
 437     for tt in tts {
 438         match *tt {
 439             TokenTree::Token(..) | TokenTree::MetaVar(..) | TokenTree::MetaVarDecl(..) => (),
 440             TokenTree::Delimited(_, ref del) => if !check_lhs_no_empty_seq(sess, &del.tts) {
 441                 return false;
 442             },
 443             TokenTree::Sequence(span, ref seq) => {
 444                 if seq.separator.is_none() && seq.tts.iter().all(|seq_tt| {
 445                     match *seq_tt {
 446                         TokenTree::MetaVarDecl(_, _, id) => id.name == "vis",
 447                         TokenTree::Sequence(_, ref sub_seq) =>
 448                             sub_seq.op == quoted::KleeneOp::ZeroOrMore,
 449                         _ => false,
 450                     }
 451                 }) {
 452                     let sp = span.entire();
 453                     sess.span_diagnostic.span_err(sp, "repetition matches empty token tree");
 454                     return false;
 455                 }
 456                 if !check_lhs_no_empty_seq(sess, &seq.tts) {
 457                     return false;
 458                 }
 459             }
 460         }
 461     }
 462
 463     true
 464 }
 465
 466 fn check_rhs(sess: &ParseSess, rhs: &quoted::TokenTree) -> bool {
 467     match *rhs {
 468         quoted::TokenTree::Delimited(..) => return true,
 469         _ => sess.span_diagnostic.span_err(rhs.span(), "macro rhs must be delimited")
 470     }
 471     false
 472 }
 473
 474 fn check_matcher(sess: &ParseSess,
 475                  features: &Features,
 476                  attrs: &[ast::Attribute],
 477                  matcher: &[quoted::TokenTree]) -> bool {
 478     let first_sets = FirstSets::new(matcher);
 479     let empty_suffix = TokenSet::empty();
 480     let err = sess.span_diagnostic.err_count();
 481     check_matcher_core(sess, features, attrs, &first_sets, matcher, &empty_suffix);
 482     err == sess.span_diagnostic.err_count()
 483 }
 484
 485 // The FirstSets for a matcher is a mapping from subsequences in the
 486 // matcher to the FIRST set for that subsequence.
 487 //
 488 // This mapping is partially precomputed via a backwards scan over the
 489 // token trees of the matcher, which provides a mapping from each
 490 // repetition sequence to its FIRST set.
 491 //
 492 // (Hypothetically sequences should be uniquely identifiable via their
 493 // spans, though perhaps that is false e.g. for macro-generated macros
 494 // that do not try to inject artificial span information. My plan is
 495 // to try to catch such cases ahead of time and not include them in
 496 // the precomputed mapping.)
 497 struct FirstSets {
 498     // this maps each TokenTree::Sequence `$(tt ...) SEP OP` that is uniquely identified by its
 499     // span in the original matcher to the First set for the inner sequence `tt ...`.
 500     //
 501     // If two sequences have the same span in a matcher, then map that
 502     // span to None (invalidating the mapping here and forcing the code to
 503     // use a slow path).
 504     first: FxHashMap<Span, Option<TokenSet>>,
 505 }
 506
 507 impl FirstSets {
 508     fn new(tts: &[quoted::TokenTree]) -> FirstSets {
 509         use self::quoted::TokenTree;
 510
 511         let mut sets = FirstSets { first: FxHashMap::default() };
 512         build_recur(&mut sets, tts);
 513         return sets;
 514
 515         // walks backward over `tts`, returning the FIRST for `tts`
 516         // and updating `sets` at the same time for all sequence
 517         // substructure we find within `tts`.
 518         fn build_recur(sets: &mut FirstSets, tts: &[TokenTree]) -> TokenSet {
 519             let mut first = TokenSet::empty();
 520             for tt in tts.iter().rev() {
 521                 match *tt {
 522                     TokenTree::Token(..) | TokenTree::MetaVar(..) | TokenTree::MetaVarDecl(..) => {
 523                         first.replace_with(tt.clone());
 524                     }
 525                     TokenTree::Delimited(span, ref delimited) => {
 526                         build_recur(sets, &delimited.tts[..]);
 527                         first.replace_with(delimited.open_tt(span.open));
 528                     }
 529                     TokenTree::Sequence(sp, ref seq_rep) => {
 530                         let subfirst = build_recur(sets, &seq_rep.tts[..]);
 531
 532                         match sets.first.entry(sp.entire()) {
 533                             Entry::Vacant(vac) => {
 534                                 vac.insert(Some(subfirst.clone()));
 535                             }
 536                             Entry::Occupied(mut occ) => {
 537                                 // if there is already an entry, then a span must have collided.
 538                                 // This should not happen with typical macro_rules macros,
 539                                 // but syntax extensions need not maintain distinct spans,
 540                                 // so distinct syntax trees can be assigned the same span.
 541                                 // In such a case, the map cannot be trusted; so mark this
 542                                 // entry as unusable.
 543                                 occ.insert(None);
 544                             }
 545                         }
 546
 547                         // If the sequence contents can be empty, then the first
 548                         // token could be the separator token itself.
 549
 550                         if let (Some(ref sep), true) = (seq_rep.separator.clone(),
 551                                                         subfirst.maybe_empty) {
 552                             first.add_one_maybe(TokenTree::Token(sp.entire(), sep.clone()));
 553                         }
 554
 555                         // Reverse scan: Sequence comes before `first`.
 556                         if subfirst.maybe_empty || seq_rep.op == quoted::KleeneOp::ZeroOrMore {
 557                             // If sequence is potentially empty, then
 558                             // union them (preserving first emptiness).
 559                             first.add_all(&TokenSet { maybe_empty: true, ..subfirst });
 560                         } else {
 561                             // Otherwise, sequence guaranteed
 562                             // non-empty; replace first.
 563                             first = subfirst;
 564                         }
 565                     }
 566                 }
 567             }
 568
 569             first
 570         }
 571     }
 572
 573     // walks forward over `tts` until all potential FIRST tokens are
 574     // identified.
 575     fn first(&self, tts: &[quoted::TokenTree]) -> TokenSet {
 576         use self::quoted::TokenTree;
 577
 578         let mut first = TokenSet::empty();
 579         for tt in tts.iter() {
 580             assert!(first.maybe_empty);
 581             match *tt {
 582                 TokenTree::Token(..) | TokenTree::MetaVar(..) | TokenTree::MetaVarDecl(..) => {
 583                     first.add_one(tt.clone());
 584                     return first;
 585                 }
 586                 TokenTree::Delimited(span, ref delimited) => {
 587                     first.add_one(delimited.open_tt(span.open));
 588                     return first;
 589                 }
 590                 TokenTree::Sequence(sp, ref seq_rep) => {
 591                     match self.first.get(&sp.entire()) {
 592                         Some(&Some(ref subfirst)) => {
 593
 594                             // If the sequence contents can be empty, then the first
 595                             // token could be the separator token itself.
 596
 597                             if let (Some(ref sep), true) = (seq_rep.separator.clone(),
 598                                                             subfirst.maybe_empty) {
 599                                 first.add_one_maybe(TokenTree::Token(sp.entire(), sep.clone()));
 600                             }
 601
 602                             assert!(first.maybe_empty);
 603                             first.add_all(subfirst);
 604                             if subfirst.maybe_empty ||
 605                                seq_rep.op == quoted::KleeneOp::ZeroOrMore {
 606                                 // continue scanning for more first
 607                                 // tokens, but also make sure we
 608                                 // restore empty-tracking state
 609                                 first.maybe_empty = true;
 610                                 continue;
 611                             } else {
 612                                 return first;
 613                             }
 614                         }
 615
 616                         Some(&None) => {
 617                             panic!("assume all sequences have (unique) spans for now");
 618                         }
 619
 620                         None => {
 621                             panic!("We missed a sequence during FirstSets construction");
 622                         }
 623                     }
 624                 }
 625             }
 626         }
 627
 628         // we only exit the loop if `tts` was empty or if every
 629         // element of `tts` matches the empty sequence.
 630         assert!(first.maybe_empty);
 631         first
 632     }
 633 }
 634
 635 // A set of `quoted::TokenTree`s, which may include `TokenTree::Match`s
 636 // (for macro-by-example syntactic variables). It also carries the
 637 // `maybe_empty` flag; that is true if and only if the matcher can
 638 // match an empty token sequence.
 639 //
 640 // The First set is computed on submatchers like `$($a:expr b),* $(c)* d`,
 641 // which has corresponding FIRST = {$a:expr, c, d}.
 642 // Likewise, `$($a:expr b),* $(c)+ d` has FIRST = {$a:expr, c}.
 643 //
 644 // (Notably, we must allow for *-op to occur zero times.)
 645 #[derive(Clone, Debug)]
 646 struct TokenSet {
 647     tokens: Vec<quoted::TokenTree>,
 648     maybe_empty: bool,
 649 }
 650
 651 impl TokenSet {
 652     // Returns a set for the empty sequence.
 653     fn empty() -> Self { TokenSet { tokens: Vec::new(), maybe_empty: true } }
 654
 655     // Returns the set `{ tok }` for the single-token (and thus
 656     // non-empty) sequence [tok].
 657     fn singleton(tok: quoted::TokenTree) -> Self {
 658         TokenSet { tokens: vec![tok], maybe_empty: false }
 659     }
 660
 661     // Changes self to be the set `{ tok }`.
 662     // Since `tok` is always present, marks self as non-empty.
 663     fn replace_with(&mut self, tok: quoted::TokenTree) {
 664         self.tokens.clear();
 665         self.tokens.push(tok);
 666         self.maybe_empty = false;
 667     }
 668
 669     // Changes self to be the empty set `{}`; meant for use when
 670     // the particular token does not matter, but we want to
 671     // record that it occurs.
 672     fn replace_with_irrelevant(&mut self) {
 673         self.tokens.clear();
 674         self.maybe_empty = false;
 675     }
 676
 677     // Adds `tok` to the set for `self`, marking sequence as non-empy.
 678     fn add_one(&mut self, tok: quoted::TokenTree) {
 679         if !self.tokens.contains(&tok) {
 680             self.tokens.push(tok);
 681         }
 682         self.maybe_empty = false;
 683     }
 684
 685     // Adds `tok` to the set for `self`. (Leaves `maybe_empty` flag alone.)
 686     fn add_one_maybe(&mut self, tok: quoted::TokenTree) {
 687         if !self.tokens.contains(&tok) {
 688             self.tokens.push(tok);
 689         }
 690     }
 691
 692     // Adds all elements of `other` to this.
 693     //
 694     // (Since this is a set, we filter out duplicates.)
 695     //
 696     // If `other` is potentially empty, then preserves the previous
 697     // setting of the empty flag of `self`. If `other` is guaranteed
 698     // non-empty, then `self` is marked non-empty.
 699     fn add_all(&mut self, other: &Self) {
 700         for tok in &other.tokens {
 701             if !self.tokens.contains(tok) {
 702                 self.tokens.push(tok.clone());
 703             }
 704         }
 705         if !other.maybe_empty {
 706             self.maybe_empty = false;
 707         }
 708     }
 709 }
 710
 711 // Checks that `matcher` is internally consistent and that it
 712 // can legally by followed by a token N, for all N in `follow`.
 713 // (If `follow` is empty, then it imposes no constraint on
 714 // the `matcher`.)
 715 //
 716 // Returns the set of NT tokens that could possibly come last in
 717 // `matcher`. (If `matcher` matches the empty sequence, then
 718 // `maybe_empty` will be set to true.)
 719 //
 720 // Requires that `first_sets` is pre-computed for `matcher`;
 721 // see `FirstSets::new`.
 722 fn check_matcher_core(sess: &ParseSess,
 723                       features: &Features,
 724                       attrs: &[ast::Attribute],
 725                       first_sets: &FirstSets,
 726                       matcher: &[quoted::TokenTree],
 727                       follow: &TokenSet) -> TokenSet {
 728     use self::quoted::TokenTree;
 729
 730     let mut last = TokenSet::empty();
 731
 732     // 2. For each token and suffix  [T, SUFFIX] in M:
 733     // ensure that T can be followed by SUFFIX, and if SUFFIX may be empty,
 734     // then ensure T can also be followed by any element of FOLLOW.
 735     'each_token: for i in 0..matcher.len() {
 736         let token = &matcher[i];
 737         let suffix = &matcher[i+1..];
 738
 739         let build_suffix_first = || {
 740             let mut s = first_sets.first(suffix);
 741             if s.maybe_empty { s.add_all(follow); }
 742             s
 743         };
 744
 745         // (we build `suffix_first` on demand below; you can tell
 746         // which cases are supposed to fall through by looking for the
 747         // initialization of this variable.)
 748         let suffix_first;
 749
 750         // First, update `last` so that it corresponds to the set
 751         // of NT tokens that might end the sequence `... token`.
 752         match *token {
 753             TokenTree::Token(..) | TokenTree::MetaVar(..) | TokenTree::MetaVarDecl(..) => {
 754                 let can_be_followed_by_any;
 755                 if let Err(bad_frag) = has_legal_fragment_specifier(sess, features, attrs, token) {
 756                     let msg = format!("invalid fragment specifier `{}`", bad_frag);
 757                     sess.span_diagnostic.struct_span_err(token.span(), &msg)
 758                         .help(VALID_FRAGMENT_NAMES_MSG)
 759                         .emit();
 760                     // (This eliminates false positives and duplicates
 761                     // from error messages.)
 762                     can_be_followed_by_any = true;
 763                 } else {
 764                     can_be_followed_by_any = token_can_be_followed_by_any(token);
 765                 }
 766
 767                 if can_be_followed_by_any {
 768                     // don't need to track tokens that work with any,
 769                     last.replace_with_irrelevant();
 770                     // ... and don't need to check tokens that can be
 771                     // followed by anything against SUFFIX.
 772                     continue 'each_token;
 773                 } else {
 774                     last.replace_with(token.clone());
 775                     suffix_first = build_suffix_first();
 776                 }
 777             }
 778             TokenTree::Delimited(span, ref d) => {
 779                 let my_suffix = TokenSet::singleton(d.close_tt(span.close));
 780                 check_matcher_core(sess, features, attrs, first_sets, &d.tts, &my_suffix);
 781                 // don't track non NT tokens
 782                 last.replace_with_irrelevant();
 783
 784                 // also, we don't need to check delimited sequences
 785                 // against SUFFIX
 786                 continue 'each_token;
 787             }
 788             TokenTree::Sequence(sp, ref seq_rep) => {
 789                 suffix_first = build_suffix_first();
 790                 // The trick here: when we check the interior, we want
 791                 // to include the separator (if any) as a potential
 792                 // (but not guaranteed) element of FOLLOW. So in that
 793                 // case, we make a temp copy of suffix and stuff
 794                 // delimiter in there.
 795                 //
 796                 // FIXME: Should I first scan suffix_first to see if
 797                 // delimiter is already in it before I go through the
 798                 // work of cloning it? But then again, this way I may
 799                 // get a "tighter" span?
 800                 let mut new;
 801                 let my_suffix = if let Some(ref u) = seq_rep.separator {
 802                     new = suffix_first.clone();
 803                     new.add_one_maybe(TokenTree::Token(sp.entire(), u.clone()));
 804                     &new
 805                 } else {
 806                     &suffix_first
 807                 };
 808
 809                 // At this point, `suffix_first` is built, and
 810                 // `my_suffix` is some TokenSet that we can use
 811                 // for checking the interior of `seq_rep`.
 812                 let next = check_matcher_core(sess,
 813                                               features,
 814                                               attrs,
 815                                               first_sets,
 816                                               &seq_rep.tts,
 817                                               my_suffix);
 818                 if next.maybe_empty {
 819                     last.add_all(&next);
 820                 } else {
 821                     last = next;
 822                 }
 823
 824                 // the recursive call to check_matcher_core already ran the 'each_last
 825                 // check below, so we can just keep going forward here.
 826                 continue 'each_token;
 827             }
 828         }
 829
 830         // (`suffix_first` guaranteed initialized once reaching here.)
 831
 832         // Now `last` holds the complete set of NT tokens that could
 833         // end the sequence before SUFFIX. Check that every one works with `suffix`.
 834         'each_last: for token in &last.tokens {
 835             if let TokenTree::MetaVarDecl(_, ref name, ref frag_spec) = *token {
 836                 for next_token in &suffix_first.tokens {
 837                     match is_in_follow(next_token, &frag_spec.as_str()) {
 838                         IsInFollow::Invalid(msg, help) => {
 839                             sess.span_diagnostic.struct_span_err(next_token.span(), &msg)
 840                                 .help(help).emit();
 841                             // don't bother reporting every source of
 842                             // conflict for a particular element of `last`.
 843                             continue 'each_last;
 844                         }
 845                         IsInFollow::Yes => {}
 846                         IsInFollow::No(ref possible) => {
 847                             let may_be = if last.tokens.len() == 1 &&
 848                                 suffix_first.tokens.len() == 1
 849                             {
 850                                 "is"
 851                             } else {
 852                                 "may be"
 853                             };
 854
 855                             let sp = next_token.span();
 856                             let mut err = sess.span_diagnostic.struct_span_err(
 857                                 sp,
 858                                 &format!("`${name}:{frag}` {may_be} followed by `{next}`, which \
 859                                           is not allowed for `{frag}` fragments",
 860                                          name=name,
 861                                          frag=frag_spec,
 862                                          next=quoted_tt_to_string(next_token),
 863                                          may_be=may_be),
 864                             );
 865                             err.span_label(
 866                                 sp,
 867                                 format!("not allowed after `{}` fragments", frag_spec),
 868                             );
 869                             let msg = "allowed there are: ";
 870                             match &possible[..] {
 871                                 &[] => {}
 872                                 &[t] => {
 873                                     err.note(&format!(
 874                                         "only {} is allowed after `{}` fragments",
 875                                         t,
 876                                         frag_spec,
 877                                     ));
 878                                 }
 879                                 ts => {
 880                                     err.note(&format!(
 881                                         "{}{} or {}",
 882                                         msg,
 883                                         ts[..ts.len() - 1].iter().map(|s| *s)
 884                                             .collect::<Vec<_>>().join(", "),
 885                                         ts[ts.len() - 1],
 886                                     ));
 887                                 }
 888                             }
 889                             err.emit();
 890                         }
 891                     }
 892                 }
 893             }
 894         }
 895     }
 896     last
 897 }
 898
 899 fn token_can_be_followed_by_any(tok: &quoted::TokenTree) -> bool {
 900     if let quoted::TokenTree::MetaVarDecl(_, _, frag_spec) = *tok {
 901         frag_can_be_followed_by_any(&frag_spec.as_str())
 902     } else {
 903         // (Non NT's can always be followed by anthing in matchers.)
 904         true
 905     }
 906 }
 907
 908 /// True if a fragment of type `frag` can be followed by any sort of
 909 /// token.  We use this (among other things) as a useful approximation
 910 /// for when `frag` can be followed by a repetition like `$(...)*` or
 911 /// `$(...)+`. In general, these can be a bit tricky to reason about,
 912 /// so we adopt a conservative position that says that any fragment
 913 /// specifier which consumes at most one token tree can be followed by
 914 /// a fragment specifier (indeed, these fragments can be followed by
 915 /// ANYTHING without fear of future compatibility hazards).
 916 fn frag_can_be_followed_by_any(frag: &str) -> bool {
 917     match frag {
 918         "item"     | // always terminated by `}` or `;`
 919         "block"    | // exactly one token tree
 920         "ident"    | // exactly one token tree
 921         "literal"  | // exactly one token tree
 922         "meta"     | // exactly one token tree
 923         "lifetime" | // exactly one token tree
 924         "tt" =>   // exactly one token tree
 925             true,
 926
 927         _ =>
 928             false,
 929     }
 930 }
 931
 932 enum IsInFollow {
 933     Yes,
 934     No(Vec<&'static str>),
 935     Invalid(String, &'static str),
 936 }
 937
 938 /// True if `frag` can legally be followed by the token `tok`. For
 939 /// fragments that can consume an unbounded number of tokens, `tok`
 940 /// must be within a well-defined follow set. This is intended to
 941 /// guarantee future compatibility: for example, without this rule, if
 942 /// we expanded `expr` to include a new binary operator, we might
 943 /// break macros that were relying on that binary operator as a
 944 /// separator.
 945 // when changing this do not forget to update doc/book/macros.md!
 946 fn is_in_follow(tok: &quoted::TokenTree, frag: &str) -> IsInFollow {
 947     use self::quoted::TokenTree;
 948
 949     if let TokenTree::Token(_, token::CloseDelim(_)) = *tok {
 950         // closing a token tree can never be matched by any fragment;
 951         // iow, we always require that `(` and `)` match, etc.
 952         IsInFollow::Yes
 953     } else {
 954         match frag {
 955             "item" => {
 956                 // since items *must* be followed by either a `;` or a `}`, we can
 957                 // accept anything after them
 958                 IsInFollow::Yes
 959             },
 960             "block" => {
 961                 // anything can follow block, the braces provide an easy boundary to
 962                 // maintain
 963                 IsInFollow::Yes
 964             },
 965             "stmt" | "expr"  => {
 966                 let tokens = vec!["`=>`", "`,`", "`;`"];
 967                 match *tok {
 968                     TokenTree::Token(_, ref tok) => match *tok {
 969                         FatArrow | Comma | Semi => IsInFollow::Yes,
 970                         _ => IsInFollow::No(tokens),
 971                     },
 972                     _ => IsInFollow::No(tokens),
 973                 }
 974             },
 975             "pat" => {
 976                 let tokens = vec!["`=>`", "`,`", "`=`", "`|`", "`if`", "`in`"];
 977                 match *tok {
 978                     TokenTree::Token(_, ref tok) => match *tok {
 979                         FatArrow | Comma | Eq | BinOp(token::Or) => IsInFollow::Yes,
 980                         Ident(i, false) if i.name == "if" || i.name == "in" => IsInFollow::Yes,
 981                         _ => IsInFollow::No(tokens),
 982                     },
 983                     _ => IsInFollow::No(tokens),
 984                 }
 985             },
 986             "path" | "ty" => {
 987                 let tokens = vec![
 988                     "`{`", "`[`", "`=>`", "`,`", "`>`","`=`", "`:`", "`;`", "`|`", "`as`",
 989                     "`where`",
 990                 ];
 991                 match *tok {
 992                     TokenTree::Token(_, ref tok) => match *tok {
 993                         OpenDelim(token::DelimToken::Brace) |
 994                         OpenDelim(token::DelimToken::Bracket) |
 995                         Comma | FatArrow | Colon | Eq | Gt | BinOp(token::Shr) | Semi |
 996                         BinOp(token::Or) => IsInFollow::Yes,
 997                         Ident(i, false) if i.name == "as" || i.name == "where" => IsInFollow::Yes,
 998                         _ => IsInFollow::No(tokens),
 999                     },
1000                     TokenTree::MetaVarDecl(_, _, frag) if frag.name == "block" => IsInFollow::Yes,
1001                     _ => IsInFollow::No(tokens),
1002                 }
1003             },
1004             "ident" | "lifetime" => {
1005                 // being a single token, idents and lifetimes are harmless
1006                 IsInFollow::Yes
1007             },
1008             "literal" => {
1009                 // literals may be of a single token, or two tokens (negative numbers)
1010                 IsInFollow::Yes
1011             },
1012             "meta" | "tt" => {
1013                 // being either a single token or a delimited sequence, tt is
1014                 // harmless
1015                 IsInFollow::Yes
1016             },
1017             "vis" => {
1018                 // Explicitly disallow `priv`, on the off chance it comes back.
1019                 let tokens = vec!["`,`", "an ident", "a type"];
1020                 match *tok {
1021                     TokenTree::Token(_, ref tok) => match *tok {
1022                         Comma => IsInFollow::Yes,
1023                         Ident(i, is_raw) if is_raw || i.name != "priv" => IsInFollow::Yes,
1024                         ref tok => if tok.can_begin_type() {
1025                             IsInFollow::Yes
1026                         } else {
1027                             IsInFollow::No(tokens)
1028                         }
1029                     },
1030                     TokenTree::MetaVarDecl(_, _, frag) if frag.name == "ident"
1031                                                        || frag.name == "ty"
1032                                                        || frag.name == "path" => IsInFollow::Yes,
1033                     _ => IsInFollow::No(tokens),
1034                 }
1035             },
1036             "" => IsInFollow::Yes, // keywords::Invalid
1037             _ => IsInFollow::Invalid(format!("invalid fragment specifier `{}`", frag),
1038                                      VALID_FRAGMENT_NAMES_MSG),
1039         }
1040     }
1041 }
1042
1043 fn has_legal_fragment_specifier(sess: &ParseSess,
1044                                 features: &Features,
1045                                 attrs: &[ast::Attribute],
1046                                 tok: &quoted::TokenTree) -> Result<(), String> {
1047     debug!("has_legal_fragment_specifier({:?})", tok);
1048     if let quoted::TokenTree::MetaVarDecl(_, _, ref frag_spec) = *tok {
1049         let frag_name = frag_spec.as_str();
1050         let frag_span = tok.span();
1051         if !is_legal_fragment_specifier(sess, features, attrs, &frag_name, frag_span) {
1052             return Err(frag_name.to_string());
1053         }
1054     }
1055     Ok(())
1056 }
1057
1058 fn is_legal_fragment_specifier(_sess: &ParseSess,
1059                                _features: &Features,
1060                                _attrs: &[ast::Attribute],
1061                                frag_name: &str,
1062                                _frag_span: Span) -> bool {
1063     /*
1064      * If new fragment specifiers are invented in nightly, `_sess`,
1065      * `_features`, `_attrs`, and `_frag_span` will be useful here
1066      * for checking against feature gates. See past versions of
1067      * this function.
1068      */
1069     match frag_name {
1070         "item" | "block" | "stmt" | "expr" | "pat" | "lifetime" |
1071         "path" | "ty" | "ident" | "meta" | "tt" | "vis" | "literal" |
1072         "" => true,
1073         _ => false,
1074     }
1075 }
1076
1077 fn quoted_tt_to_string(tt: &quoted::TokenTree) -> String {
1078     match *tt {
1079         quoted::TokenTree::Token(_, ref tok) => ::print::pprust::token_to_string(tok),
1080         quoted::TokenTree::MetaVar(_, name) => format!("${}", name),
1081         quoted::TokenTree::MetaVarDecl(_, name, kind) => format!("${}:{}", name, kind),
1082         _ => panic!("unexpected quoted::TokenTree::{{Sequence or Delimited}} \
1083                      in follow set checker"),
1084     }
1085 }