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