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 fn trace_macros_note(cx: &mut ExtCtxt, sp: Span, message: String) {
  88     let sp = sp.macro_backtrace().last().map(|trace| trace.call_site).unwrap_or(sp);
  89     let mut values: &mut Vec<String> = cx.expansions.entry(sp).or_insert_with(Vec::new);
  90     values.push(message);
  91 }
  92
  93 /// Given `lhses` and `rhses`, this is the new macro we create
  94 fn generic_extension<'cx>(cx: &'cx mut ExtCtxt,
  95                           sp: Span,
  96                           name: ast::Ident,
  97                           arg: TokenStream,
  98                           lhses: &[quoted::TokenTree],
  99                           rhses: &[quoted::TokenTree])
 100                           -> Box<MacResult+'cx> {
 101     if cx.trace_macros() {
 102         trace_macros_note(cx, sp, format!("expanding `{}! {{ {} }}`", name, arg));
 103     }
 104
 105     // Which arm's failure should we report? (the one furthest along)
 106     let mut best_fail_spot = DUMMY_SP;
 107     let mut best_fail_tok = None;
 108
 109     for (i, lhs) in lhses.iter().enumerate() { // try each arm's matchers
 110         let lhs_tt = match *lhs {
 111             quoted::TokenTree::Delimited(_, ref delim) => &delim.tts[..],
 112             _ => cx.span_bug(sp, "malformed macro lhs")
 113         };
 114
 115         match TokenTree::parse(cx, lhs_tt, arg.clone()) {
 116             Success(named_matches) => {
 117                 let rhs = match rhses[i] {
 118                     // ignore delimiters
 119                     quoted::TokenTree::Delimited(_, ref delimed) => delimed.tts.clone(),
 120                     _ => cx.span_bug(sp, "malformed macro rhs"),
 121                 };
 122                 // rhs has holes ( `$id` and `$(...)` that need filled)
 123                 let tts = transcribe(&cx.parse_sess.span_diagnostic, Some(named_matches), rhs);
 124
 125                 if cx.trace_macros() {
 126                     trace_macros_note(cx, sp, format!("to `{}`", tts));
 127                 }
 128
 129                 let directory = Directory {
 130                     path: cx.current_expansion.module.directory.clone(),
 131                     ownership: cx.current_expansion.directory_ownership,
 132                 };
 133                 let mut p = Parser::new(cx.parse_sess(), tts, Some(directory), true, false);
 134                 p.root_module_name = cx.current_expansion.module.mod_path.last()
 135                     .map(|id| id.name.as_str().to_string());
 136
 137                 p.process_potential_macro_variable();
 138                 // Let the context choose how to interpret the result.
 139                 // Weird, but useful for X-macros.
 140                 return Box::new(ParserAnyMacro {
 141                     parser: p,
 142
 143                     // Pass along the original expansion site and the name of the macro
 144                     // so we can print a useful error message if the parse of the expanded
 145                     // macro leaves unparsed tokens.
 146                     site_span: sp,
 147                     macro_ident: name
 148                 })
 149             }
 150             Failure(sp, tok) => if sp.lo >= best_fail_spot.lo {
 151                 best_fail_spot = sp;
 152                 best_fail_tok = Some(tok);
 153             },
 154             Error(err_sp, ref msg) => {
 155                 cx.span_fatal(err_sp.substitute_dummy(sp), &msg[..])
 156             }
 157         }
 158     }
 159
 160     let best_fail_msg = parse_failure_msg(best_fail_tok.expect("ran no matchers"));
 161     cx.span_fatal(best_fail_spot.substitute_dummy(sp), &best_fail_msg);
 162 }
 163
 164 // Note that macro-by-example's input is also matched against a token tree:
 165 //                   $( $lhs:tt => $rhs:tt );+
 166 //
 167 // Holy self-referential!
 168
 169 /// Converts a `macro_rules!` invocation into a syntax extension.
 170 pub fn compile(sess: &ParseSess, features: &RefCell<Features>, def: &ast::Item) -> SyntaxExtension {
 171     let lhs_nm = ast::Ident::with_empty_ctxt(Symbol::gensym("lhs"));
 172     let rhs_nm = ast::Ident::with_empty_ctxt(Symbol::gensym("rhs"));
 173
 174     // Parse the macro_rules! invocation
 175     let body = match def.node {
 176         ast::ItemKind::MacroDef(ref body) => body,
 177         _ => unreachable!(),
 178     };
 179
 180     // The pattern that macro_rules matches.
 181     // The grammar for macro_rules! is:
 182     // $( $lhs:tt => $rhs:tt );+
 183     // ...quasiquoting this would be nice.
 184     // These spans won't matter, anyways
 185     let argument_gram = vec![
 186         quoted::TokenTree::Sequence(DUMMY_SP, Rc::new(quoted::SequenceRepetition {
 187             tts: vec![
 188                 quoted::TokenTree::MetaVarDecl(DUMMY_SP, lhs_nm, ast::Ident::from_str("tt")),
 189                 quoted::TokenTree::Token(DUMMY_SP, token::FatArrow),
 190                 quoted::TokenTree::MetaVarDecl(DUMMY_SP, rhs_nm, ast::Ident::from_str("tt")),
 191             ],
 192             separator: Some(if body.legacy { token::Semi } else { token::Comma }),
 193             op: quoted::KleeneOp::OneOrMore,
 194             num_captures: 2,
 195         })),
 196         // to phase into semicolon-termination instead of semicolon-separation
 197         quoted::TokenTree::Sequence(DUMMY_SP, Rc::new(quoted::SequenceRepetition {
 198             tts: vec![quoted::TokenTree::Token(DUMMY_SP, token::Semi)],
 199             separator: None,
 200             op: quoted::KleeneOp::ZeroOrMore,
 201             num_captures: 0
 202         })),
 203     ];
 204
 205     let argument_map = match parse(sess, body.stream(), &argument_gram, None, true) {
 206         Success(m) => m,
 207         Failure(sp, tok) => {
 208             let s = parse_failure_msg(tok);
 209             panic!(sess.span_diagnostic.span_fatal(sp.substitute_dummy(def.span), &s));
 210         }
 211         Error(sp, s) => {
 212             panic!(sess.span_diagnostic.span_fatal(sp.substitute_dummy(def.span), &s));
 213         }
 214     };
 215
 216     let mut valid = true;
 217
 218     // Extract the arguments:
 219     let lhses = match *argument_map[&lhs_nm] {
 220         MatchedSeq(ref s, _) => {
 221             s.iter().map(|m| {
 222                 if let MatchedNonterminal(ref nt) = **m {
 223                     if let NtTT(ref tt) = **nt {
 224                         let tt = quoted::parse(tt.clone().into(), true, sess).pop().unwrap();
 225                         valid &= check_lhs_nt_follows(sess, features, &tt);
 226                         return tt;
 227                     }
 228                 }
 229                 sess.span_diagnostic.span_bug(def.span, "wrong-structured lhs")
 230             }).collect::<Vec<quoted::TokenTree>>()
 231         }
 232         _ => sess.span_diagnostic.span_bug(def.span, "wrong-structured lhs")
 233     };
 234
 235     let rhses = match *argument_map[&rhs_nm] {
 236         MatchedSeq(ref s, _) => {
 237             s.iter().map(|m| {
 238                 if let MatchedNonterminal(ref nt) = **m {
 239                     if let NtTT(ref tt) = **nt {
 240                         return quoted::parse(tt.clone().into(), false, sess).pop().unwrap();
 241                     }
 242                 }
 243                 sess.span_diagnostic.span_bug(def.span, "wrong-structured lhs")
 244             }).collect::<Vec<quoted::TokenTree>>()
 245         }
 246         _ => sess.span_diagnostic.span_bug(def.span, "wrong-structured rhs")
 247     };
 248
 249     for rhs in &rhses {
 250         valid &= check_rhs(sess, rhs);
 251     }
 252
 253     // don't abort iteration early, so that errors for multiple lhses can be reported
 254     for lhs in &lhses {
 255         valid &= check_lhs_no_empty_seq(sess, &[lhs.clone()])
 256     }
 257
 258     let exp: Box<_> = Box::new(MacroRulesMacroExpander {
 259         name: def.ident,
 260         lhses: lhses,
 261         rhses: rhses,
 262         valid: valid,
 263     });
 264
 265     if body.legacy {
 266         let allow_internal_unstable = attr::contains_name(&def.attrs, "allow_internal_unstable");
 267         NormalTT(exp, Some((def.id, def.span)), allow_internal_unstable)
 268     } else {
 269         SyntaxExtension::DeclMacro(exp, Some(def.span))
 270     }
 271 }
 272
 273 fn check_lhs_nt_follows(sess: &ParseSess,
 274                         features: &RefCell<Features>,
 275                         lhs: &quoted::TokenTree) -> bool {
 276     // lhs is going to be like TokenTree::Delimited(...), where the
 277     // entire lhs is those tts. Or, it can be a "bare sequence", not wrapped in parens.
 278     if let quoted::TokenTree::Delimited(_, ref tts) = *lhs {
 279         check_matcher(sess, features, &tts.tts)
 280     } else {
 281         let msg = "invalid macro matcher; matchers must be contained in balanced delimiters";
 282         sess.span_diagnostic.span_err(lhs.span(), msg);
 283         false
 284     }
 285     // we don't abort on errors on rejection, the driver will do that for us
 286     // after parsing/expansion. we can report every error in every macro this way.
 287 }
 288
 289 /// Check that the lhs contains no repetition which could match an empty token
 290 /// tree, because then the matcher would hang indefinitely.
 291 fn check_lhs_no_empty_seq(sess: &ParseSess, tts: &[quoted::TokenTree]) -> bool {
 292     use self::quoted::TokenTree;
 293     for tt in tts {
 294         match *tt {
 295             TokenTree::Token(..) | TokenTree::MetaVarDecl(..) => (),
 296             TokenTree::Delimited(_, ref del) => if !check_lhs_no_empty_seq(sess, &del.tts) {
 297                 return false;
 298             },
 299             TokenTree::Sequence(span, ref seq) => {
 300                 if seq.separator.is_none() && seq.tts.iter().all(|seq_tt| {
 301                     match *seq_tt {
 302                         TokenTree::Sequence(_, ref sub_seq) =>
 303                             sub_seq.op == quoted::KleeneOp::ZeroOrMore,
 304                         _ => false,
 305                     }
 306                 }) {
 307                     sess.span_diagnostic.span_err(span, "repetition matches empty token tree");
 308                     return false;
 309                 }
 310                 if !check_lhs_no_empty_seq(sess, &seq.tts) {
 311                     return false;
 312                 }
 313             }
 314         }
 315     }
 316
 317     true
 318 }
 319
 320 fn check_rhs(sess: &ParseSess, rhs: &quoted::TokenTree) -> bool {
 321     match *rhs {
 322         quoted::TokenTree::Delimited(..) => return true,
 323         _ => sess.span_diagnostic.span_err(rhs.span(), "macro rhs must be delimited")
 324     }
 325     false
 326 }
 327
 328 fn check_matcher(sess: &ParseSess,
 329                  features: &RefCell<Features>,
 330                  matcher: &[quoted::TokenTree]) -> bool {
 331     let first_sets = FirstSets::new(matcher);
 332     let empty_suffix = TokenSet::empty();
 333     let err = sess.span_diagnostic.err_count();
 334     check_matcher_core(sess, features, &first_sets, matcher, &empty_suffix);
 335     err == sess.span_diagnostic.err_count()
 336 }
 337
 338 // The FirstSets for a matcher is a mapping from subsequences in the
 339 // matcher to the FIRST set for that subsequence.
 340 //
 341 // This mapping is partially precomputed via a backwards scan over the
 342 // token trees of the matcher, which provides a mapping from each
 343 // repetition sequence to its FIRST set.
 344 //
 345 // (Hypothetically sequences should be uniquely identifiable via their
 346 // spans, though perhaps that is false e.g. for macro-generated macros
 347 // that do not try to inject artificial span information. My plan is
 348 // to try to catch such cases ahead of time and not include them in
 349 // the precomputed mapping.)
 350 struct FirstSets {
 351     // this maps each TokenTree::Sequence `$(tt ...) SEP OP` that is uniquely identified by its
 352     // span in the original matcher to the First set for the inner sequence `tt ...`.
 353     //
 354     // If two sequences have the same span in a matcher, then map that
 355     // span to None (invalidating the mapping here and forcing the code to
 356     // use a slow path).
 357     first: HashMap<Span, Option<TokenSet>>,
 358 }
 359
 360 impl FirstSets {
 361     fn new(tts: &[quoted::TokenTree]) -> FirstSets {
 362         use self::quoted::TokenTree;
 363
 364         let mut sets = FirstSets { first: HashMap::new() };
 365         build_recur(&mut sets, tts);
 366         return sets;
 367
 368         // walks backward over `tts`, returning the FIRST for `tts`
 369         // and updating `sets` at the same time for all sequence
 370         // substructure we find within `tts`.
 371         fn build_recur(sets: &mut FirstSets, tts: &[TokenTree]) -> TokenSet {
 372             let mut first = TokenSet::empty();
 373             for tt in tts.iter().rev() {
 374                 match *tt {
 375                     TokenTree::Token(..) | TokenTree::MetaVarDecl(..) => {
 376                         first.replace_with(tt.clone());
 377                     }
 378                     TokenTree::Delimited(span, ref delimited) => {
 379                         build_recur(sets, &delimited.tts[..]);
 380                         first.replace_with(delimited.open_tt(span));
 381                     }
 382                     TokenTree::Sequence(sp, ref seq_rep) => {
 383                         let subfirst = build_recur(sets, &seq_rep.tts[..]);
 384
 385                         match sets.first.entry(sp) {
 386                             Entry::Vacant(vac) => {
 387                                 vac.insert(Some(subfirst.clone()));
 388                             }
 389                             Entry::Occupied(mut occ) => {
 390                                 // if there is already an entry, then a span must have collided.
 391                                 // This should not happen with typical macro_rules macros,
 392                                 // but syntax extensions need not maintain distinct spans,
 393                                 // so distinct syntax trees can be assigned the same span.
 394                                 // In such a case, the map cannot be trusted; so mark this
 395                                 // entry as unusable.
 396                                 occ.insert(None);
 397                             }
 398                         }
 399
 400                         // If the sequence contents can be empty, then the first
 401                         // token could be the separator token itself.
 402
 403                         if let (Some(ref sep), true) = (seq_rep.separator.clone(),
 404                                                         subfirst.maybe_empty) {
 405                             first.add_one_maybe(TokenTree::Token(sp, sep.clone()));
 406                         }
 407
 408                         // Reverse scan: Sequence comes before `first`.
 409                         if subfirst.maybe_empty || seq_rep.op == quoted::KleeneOp::ZeroOrMore {
 410                             // If sequence is potentially empty, then
 411                             // union them (preserving first emptiness).
 412                             first.add_all(&TokenSet { maybe_empty: true, ..subfirst });
 413                         } else {
 414                             // Otherwise, sequence guaranteed
 415                             // non-empty; replace first.
 416                             first = subfirst;
 417                         }
 418                     }
 419                 }
 420             }
 421
 422             first
 423         }
 424     }
 425
 426     // walks forward over `tts` until all potential FIRST tokens are
 427     // identified.
 428     fn first(&self, tts: &[quoted::TokenTree]) -> TokenSet {
 429         use self::quoted::TokenTree;
 430
 431         let mut first = TokenSet::empty();
 432         for tt in tts.iter() {
 433             assert!(first.maybe_empty);
 434             match *tt {
 435                 TokenTree::Token(..) | TokenTree::MetaVarDecl(..) => {
 436                     first.add_one(tt.clone());
 437                     return first;
 438                 }
 439                 TokenTree::Delimited(span, ref delimited) => {
 440                     first.add_one(delimited.open_tt(span));
 441                     return first;
 442                 }
 443                 TokenTree::Sequence(sp, ref seq_rep) => {
 444                     match self.first.get(&sp) {
 445                         Some(&Some(ref subfirst)) => {
 446
 447                             // If the sequence contents can be empty, then the first
 448                             // token could be the separator token itself.
 449
 450                             if let (Some(ref sep), true) = (seq_rep.separator.clone(),
 451                                                             subfirst.maybe_empty) {
 452                                 first.add_one_maybe(TokenTree::Token(sp, sep.clone()));
 453                             }
 454
 455                             assert!(first.maybe_empty);
 456                             first.add_all(subfirst);
 457                             if subfirst.maybe_empty ||
 458                                seq_rep.op == quoted::KleeneOp::ZeroOrMore {
 459                                 // continue scanning for more first
 460                                 // tokens, but also make sure we
 461                                 // restore empty-tracking state
 462                                 first.maybe_empty = true;
 463                                 continue;
 464                             } else {
 465                                 return first;
 466                             }
 467                         }
 468
 469                         Some(&None) => {
 470                             panic!("assume all sequences have (unique) spans for now");
 471                         }
 472
 473                         None => {
 474                             panic!("We missed a sequence during FirstSets construction");
 475                         }
 476                     }
 477                 }
 478             }
 479         }
 480
 481         // we only exit the loop if `tts` was empty or if every
 482         // element of `tts` matches the empty sequence.
 483         assert!(first.maybe_empty);
 484         first
 485     }
 486 }
 487
 488 // A set of `quoted::TokenTree`s, which may include `TokenTree::Match`s
 489 // (for macro-by-example syntactic variables). It also carries the
 490 // `maybe_empty` flag; that is true if and only if the matcher can
 491 // match an empty token sequence.
 492 //
 493 // The First set is computed on submatchers like `$($a:expr b),* $(c)* d`,
 494 // which has corresponding FIRST = {$a:expr, c, d}.
 495 // Likewise, `$($a:expr b),* $(c)+ d` has FIRST = {$a:expr, c}.
 496 //
 497 // (Notably, we must allow for *-op to occur zero times.)
 498 #[derive(Clone, Debug)]
 499 struct TokenSet {
 500     tokens: Vec<quoted::TokenTree>,
 501     maybe_empty: bool,
 502 }
 503
 504 impl TokenSet {
 505     // Returns a set for the empty sequence.
 506     fn empty() -> Self { TokenSet { tokens: Vec::new(), maybe_empty: true } }
 507
 508     // Returns the set `{ tok }` for the single-token (and thus
 509     // non-empty) sequence [tok].
 510     fn singleton(tok: quoted::TokenTree) -> Self {
 511         TokenSet { tokens: vec![tok], maybe_empty: false }
 512     }
 513
 514     // Changes self to be the set `{ tok }`.
 515     // Since `tok` is always present, marks self as non-empty.
 516     fn replace_with(&mut self, tok: quoted::TokenTree) {
 517         self.tokens.clear();
 518         self.tokens.push(tok);
 519         self.maybe_empty = false;
 520     }
 521
 522     // Changes self to be the empty set `{}`; meant for use when
 523     // the particular token does not matter, but we want to
 524     // record that it occurs.
 525     fn replace_with_irrelevant(&mut self) {
 526         self.tokens.clear();
 527         self.maybe_empty = false;
 528     }
 529
 530     // Adds `tok` to the set for `self`, marking sequence as non-empy.
 531     fn add_one(&mut self, tok: quoted::TokenTree) {
 532         if !self.tokens.contains(&tok) {
 533             self.tokens.push(tok);
 534         }
 535         self.maybe_empty = false;
 536     }
 537
 538     // Adds `tok` to the set for `self`. (Leaves `maybe_empty` flag alone.)
 539     fn add_one_maybe(&mut self, tok: quoted::TokenTree) {
 540         if !self.tokens.contains(&tok) {
 541             self.tokens.push(tok);
 542         }
 543     }
 544
 545     // Adds all elements of `other` to this.
 546     //
 547     // (Since this is a set, we filter out duplicates.)
 548     //
 549     // If `other` is potentially empty, then preserves the previous
 550     // setting of the empty flag of `self`. If `other` is guaranteed
 551     // non-empty, then `self` is marked non-empty.
 552     fn add_all(&mut self, other: &Self) {
 553         for tok in &other.tokens {
 554             if !self.tokens.contains(tok) {
 555                 self.tokens.push(tok.clone());
 556             }
 557         }
 558         if !other.maybe_empty {
 559             self.maybe_empty = false;
 560         }
 561     }
 562 }
 563
 564 // Checks that `matcher` is internally consistent and that it
 565 // can legally by followed by a token N, for all N in `follow`.
 566 // (If `follow` is empty, then it imposes no constraint on
 567 // the `matcher`.)
 568 //
 569 // Returns the set of NT tokens that could possibly come last in
 570 // `matcher`. (If `matcher` matches the empty sequence, then
 571 // `maybe_empty` will be set to true.)
 572 //
 573 // Requires that `first_sets` is pre-computed for `matcher`;
 574 // see `FirstSets::new`.
 575 fn check_matcher_core(sess: &ParseSess,
 576                       features: &RefCell<Features>,
 577                       first_sets: &FirstSets,
 578                       matcher: &[quoted::TokenTree],
 579                       follow: &TokenSet) -> TokenSet {
 580     use self::quoted::TokenTree;
 581
 582     let mut last = TokenSet::empty();
 583
 584     // 2. For each token and suffix  [T, SUFFIX] in M:
 585     // ensure that T can be followed by SUFFIX, and if SUFFIX may be empty,
 586     // then ensure T can also be followed by any element of FOLLOW.
 587     'each_token: for i in 0..matcher.len() {
 588         let token = &matcher[i];
 589         let suffix = &matcher[i+1..];
 590
 591         let build_suffix_first = || {
 592             let mut s = first_sets.first(suffix);
 593             if s.maybe_empty { s.add_all(follow); }
 594             s
 595         };
 596
 597         // (we build `suffix_first` on demand below; you can tell
 598         // which cases are supposed to fall through by looking for the
 599         // initialization of this variable.)
 600         let suffix_first;
 601
 602         // First, update `last` so that it corresponds to the set
 603         // of NT tokens that might end the sequence `... token`.
 604         match *token {
 605             TokenTree::Token(..) | TokenTree::MetaVarDecl(..) => {
 606                 let can_be_followed_by_any;
 607                 if let Err(bad_frag) = has_legal_fragment_specifier(sess, features, token) {
 608                     let msg = format!("invalid fragment specifier `{}`", bad_frag);
 609                     sess.span_diagnostic.struct_span_err(token.span(), &msg)
 610                         .help("valid fragment specifiers are `ident`, `block`, `stmt`, `expr`, \
 611                               `pat`, `ty`, `path`, `meta`, `tt`, `item` and `vis`")
 612                         .emit();
 613                     // (This eliminates false positives and duplicates
 614                     // from error messages.)
 615                     can_be_followed_by_any = true;
 616                 } else {
 617                     can_be_followed_by_any = token_can_be_followed_by_any(token);
 618                 }
 619
 620                 if can_be_followed_by_any {
 621                     // don't need to track tokens that work with any,
 622                     last.replace_with_irrelevant();
 623                     // ... and don't need to check tokens that can be
 624                     // followed by anything against SUFFIX.
 625                     continue 'each_token;
 626                 } else {
 627                     last.replace_with(token.clone());
 628                     suffix_first = build_suffix_first();
 629                 }
 630             }
 631             TokenTree::Delimited(span, ref d) => {
 632                 let my_suffix = TokenSet::singleton(d.close_tt(span));
 633                 check_matcher_core(sess, features, first_sets, &d.tts, &my_suffix);
 634                 // don't track non NT tokens
 635                 last.replace_with_irrelevant();
 636
 637                 // also, we don't need to check delimited sequences
 638                 // against SUFFIX
 639                 continue 'each_token;
 640             }
 641             TokenTree::Sequence(sp, ref seq_rep) => {
 642                 suffix_first = build_suffix_first();
 643                 // The trick here: when we check the interior, we want
 644                 // to include the separator (if any) as a potential
 645                 // (but not guaranteed) element of FOLLOW. So in that
 646                 // case, we make a temp copy of suffix and stuff
 647                 // delimiter in there.
 648                 //
 649                 // FIXME: Should I first scan suffix_first to see if
 650                 // delimiter is already in it before I go through the
 651                 // work of cloning it? But then again, this way I may
 652                 // get a "tighter" span?
 653                 let mut new;
 654                 let my_suffix = if let Some(ref u) = seq_rep.separator {
 655                     new = suffix_first.clone();
 656                     new.add_one_maybe(TokenTree::Token(sp, u.clone()));
 657                     &new
 658                 } else {
 659                     &suffix_first
 660                 };
 661
 662                 // At this point, `suffix_first` is built, and
 663                 // `my_suffix` is some TokenSet that we can use
 664                 // for checking the interior of `seq_rep`.
 665                 let next = check_matcher_core(sess, features, first_sets, &seq_rep.tts, my_suffix);
 666                 if next.maybe_empty {
 667                     last.add_all(&next);
 668                 } else {
 669                     last = next;
 670                 }
 671
 672                 // the recursive call to check_matcher_core already ran the 'each_last
 673                 // check below, so we can just keep going forward here.
 674                 continue 'each_token;
 675             }
 676         }
 677
 678         // (`suffix_first` guaranteed initialized once reaching here.)
 679
 680         // Now `last` holds the complete set of NT tokens that could
 681         // end the sequence before SUFFIX. Check that every one works with `suffix`.
 682         'each_last: for token in &last.tokens {
 683             if let TokenTree::MetaVarDecl(_, ref name, ref frag_spec) = *token {
 684                 for next_token in &suffix_first.tokens {
 685                     match is_in_follow(next_token, &frag_spec.name.as_str()) {
 686                         Err((msg, help)) => {
 687                             sess.span_diagnostic.struct_span_err(next_token.span(), &msg)
 688                                 .help(help).emit();
 689                             // don't bother reporting every source of
 690                             // conflict for a particular element of `last`.
 691                             continue 'each_last;
 692                         }
 693                         Ok(true) => {}
 694                         Ok(false) => {
 695                             let may_be = if last.tokens.len() == 1 &&
 696                                 suffix_first.tokens.len() == 1
 697                             {
 698                                 "is"
 699                             } else {
 700                                 "may be"
 701                             };
 702
 703                             sess.span_diagnostic.span_err(
 704                                 next_token.span(),
 705                                 &format!("`${name}:{frag}` {may_be} followed by `{next}`, which \
 706                                           is not allowed for `{frag}` fragments",
 707                                          name=name,
 708                                          frag=frag_spec,
 709                                          next=quoted_tt_to_string(next_token),
 710                                          may_be=may_be)
 711                             );
 712                         }
 713                     }
 714                 }
 715             }
 716         }
 717     }
 718     last
 719 }
 720
 721 fn token_can_be_followed_by_any(tok: &quoted::TokenTree) -> bool {
 722     if let quoted::TokenTree::MetaVarDecl(_, _, frag_spec) = *tok {
 723         frag_can_be_followed_by_any(&frag_spec.name.as_str())
 724     } else {
 725         // (Non NT's can always be followed by anthing in matchers.)
 726         true
 727     }
 728 }
 729
 730 /// True if a fragment of type `frag` can be followed by any sort of
 731 /// token.  We use this (among other things) as a useful approximation
 732 /// for when `frag` can be followed by a repetition like `$(...)*` or
 733 /// `$(...)+`. In general, these can be a bit tricky to reason about,
 734 /// so we adopt a conservative position that says that any fragment
 735 /// specifier which consumes at most one token tree can be followed by
 736 /// a fragment specifier (indeed, these fragments can be followed by
 737 /// ANYTHING without fear of future compatibility hazards).
 738 fn frag_can_be_followed_by_any(frag: &str) -> bool {
 739     match frag {
 740         "item"  | // always terminated by `}` or `;`
 741         "block" | // exactly one token tree
 742         "ident" | // exactly one token tree
 743         "meta"  | // exactly one token tree
 744         "tt" =>   // exactly one token tree
 745             true,
 746
 747         _ =>
 748             false,
 749     }
 750 }
 751
 752 /// True if `frag` can legally be followed by the token `tok`. For
 753 /// fragments that can consume an unbounded number of tokens, `tok`
 754 /// must be within a well-defined follow set. This is intended to
 755 /// guarantee future compatibility: for example, without this rule, if
 756 /// we expanded `expr` to include a new binary operator, we might
 757 /// break macros that were relying on that binary operator as a
 758 /// separator.
 759 // when changing this do not forget to update doc/book/macros.md!
 760 fn is_in_follow(tok: &quoted::TokenTree, frag: &str) -> Result<bool, (String, &'static str)> {
 761     use self::quoted::TokenTree;
 762
 763     if let TokenTree::Token(_, token::CloseDelim(_)) = *tok {
 764         // closing a token tree can never be matched by any fragment;
 765         // iow, we always require that `(` and `)` match, etc.
 766         Ok(true)
 767     } else {
 768         match frag {
 769             "item" => {
 770                 // since items *must* be followed by either a `;` or a `}`, we can
 771                 // accept anything after them
 772                 Ok(true)
 773             },
 774             "block" => {
 775                 // anything can follow block, the braces provide an easy boundary to
 776                 // maintain
 777                 Ok(true)
 778             },
 779             "stmt" | "expr"  => match *tok {
 780                 TokenTree::Token(_, ref tok) => match *tok {
 781                     FatArrow | Comma | Semi => Ok(true),
 782                     _ => Ok(false)
 783                 },
 784                 _ => Ok(false),
 785             },
 786             "pat" => match *tok {
 787                 TokenTree::Token(_, ref tok) => match *tok {
 788                     FatArrow | Comma | Eq | BinOp(token::Or) => Ok(true),
 789                     Ident(i) if i.name == "if" || i.name == "in" => Ok(true),
 790                     _ => Ok(false)
 791                 },
 792                 _ => Ok(false),
 793             },
 794             "path" | "ty" => match *tok {
 795                 TokenTree::Token(_, ref tok) => match *tok {
 796                     OpenDelim(token::DelimToken::Brace) | OpenDelim(token::DelimToken::Bracket) |
 797                     Comma | FatArrow | Colon | Eq | Gt | Semi | BinOp(token::Or) => Ok(true),
 798                     Ident(i) if i.name == "as" || i.name == "where" => Ok(true),
 799                     _ => Ok(false)
 800                 },
 801                 TokenTree::MetaVarDecl(_, _, frag) if frag.name == "block" => Ok(true),
 802                 _ => Ok(false),
 803             },
 804             "ident" => {
 805                 // being a single token, idents are harmless
 806                 Ok(true)
 807             },
 808             "meta" | "tt" => {
 809                 // being either a single token or a delimited sequence, tt is
 810                 // harmless
 811                 Ok(true)
 812             },
 813             "vis" => {
 814                 // Explicitly disallow `priv`, on the off chance it comes back.
 815                 match *tok {
 816                     TokenTree::Token(_, ref tok) => match *tok {
 817                         Comma => Ok(true),
 818                         Ident(i) if i.name != "priv" => Ok(true),
 819                         ref tok => Ok(tok.can_begin_type())
 820                     },
 821                     TokenTree::MetaVarDecl(_, _, frag) if frag.name == "ident"
 822                                                        || frag.name == "ty"
 823                                                        || frag.name == "path" => Ok(true),
 824                     _ => Ok(false)
 825                 }
 826             },
 827             "" => Ok(true), // keywords::Invalid
 828             _ => Err((format!("invalid fragment specifier `{}`", frag),
 829                      "valid fragment specifiers are `ident`, `block`, \
 830                       `stmt`, `expr`, `pat`, `ty`, `path`, `meta`, `tt`, \
 831                       `item` and `vis`"))
 832         }
 833     }
 834 }
 835
 836 fn has_legal_fragment_specifier(sess: &ParseSess,
 837                                 features: &RefCell<Features>,
 838                                 tok: &quoted::TokenTree) -> Result<(), String> {
 839     debug!("has_legal_fragment_specifier({:?})", tok);
 840     if let quoted::TokenTree::MetaVarDecl(_, _, ref frag_spec) = *tok {
 841         let frag_name = frag_spec.name.as_str();
 842         let frag_span = tok.span();
 843         if !is_legal_fragment_specifier(sess, features, &frag_name, frag_span) {
 844             return Err(frag_name.to_string());
 845         }
 846     }
 847     Ok(())
 848 }
 849
 850 fn is_legal_fragment_specifier(sess: &ParseSess,
 851                                features: &RefCell<Features>,
 852                                frag_name: &str,
 853                                frag_span: Span) -> bool {
 854     match frag_name {
 855         "item" | "block" | "stmt" | "expr" | "pat" |
 856         "path" | "ty" | "ident" | "meta" | "tt" | "" => true,
 857         "vis" => {
 858             if !features.borrow().macro_vis_matcher {
 859                 let explain = feature_gate::EXPLAIN_VIS_MATCHER;
 860                 emit_feature_err(sess,
 861                                  "macro_vis_matcher",
 862                                  frag_span,
 863                                  GateIssue::Language,
 864                                  explain);
 865             }
 866             true
 867         },
 868         _ => false,
 869     }
 870 }
 871
 872 fn quoted_tt_to_string(tt: &quoted::TokenTree) -> String {
 873     match *tt {
 874         quoted::TokenTree::Token(_, ref tok) => ::print::pprust::token_to_string(tok),
 875         quoted::TokenTree::MetaVarDecl(_, name, kind) => format!("${}:{}", name, kind),
 876         _ => panic!("unexpected quoted::TokenTree::{{Sequence or Delimited}} \
 877                      in follow set checker"),
 878     }
 879 }