compiler/rustc_expand/src/mbe/macro_check.rs

   1 //! Checks that meta-variables in macro definition are correctly declared and used.
   2 //!
   3 //! # What is checked
   4 //!
   5 //! ## Meta-variables must not be bound twice
   6 //!
   7 //! ```
   8 //! macro_rules! foo { ($x:tt $x:tt) => { $x }; }
   9 //! ```
  10 //!
  11 //! This check is sound (no false-negative) and complete (no false-positive).
  12 //!
  13 //! ## Meta-variables must not be free
  14 //!
  15 //! ```
  16 //! macro_rules! foo { () => { $x }; }
  17 //! ```
  18 //!
  19 //! This check is also done at macro instantiation but only if the branch is taken.
  20 //!
  21 //! ## Meta-variables must repeat at least as many times as their binder
  22 //!
  23 //! ```
  24 //! macro_rules! foo { ($($x:tt)*) => { $x }; }
  25 //! ```
  26 //!
  27 //! This check is also done at macro instantiation but only if the branch is taken.
  28 //!
  29 //! ## Meta-variables must repeat with the same Kleene operators as their binder
  30 //!
  31 //! ```
  32 //! macro_rules! foo { ($($x:tt)+) => { $($x)* }; }
  33 //! ```
  34 //!
  35 //! This check is not done at macro instantiation.
  36 //!
  37 //! # Disclaimer
  38 //!
  39 //! In the presence of nested macros (a macro defined in a macro), those checks may have false
  40 //! positives and false negatives. We try to detect those cases by recognizing potential macro
  41 //! definitions in RHSes, but nested macros may be hidden through the use of particular values of
  42 //! meta-variables.
  43 //!
  44 //! ## Examples of false positive
  45 //!
  46 //! False positives can come from cases where we don't recognize a nested macro, because it depends
  47 //! on particular values of meta-variables. In the following example, we think both instances of
  48 //! `$x` are free, which is a correct statement if `$name` is anything but `macro_rules`. But when
  49 //! `$name` is `macro_rules`, like in the instantiation below, then `$x:tt` is actually a binder of
  50 //! the nested macro and `$x` is bound to it.
  51 //!
  52 //! ```
  53 //! macro_rules! foo { ($name:ident) => { $name! bar { ($x:tt) => { $x }; } }; }
  54 //! foo!(macro_rules);
  55 //! ```
  56 //!
  57 //! False positives can also come from cases where we think there is a nested macro while there
  58 //! isn't. In the following example, we think `$x` is free, which is incorrect because `bar` is not
  59 //! a nested macro since it is not evaluated as code by `stringify!`.
  60 //!
  61 //! ```
  62 //! macro_rules! foo { () => { stringify!(macro_rules! bar { () => { $x }; }) }; }
  63 //! ```
  64 //!
  65 //! ## Examples of false negative
  66 //!
  67 //! False negatives can come from cases where we don't recognize a meta-variable, because it depends
  68 //! on particular values of meta-variables. In the following examples, we don't see that if `$d` is
  69 //! instantiated with `$` then `$d z` becomes `$z` in the nested macro definition and is thus a free
  70 //! meta-variable. Note however, that if `foo` is instantiated, then we would check the definition
  71 //! of `bar` and would see the issue.
  72 //!
  73 //! ```
  74 //! macro_rules! foo { ($d:tt) => { macro_rules! bar { ($y:tt) => { $d z }; } }; }
  75 //! ```
  76 //!
  77 //! # How it is checked
  78 //!
  79 //! There are 3 main functions: `check_binders`, `check_occurrences`, and `check_nested_macro`. They
  80 //! all need some kind of environment.
  81 //!
  82 //! ## Environments
  83 //!
  84 //! Environments are used to pass information.
  85 //!
  86 //! ### From LHS to RHS
  87 //!
  88 //! When checking a LHS with `check_binders`, we produce (and use) an environment for binders,
  89 //! namely `Binders`. This is a mapping from binder name to information about that binder: the span
  90 //! of the binder for error messages and the stack of Kleene operators under which it was bound in
  91 //! the LHS.
  92 //!
  93 //! This environment is used by both the LHS and RHS. The LHS uses it to detect duplicate binders.
  94 //! The RHS uses it to detect the other errors.
  95 //!
  96 //! ### From outer macro to inner macro
  97 //!
  98 //! When checking the RHS of an outer macro and we detect a nested macro definition, we push the
  99 //! current state, namely `MacroState`, to an environment of nested macro definitions. Each state
 100 //! stores the LHS binders when entering the macro definition as well as the stack of Kleene
 101 //! operators under which the inner macro is defined in the RHS.
 102 //!
 103 //! This environment is a stack representing the nesting of macro definitions. As such, the stack of
 104 //! Kleene operators under which a meta-variable is repeating is the concatenation of the stacks
 105 //! stored when entering a macro definition starting from the state in which the meta-variable is
 106 //! bound.
 107 use crate::mbe::{KleeneToken, TokenTree};
 108
 109 use rustc_ast::token::{DelimToken, Token, TokenKind};
 110 use rustc_ast::{NodeId, DUMMY_NODE_ID};
 111 use rustc_data_structures::fx::FxHashMap;
 112 use rustc_session::lint::builtin::META_VARIABLE_MISUSE;
 113 use rustc_session::parse::ParseSess;
 114 use rustc_span::symbol::kw;
 115 use rustc_span::{symbol::MacroRulesNormalizedIdent, MultiSpan, Span};
 116
 117 use smallvec::SmallVec;
 118
 119 use std::iter;
 120
 121 /// Stack represented as linked list.
 122 ///
 123 /// Those are used for environments because they grow incrementally and are not mutable.
 124 enum Stack<'a, T> {
 125     /// Empty stack.
 126     Empty,
 127     /// A non-empty stack.
 128     Push {
 129         /// The top element.
 130         top: T,
 131         /// The previous elements.
 132         prev: &'a Stack<'a, T>,
 133     },
 134 }
 135
 136 impl<'a, T> Stack<'a, T> {
 137     /// Returns whether a stack is empty.
 138     fn is_empty(&self) -> bool {
 139         matches!(*self, Stack::Empty)
 140     }
 141
 142     /// Returns a new stack with an element of top.
 143     fn push(&'a self, top: T) -> Stack<'a, T> {
 144         Stack::Push { top, prev: self }
 145     }
 146 }
 147
 148 impl<'a, T> Iterator for &'a Stack<'a, T> {
 149     type Item = &'a T;
 150
 151     // Iterates from top to bottom of the stack.
 152     fn next(&mut self) -> Option<&'a T> {
 153         match *self {
 154             Stack::Empty => None,
 155             Stack::Push { ref top, ref prev } => {
 156                 *self = prev;
 157                 Some(top)
 158             }
 159         }
 160     }
 161 }
 162
 163 impl From<&Stack<'_, KleeneToken>> for SmallVec<[KleeneToken; 1]> {
 164     fn from(ops: &Stack<'_, KleeneToken>) -> SmallVec<[KleeneToken; 1]> {
 165         let mut ops: SmallVec<[KleeneToken; 1]> = ops.cloned().collect();
 166         // The stack is innermost on top. We want outermost first.
 167         ops.reverse();
 168         ops
 169     }
 170 }
 171
 172 /// Information attached to a meta-variable binder in LHS.
 173 struct BinderInfo {
 174     /// The span of the meta-variable in LHS.
 175     span: Span,
 176     /// The stack of Kleene operators (outermost first).
 177     ops: SmallVec<[KleeneToken; 1]>,
 178 }
 179
 180 /// An environment of meta-variables to their binder information.
 181 type Binders = FxHashMap<MacroRulesNormalizedIdent, BinderInfo>;
 182
 183 /// The state at which we entered a macro definition in the RHS of another macro definition.
 184 struct MacroState<'a> {
 185     /// The binders of the branch where we entered the macro definition.
 186     binders: &'a Binders,
 187     /// The stack of Kleene operators (outermost first) where we entered the macro definition.
 188     ops: SmallVec<[KleeneToken; 1]>,
 189 }
 190
 191 /// Checks that meta-variables are used correctly in a macro definition.
 192 ///
 193 /// Arguments:
 194 /// - `sess` is used to emit diagnostics and lints
 195 /// - `node_id` is used to emit lints
 196 /// - `span` is used when no spans are available
 197 /// - `lhses` and `rhses` should have the same length and represent the macro definition
 198 pub(super) fn check_meta_variables(
 199     sess: &ParseSess,
 200     node_id: NodeId,
 201     span: Span,
 202     lhses: &[TokenTree],
 203     rhses: &[TokenTree],
 204 ) -> bool {
 205     if lhses.len() != rhses.len() {
 206         sess.span_diagnostic.span_bug(span, "length mismatch between LHSes and RHSes")
 207     }
 208     let mut valid = true;
 209     for (lhs, rhs) in iter::zip(lhses, rhses) {
 210         let mut binders = Binders::default();
 211         check_binders(sess, node_id, lhs, &Stack::Empty, &mut binders, &Stack::Empty, &mut valid);
 212         check_occurrences(sess, node_id, rhs, &Stack::Empty, &binders, &Stack::Empty, &mut valid);
 213     }
 214     valid
 215 }
 216
 217 /// Checks `lhs` as part of the LHS of a macro definition, extends `binders` with new binders, and
 218 /// sets `valid` to false in case of errors.
 219 ///
 220 /// Arguments:
 221 /// - `sess` is used to emit diagnostics and lints
 222 /// - `node_id` is used to emit lints
 223 /// - `lhs` is checked as part of a LHS
 224 /// - `macros` is the stack of possible outer macros
 225 /// - `binders` contains the binders of the LHS
 226 /// - `ops` is the stack of Kleene operators from the LHS
 227 /// - `valid` is set in case of errors
 228 fn check_binders(
 229     sess: &ParseSess,
 230     node_id: NodeId,
 231     lhs: &TokenTree,
 232     macros: &Stack<'_, MacroState<'_>>,
 233     binders: &mut Binders,
 234     ops: &Stack<'_, KleeneToken>,
 235     valid: &mut bool,
 236 ) {
 237     match *lhs {
 238         TokenTree::Token(..) => {}
 239         // This can only happen when checking a nested macro because this LHS is then in the RHS of
 240         // the outer macro. See ui/macros/macro-of-higher-order.rs where $y:$fragment in the
 241         // LHS of the nested macro (and RHS of the outer macro) is parsed as MetaVar(y) Colon
 242         // MetaVar(fragment) and not as MetaVarDecl(y, fragment).
 243         TokenTree::MetaVar(span, name) => {
 244             if macros.is_empty() {
 245                 sess.span_diagnostic.span_bug(span, "unexpected MetaVar in lhs");
 246             }
 247             let name = MacroRulesNormalizedIdent::new(name);
 248             // There are 3 possibilities:
 249             if let Some(prev_info) = binders.get(&name) {
 250                 // 1. The meta-variable is already bound in the current LHS: This is an error.
 251                 let mut span = MultiSpan::from_span(span);
 252                 span.push_span_label(prev_info.span, "previous declaration".into());
 253                 buffer_lint(sess, span, node_id, "duplicate matcher binding");
 254             } else if get_binder_info(macros, binders, name).is_none() {
 255                 // 2. The meta-variable is free: This is a binder.
 256                 binders.insert(name, BinderInfo { span, ops: ops.into() });
 257             } else {
 258                 // 3. The meta-variable is bound: This is an occurrence.
 259                 check_occurrences(sess, node_id, lhs, macros, binders, ops, valid);
 260             }
 261         }
 262         // Similarly, this can only happen when checking a toplevel macro.
 263         TokenTree::MetaVarDecl(span, name, _kind) => {
 264             if !macros.is_empty() {
 265                 sess.span_diagnostic.span_bug(span, "unexpected MetaVarDecl in nested lhs");
 266             }
 267             let name = MacroRulesNormalizedIdent::new(name);
 268             if let Some(prev_info) = get_binder_info(macros, binders, name) {
 269                 // Duplicate binders at the top-level macro definition are errors. The lint is only
 270                 // for nested macro definitions.
 271                 sess.span_diagnostic
 272                     .struct_span_err(span, "duplicate matcher binding")
 273                     .span_label(span, "duplicate binding")
 274                     .span_label(prev_info.span, "previous binding")
 275                     .emit();
 276                 *valid = false;
 277             } else {
 278                 binders.insert(name, BinderInfo { span, ops: ops.into() });
 279             }
 280         }
 281         TokenTree::Delimited(_, ref del) => {
 282             for tt in &del.tts {
 283                 check_binders(sess, node_id, tt, macros, binders, ops, valid);
 284             }
 285         }
 286         TokenTree::Sequence(_, ref seq) => {
 287             let ops = ops.push(seq.kleene);
 288             for tt in &seq.tts {
 289                 check_binders(sess, node_id, tt, macros, binders, &ops, valid);
 290             }
 291         }
 292     }
 293 }
 294
 295 /// Returns the binder information of a meta-variable.
 296 ///
 297 /// Arguments:
 298 /// - `macros` is the stack of possible outer macros
 299 /// - `binders` contains the current binders
 300 /// - `name` is the name of the meta-variable we are looking for
 301 fn get_binder_info<'a>(
 302     mut macros: &'a Stack<'a, MacroState<'a>>,
 303     binders: &'a Binders,
 304     name: MacroRulesNormalizedIdent,
 305 ) -> Option<&'a BinderInfo> {
 306     binders.get(&name).or_else(|| macros.find_map(|state| state.binders.get(&name)))
 307 }
 308
 309 /// Checks `rhs` as part of the RHS of a macro definition and sets `valid` to false in case of
 310 /// errors.
 311 ///
 312 /// Arguments:
 313 /// - `sess` is used to emit diagnostics and lints
 314 /// - `node_id` is used to emit lints
 315 /// - `rhs` is checked as part of a RHS
 316 /// - `macros` is the stack of possible outer macros
 317 /// - `binders` contains the binders of the associated LHS
 318 /// - `ops` is the stack of Kleene operators from the RHS
 319 /// - `valid` is set in case of errors
 320 fn check_occurrences(
 321     sess: &ParseSess,
 322     node_id: NodeId,
 323     rhs: &TokenTree,
 324     macros: &Stack<'_, MacroState<'_>>,
 325     binders: &Binders,
 326     ops: &Stack<'_, KleeneToken>,
 327     valid: &mut bool,
 328 ) {
 329     match *rhs {
 330         TokenTree::Token(..) => {}
 331         TokenTree::MetaVarDecl(span, _name, _kind) => {
 332             sess.span_diagnostic.span_bug(span, "unexpected MetaVarDecl in rhs")
 333         }
 334         TokenTree::MetaVar(span, name) => {
 335             let name = MacroRulesNormalizedIdent::new(name);
 336             check_ops_is_prefix(sess, node_id, macros, binders, ops, span, name);
 337         }
 338         TokenTree::Delimited(_, ref del) => {
 339             check_nested_occurrences(sess, node_id, &del.tts, macros, binders, ops, valid);
 340         }
 341         TokenTree::Sequence(_, ref seq) => {
 342             let ops = ops.push(seq.kleene);
 343             check_nested_occurrences(sess, node_id, &seq.tts, macros, binders, &ops, valid);
 344         }
 345     }
 346 }
 347
 348 /// Represents the processed prefix of a nested macro.
 349 #[derive(Clone, Copy, PartialEq, Eq)]
 350 enum NestedMacroState {
 351     /// Nothing that matches a nested macro definition was processed yet.
 352     Empty,
 353     /// The token `macro_rules` was processed.
 354     MacroRules,
 355     /// The tokens `macro_rules!` were processed.
 356     MacroRulesNot,
 357     /// The tokens `macro_rules!` followed by a name were processed. The name may be either directly
 358     /// an identifier or a meta-variable (that hopefully would be instantiated by an identifier).
 359     MacroRulesNotName,
 360     /// The keyword `macro` was processed.
 361     Macro,
 362     /// The keyword `macro` followed by a name was processed.
 363     MacroName,
 364     /// The keyword `macro` followed by a name and a token delimited by parentheses was processed.
 365     MacroNameParen,
 366 }
 367
 368 /// Checks `tts` as part of the RHS of a macro definition, tries to recognize nested macro
 369 /// definitions, and sets `valid` to false in case of errors.
 370 ///
 371 /// Arguments:
 372 /// - `sess` is used to emit diagnostics and lints
 373 /// - `node_id` is used to emit lints
 374 /// - `tts` is checked as part of a RHS and may contain macro definitions
 375 /// - `macros` is the stack of possible outer macros
 376 /// - `binders` contains the binders of the associated LHS
 377 /// - `ops` is the stack of Kleene operators from the RHS
 378 /// - `valid` is set in case of errors
 379 fn check_nested_occurrences(
 380     sess: &ParseSess,
 381     node_id: NodeId,
 382     tts: &[TokenTree],
 383     macros: &Stack<'_, MacroState<'_>>,
 384     binders: &Binders,
 385     ops: &Stack<'_, KleeneToken>,
 386     valid: &mut bool,
 387 ) {
 388     let mut state = NestedMacroState::Empty;
 389     let nested_macros = macros.push(MacroState { binders, ops: ops.into() });
 390     let mut nested_binders = Binders::default();
 391     for tt in tts {
 392         match (state, tt) {
 393             (
 394                 NestedMacroState::Empty,
 395                 &TokenTree::Token(Token { kind: TokenKind::Ident(name, false), .. }),
 396             ) => {
 397                 if name == kw::MacroRules {
 398                     state = NestedMacroState::MacroRules;
 399                 } else if name == kw::Macro {
 400                     state = NestedMacroState::Macro;
 401                 }
 402             }
 403             (
 404                 NestedMacroState::MacroRules,
 405                 &TokenTree::Token(Token { kind: TokenKind::Not, .. }),
 406             ) => {
 407                 state = NestedMacroState::MacroRulesNot;
 408             }
 409             (
 410                 NestedMacroState::MacroRulesNot,
 411                 &TokenTree::Token(Token { kind: TokenKind::Ident(..), .. }),
 412             ) => {
 413                 state = NestedMacroState::MacroRulesNotName;
 414             }
 415             (NestedMacroState::MacroRulesNot, &TokenTree::MetaVar(..)) => {
 416                 state = NestedMacroState::MacroRulesNotName;
 417                 // We check that the meta-variable is correctly used.
 418                 check_occurrences(sess, node_id, tt, macros, binders, ops, valid);
 419             }
 420             (NestedMacroState::MacroRulesNotName, &TokenTree::Delimited(_, ref del))
 421             | (NestedMacroState::MacroName, &TokenTree::Delimited(_, ref del))
 422                 if del.delim == DelimToken::Brace =>
 423             {
 424                 let macro_rules = state == NestedMacroState::MacroRulesNotName;
 425                 state = NestedMacroState::Empty;
 426                 let rest =
 427                     check_nested_macro(sess, node_id, macro_rules, &del.tts, &nested_macros, valid);
 428                 // If we did not check the whole macro definition, then check the rest as if outside
 429                 // the macro definition.
 430                 check_nested_occurrences(
 431                     sess,
 432                     node_id,
 433                     &del.tts[rest..],
 434                     macros,
 435                     binders,
 436                     ops,
 437                     valid,
 438                 );
 439             }
 440             (
 441                 NestedMacroState::Macro,
 442                 &TokenTree::Token(Token { kind: TokenKind::Ident(..), .. }),
 443             ) => {
 444                 state = NestedMacroState::MacroName;
 445             }
 446             (NestedMacroState::Macro, &TokenTree::MetaVar(..)) => {
 447                 state = NestedMacroState::MacroName;
 448                 // We check that the meta-variable is correctly used.
 449                 check_occurrences(sess, node_id, tt, macros, binders, ops, valid);
 450             }
 451             (NestedMacroState::MacroName, &TokenTree::Delimited(_, ref del))
 452                 if del.delim == DelimToken::Paren =>
 453             {
 454                 state = NestedMacroState::MacroNameParen;
 455                 nested_binders = Binders::default();
 456                 check_binders(
 457                     sess,
 458                     node_id,
 459                     tt,
 460                     &nested_macros,
 461                     &mut nested_binders,
 462                     &Stack::Empty,
 463                     valid,
 464                 );
 465             }
 466             (NestedMacroState::MacroNameParen, &TokenTree::Delimited(_, ref del))
 467                 if del.delim == DelimToken::Brace =>
 468             {
 469                 state = NestedMacroState::Empty;
 470                 check_occurrences(
 471                     sess,
 472                     node_id,
 473                     tt,
 474                     &nested_macros,
 475                     &nested_binders,
 476                     &Stack::Empty,
 477                     valid,
 478                 );
 479             }
 480             (_, ref tt) => {
 481                 state = NestedMacroState::Empty;
 482                 check_occurrences(sess, node_id, tt, macros, binders, ops, valid);
 483             }
 484         }
 485     }
 486 }
 487
 488 /// Checks the body of nested macro, returns where the check stopped, and sets `valid` to false in
 489 /// case of errors.
 490 ///
 491 /// The token trees are checked as long as they look like a list of (LHS) => {RHS} token trees. This
 492 /// check is a best-effort to detect a macro definition. It returns the position in `tts` where we
 493 /// stopped checking because we detected we were not in a macro definition anymore.
 494 ///
 495 /// Arguments:
 496 /// - `sess` is used to emit diagnostics and lints
 497 /// - `node_id` is used to emit lints
 498 /// - `macro_rules` specifies whether the macro is `macro_rules`
 499 /// - `tts` is checked as a list of (LHS) => {RHS}
 500 /// - `macros` is the stack of outer macros
 501 /// - `valid` is set in case of errors
 502 fn check_nested_macro(
 503     sess: &ParseSess,
 504     node_id: NodeId,
 505     macro_rules: bool,
 506     tts: &[TokenTree],
 507     macros: &Stack<'_, MacroState<'_>>,
 508     valid: &mut bool,
 509 ) -> usize {
 510     let n = tts.len();
 511     let mut i = 0;
 512     let separator = if macro_rules { TokenKind::Semi } else { TokenKind::Comma };
 513     loop {
 514         // We expect 3 token trees: `(LHS) => {RHS}`. The separator is checked after.
 515         if i + 2 >= n
 516             || !tts[i].is_delimited()
 517             || !tts[i + 1].is_token(&TokenKind::FatArrow)
 518             || !tts[i + 2].is_delimited()
 519         {
 520             break;
 521         }
 522         let lhs = &tts[i];
 523         let rhs = &tts[i + 2];
 524         let mut binders = Binders::default();
 525         check_binders(sess, node_id, lhs, macros, &mut binders, &Stack::Empty, valid);
 526         check_occurrences(sess, node_id, rhs, macros, &binders, &Stack::Empty, valid);
 527         // Since the last semicolon is optional for `macro_rules` macros and decl_macro are not terminated,
 528         // we increment our checked position by how many token trees we already checked (the 3
 529         // above) before checking for the separator.
 530         i += 3;
 531         if i == n || !tts[i].is_token(&separator) {
 532             break;
 533         }
 534         // We increment our checked position for the semicolon.
 535         i += 1;
 536     }
 537     i
 538 }
 539
 540 /// Checks that a meta-variable occurrence is valid.
 541 ///
 542 /// Arguments:
 543 /// - `sess` is used to emit diagnostics and lints
 544 /// - `node_id` is used to emit lints
 545 /// - `macros` is the stack of possible outer macros
 546 /// - `binders` contains the binders of the associated LHS
 547 /// - `ops` is the stack of Kleene operators from the RHS
 548 /// - `span` is the span of the meta-variable to check
 549 /// - `name` is the name of the meta-variable to check
 550 fn check_ops_is_prefix(
 551     sess: &ParseSess,
 552     node_id: NodeId,
 553     macros: &Stack<'_, MacroState<'_>>,
 554     binders: &Binders,
 555     ops: &Stack<'_, KleeneToken>,
 556     span: Span,
 557     name: MacroRulesNormalizedIdent,
 558 ) {
 559     let macros = macros.push(MacroState { binders, ops: ops.into() });
 560     // Accumulates the stacks the operators of each state until (and including when) the
 561     // meta-variable is found. The innermost stack is first.
 562     let mut acc: SmallVec<[&SmallVec<[KleeneToken; 1]>; 1]> = SmallVec::new();
 563     for state in &macros {
 564         acc.push(&state.ops);
 565         if let Some(binder) = state.binders.get(&name) {
 566             // This variable concatenates the stack of operators from the RHS of the LHS where the
 567             // meta-variable was defined to where it is used (in possibly nested macros). The
 568             // outermost operator is first.
 569             let mut occurrence_ops: SmallVec<[KleeneToken; 2]> = SmallVec::new();
 570             // We need to iterate from the end to start with outermost stack.
 571             for ops in acc.iter().rev() {
 572                 occurrence_ops.extend_from_slice(ops);
 573             }
 574             ops_is_prefix(sess, node_id, span, name, &binder.ops, &occurrence_ops);
 575             return;
 576         }
 577     }
 578     buffer_lint(sess, span.into(), node_id, &format!("unknown macro variable `{}`", name));
 579 }
 580
 581 /// Returns whether `binder_ops` is a prefix of `occurrence_ops`.
 582 ///
 583 /// The stack of Kleene operators of a meta-variable occurrence just needs to have the stack of
 584 /// Kleene operators of its binder as a prefix.
 585 ///
 586 /// Consider $i in the following example:
 587 ///
 588 ///     ( $( $i:ident = $($j:ident),+ );* ) => { $($( $i += $j; )+)* }
 589 ///
 590 /// It occurs under the Kleene stack ["*", "+"] and is bound under ["*"] only.
 591 ///
 592 /// Arguments:
 593 /// - `sess` is used to emit diagnostics and lints
 594 /// - `node_id` is used to emit lints
 595 /// - `span` is the span of the meta-variable being check
 596 /// - `name` is the name of the meta-variable being check
 597 /// - `binder_ops` is the stack of Kleene operators for the binder
 598 /// - `occurrence_ops` is the stack of Kleene operators for the occurrence
 599 fn ops_is_prefix(
 600     sess: &ParseSess,
 601     node_id: NodeId,
 602     span: Span,
 603     name: MacroRulesNormalizedIdent,
 604     binder_ops: &[KleeneToken],
 605     occurrence_ops: &[KleeneToken],
 606 ) {
 607     for (i, binder) in binder_ops.iter().enumerate() {
 608         if i >= occurrence_ops.len() {
 609             let mut span = MultiSpan::from_span(span);
 610             span.push_span_label(binder.span, "expected repetition".into());
 611             let message = &format!("variable '{}' is still repeating at this depth", name);
 612             buffer_lint(sess, span, node_id, message);
 613             return;
 614         }
 615         let occurrence = &occurrence_ops[i];
 616         if occurrence.op != binder.op {
 617             let mut span = MultiSpan::from_span(span);
 618             span.push_span_label(binder.span, "expected repetition".into());
 619             span.push_span_label(occurrence.span, "conflicting repetition".into());
 620             let message = "meta-variable repeats with different Kleene operator";
 621             buffer_lint(sess, span, node_id, message);
 622             return;
 623         }
 624     }
 625 }
 626
 627 fn buffer_lint(sess: &ParseSess, span: MultiSpan, node_id: NodeId, message: &str) {
 628     // Macros loaded from other crates have dummy node ids.
 629     if node_id != DUMMY_NODE_ID {
 630         sess.buffer_lint(&META_VARIABLE_MISUSE, span, node_id, message);
 631     }
 632 }