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 /// Stack represented as linked list.
 120 ///
 121 /// Those are used for environments because they grow incrementally and are not mutable.
 122 enum Stack<'a, T> {
 123     /// Empty stack.
 124     Empty,
 125     /// A non-empty stack.
 126     Push {
 127         /// The top element.
 128         top: T,
 129         /// The previous elements.
 130         prev: &'a Stack<'a, T>,
 131     },
 132 }
 133
 134 impl<'a, T> Stack<'a, T> {
 135     /// Returns whether a stack is empty.
 136     fn is_empty(&self) -> bool {
 137         match *self {
 138             Stack::Empty => true,
 139             _ => false,
 140         }
 141     }
 142
 143     /// Returns a new stack with an element of top.
 144     fn push(&'a self, top: T) -> Stack<'a, T> {
 145         Stack::Push { top, prev: self }
 146     }
 147 }
 148
 149 impl<'a, T> Iterator for &'a Stack<'a, T> {
 150     type Item = &'a T;
 151
 152     // Iterates from top to bottom of the stack.
 153     fn next(&mut self) -> Option<&'a T> {
 154         match *self {
 155             Stack::Empty => None,
 156             Stack::Push { ref top, ref prev } => {
 157                 *self = prev;
 158                 Some(top)
 159             }
 160         }
 161     }
 162 }
 163
 164 impl From<&Stack<'_, KleeneToken>> for SmallVec<[KleeneToken; 1]> {
 165     fn from(ops: &Stack<'_, KleeneToken>) -> SmallVec<[KleeneToken; 1]> {
 166         let mut ops: SmallVec<[KleeneToken; 1]> = ops.cloned().collect();
 167         // The stack is innermost on top. We want outermost first.
 168         ops.reverse();
 169         ops
 170     }
 171 }
 172
 173 /// Information attached to a meta-variable binder in LHS.
 174 struct BinderInfo {
 175     /// The span of the meta-variable in LHS.
 176     span: Span,
 177     /// The stack of Kleene operators (outermost first).
 178     ops: SmallVec<[KleeneToken; 1]>,
 179 }
 180
 181 /// An environment of meta-variables to their binder information.
 182 type Binders = FxHashMap<MacroRulesNormalizedIdent, BinderInfo>;
 183
 184 /// The state at which we entered a macro definition in the RHS of another macro definition.
 185 struct MacroState<'a> {
 186     /// The binders of the branch where we entered the macro definition.
 187     binders: &'a Binders,
 188     /// The stack of Kleene operators (outermost first) where we entered the macro definition.
 189     ops: SmallVec<[KleeneToken; 1]>,
 190 }
 191
 192 /// Checks that meta-variables are used correctly in a macro definition.
 193 ///
 194 /// Arguments:
 195 /// - `sess` is used to emit diagnostics and lints
 196 /// - `node_id` is used to emit lints
 197 /// - `span` is used when no spans are available
 198 /// - `lhses` and `rhses` should have the same length and represent the macro definition
 199 pub(super) fn check_meta_variables(
 200     sess: &ParseSess,
 201     node_id: NodeId,
 202     span: Span,
 203     lhses: &[TokenTree],
 204     rhses: &[TokenTree],
 205 ) -> bool {
 206     if lhses.len() != rhses.len() {
 207         sess.span_diagnostic.span_bug(span, "length mismatch between LHSes and RHSes")
 208     }
 209     let mut valid = true;
 210     for (lhs, rhs) in lhses.iter().zip(rhses.iter()) {
 211         let mut binders = Binders::default();
 212         check_binders(sess, node_id, lhs, &Stack::Empty, &mut binders, &Stack::Empty, &mut valid);
 213         check_occurrences(sess, node_id, rhs, &Stack::Empty, &binders, &Stack::Empty, &mut valid);
 214     }
 215     valid
 216 }
 217
 218 /// Checks `lhs` as part of the LHS of a macro definition, extends `binders` with new binders, and
 219 /// sets `valid` to false in case of errors.
 220 ///
 221 /// Arguments:
 222 /// - `sess` is used to emit diagnostics and lints
 223 /// - `node_id` is used to emit lints
 224 /// - `lhs` is checked as part of a LHS
 225 /// - `macros` is the stack of possible outer macros
 226 /// - `binders` contains the binders of the LHS
 227 /// - `ops` is the stack of Kleene operators from the LHS
 228 /// - `valid` is set in case of errors
 229 fn check_binders(
 230     sess: &ParseSess,
 231     node_id: NodeId,
 232     lhs: &TokenTree,
 233     macros: &Stack<'_, MacroState<'_>>,
 234     binders: &mut Binders,
 235     ops: &Stack<'_, KleeneToken>,
 236     valid: &mut bool,
 237 ) {
 238     match *lhs {
 239         TokenTree::Token(..) => {}
 240         // This can only happen when checking a nested macro because this LHS is then in the RHS of
 241         // the outer macro. See ui/macros/macro-of-higher-order.rs where $y:$fragment in the
 242         // LHS of the nested macro (and RHS of the outer macro) is parsed as MetaVar(y) Colon
 243         // MetaVar(fragment) and not as MetaVarDecl(y, fragment).
 244         TokenTree::MetaVar(span, name) => {
 245             if macros.is_empty() {
 246                 sess.span_diagnostic.span_bug(span, "unexpected MetaVar in lhs");
 247             }
 248             let name = MacroRulesNormalizedIdent::new(name);
 249             // There are 3 possibilities:
 250             if let Some(prev_info) = binders.get(&name) {
 251                 // 1. The meta-variable is already bound in the current LHS: This is an error.
 252                 let mut span = MultiSpan::from_span(span);
 253                 span.push_span_label(prev_info.span, "previous declaration".into());
 254                 buffer_lint(sess, span, node_id, "duplicate matcher binding");
 255             } else if get_binder_info(macros, binders, name).is_none() {
 256                 // 2. The meta-variable is free: This is a binder.
 257                 binders.insert(name, BinderInfo { span, ops: ops.into() });
 258             } else {
 259                 // 3. The meta-variable is bound: This is an occurrence.
 260                 check_occurrences(sess, node_id, lhs, macros, binders, ops, valid);
 261             }
 262         }
 263         // Similarly, this can only happen when checking a toplevel macro.
 264         TokenTree::MetaVarDecl(span, name, _kind) => {
 265             if !macros.is_empty() {
 266                 sess.span_diagnostic.span_bug(span, "unexpected MetaVarDecl in nested lhs");
 267             }
 268             let name = MacroRulesNormalizedIdent::new(name);
 269             if let Some(prev_info) = get_binder_info(macros, binders, name) {
 270                 // Duplicate binders at the top-level macro definition are errors. The lint is only
 271                 // for nested macro definitions.
 272                 sess.span_diagnostic
 273                     .struct_span_err(span, "duplicate matcher binding")
 274                     .span_label(span, "duplicate binding")
 275                     .span_label(prev_info.span, "previous binding")
 276                     .emit();
 277                 *valid = false;
 278             } else {
 279                 binders.insert(name, BinderInfo { span, ops: ops.into() });
 280             }
 281         }
 282         TokenTree::Delimited(_, ref del) => {
 283             for tt in &del.tts {
 284                 check_binders(sess, node_id, tt, macros, binders, ops, valid);
 285             }
 286         }
 287         TokenTree::Sequence(_, ref seq) => {
 288             let ops = ops.push(seq.kleene);
 289             for tt in &seq.tts {
 290                 check_binders(sess, node_id, tt, macros, binders, &ops, valid);
 291             }
 292         }
 293     }
 294 }
 295
 296 /// Returns the binder information of a meta-variable.
 297 ///
 298 /// Arguments:
 299 /// - `macros` is the stack of possible outer macros
 300 /// - `binders` contains the current binders
 301 /// - `name` is the name of the meta-variable we are looking for
 302 fn get_binder_info<'a>(
 303     mut macros: &'a Stack<'a, MacroState<'a>>,
 304     binders: &'a Binders,
 305     name: MacroRulesNormalizedIdent,
 306 ) -> Option<&'a BinderInfo> {
 307     binders.get(&name).or_else(|| macros.find_map(|state| state.binders.get(&name)))
 308 }
 309
 310 /// Checks `rhs` as part of the RHS of a macro definition and sets `valid` to false in case of
 311 /// errors.
 312 ///
 313 /// Arguments:
 314 /// - `sess` is used to emit diagnostics and lints
 315 /// - `node_id` is used to emit lints
 316 /// - `rhs` is checked as part of a RHS
 317 /// - `macros` is the stack of possible outer macros
 318 /// - `binders` contains the binders of the associated LHS
 319 /// - `ops` is the stack of Kleene operators from the RHS
 320 /// - `valid` is set in case of errors
 321 fn check_occurrences(
 322     sess: &ParseSess,
 323     node_id: NodeId,
 324     rhs: &TokenTree,
 325     macros: &Stack<'_, MacroState<'_>>,
 326     binders: &Binders,
 327     ops: &Stack<'_, KleeneToken>,
 328     valid: &mut bool,
 329 ) {
 330     match *rhs {
 331         TokenTree::Token(..) => {}
 332         TokenTree::MetaVarDecl(span, _name, _kind) => {
 333             sess.span_diagnostic.span_bug(span, "unexpected MetaVarDecl in rhs")
 334         }
 335         TokenTree::MetaVar(span, name) => {
 336             let name = MacroRulesNormalizedIdent::new(name);
 337             check_ops_is_prefix(sess, node_id, macros, binders, ops, span, name);
 338         }
 339         TokenTree::Delimited(_, ref del) => {
 340             check_nested_occurrences(sess, node_id, &del.tts, macros, binders, ops, valid);
 341         }
 342         TokenTree::Sequence(_, ref seq) => {
 343             let ops = ops.push(seq.kleene);
 344             check_nested_occurrences(sess, node_id, &seq.tts, macros, binders, &ops, valid);
 345         }
 346     }
 347 }
 348
 349 /// Represents the processed prefix of a nested macro.
 350 #[derive(Clone, Copy, PartialEq, Eq)]
 351 enum NestedMacroState {
 352     /// Nothing that matches a nested macro definition was processed yet.
 353     Empty,
 354     /// The token `macro_rules` was processed.
 355     MacroRules,
 356     /// The tokens `macro_rules!` were processed.
 357     MacroRulesNot,
 358     /// The tokens `macro_rules!` followed by a name were processed. The name may be either directly
 359     /// an identifier or a meta-variable (that hopefully would be instantiated by an identifier).
 360     MacroRulesNotName,
 361     /// The keyword `macro` was processed.
 362     Macro,
 363     /// The keyword `macro` followed by a name was processed.
 364     MacroName,
 365     /// The keyword `macro` followed by a name and a token delimited by parentheses was processed.
 366     MacroNameParen,
 367 }
 368
 369 /// Checks `tts` as part of the RHS of a macro definition, tries to recognize nested macro
 370 /// definitions, and sets `valid` to false in case of errors.
 371 ///
 372 /// Arguments:
 373 /// - `sess` is used to emit diagnostics and lints
 374 /// - `node_id` is used to emit lints
 375 /// - `tts` is checked as part of a RHS and may contain macro definitions
 376 /// - `macros` is the stack of possible outer macros
 377 /// - `binders` contains the binders of the associated LHS
 378 /// - `ops` is the stack of Kleene operators from the RHS
 379 /// - `valid` is set in case of errors
 380 fn check_nested_occurrences(
 381     sess: &ParseSess,
 382     node_id: NodeId,
 383     tts: &[TokenTree],
 384     macros: &Stack<'_, MacroState<'_>>,
 385     binders: &Binders,
 386     ops: &Stack<'_, KleeneToken>,
 387     valid: &mut bool,
 388 ) {
 389     let mut state = NestedMacroState::Empty;
 390     let nested_macros = macros.push(MacroState { binders, ops: ops.into() });
 391     let mut nested_binders = Binders::default();
 392     for tt in tts {
 393         match (state, tt) {
 394             (
 395                 NestedMacroState::Empty,
 396                 &TokenTree::Token(Token { kind: TokenKind::Ident(name, false), .. }),
 397             ) => {
 398                 if name == kw::MacroRules {
 399                     state = NestedMacroState::MacroRules;
 400                 } else if name == kw::Macro {
 401                     state = NestedMacroState::Macro;
 402                 }
 403             }
 404             (
 405                 NestedMacroState::MacroRules,
 406                 &TokenTree::Token(Token { kind: TokenKind::Not, .. }),
 407             ) => {
 408                 state = NestedMacroState::MacroRulesNot;
 409             }
 410             (
 411                 NestedMacroState::MacroRulesNot,
 412                 &TokenTree::Token(Token { kind: TokenKind::Ident(..), .. }),
 413             ) => {
 414                 state = NestedMacroState::MacroRulesNotName;
 415             }
 416             (NestedMacroState::MacroRulesNot, &TokenTree::MetaVar(..)) => {
 417                 state = NestedMacroState::MacroRulesNotName;
 418                 // We check that the meta-variable is correctly used.
 419                 check_occurrences(sess, node_id, tt, macros, binders, ops, valid);
 420             }
 421             (NestedMacroState::MacroRulesNotName, &TokenTree::Delimited(_, ref del))
 422             | (NestedMacroState::MacroName, &TokenTree::Delimited(_, ref del))
 423                 if del.delim == DelimToken::Brace =>
 424             {
 425                 let macro_rules = state == NestedMacroState::MacroRulesNotName;
 426                 state = NestedMacroState::Empty;
 427                 let rest =
 428                     check_nested_macro(sess, node_id, macro_rules, &del.tts, &nested_macros, valid);
 429                 // If we did not check the whole macro definition, then check the rest as if outside
 430                 // the macro definition.
 431                 check_nested_occurrences(
 432                     sess,
 433                     node_id,
 434                     &del.tts[rest..],
 435                     macros,
 436                     binders,
 437                     ops,
 438                     valid,
 439                 );
 440             }
 441             (
 442                 NestedMacroState::Macro,
 443                 &TokenTree::Token(Token { kind: TokenKind::Ident(..), .. }),
 444             ) => {
 445                 state = NestedMacroState::MacroName;
 446             }
 447             (NestedMacroState::Macro, &TokenTree::MetaVar(..)) => {
 448                 state = NestedMacroState::MacroName;
 449                 // We check that the meta-variable is correctly used.
 450                 check_occurrences(sess, node_id, tt, macros, binders, ops, valid);
 451             }
 452             (NestedMacroState::MacroName, &TokenTree::Delimited(_, ref del))
 453                 if del.delim == DelimToken::Paren =>
 454             {
 455                 state = NestedMacroState::MacroNameParen;
 456                 nested_binders = Binders::default();
 457                 check_binders(
 458                     sess,
 459                     node_id,
 460                     tt,
 461                     &nested_macros,
 462                     &mut nested_binders,
 463                     &Stack::Empty,
 464                     valid,
 465                 );
 466             }
 467             (NestedMacroState::MacroNameParen, &TokenTree::Delimited(_, ref del))
 468                 if del.delim == DelimToken::Brace =>
 469             {
 470                 state = NestedMacroState::Empty;
 471                 check_occurrences(
 472                     sess,
 473                     node_id,
 474                     tt,
 475                     &nested_macros,
 476                     &nested_binders,
 477                     &Stack::Empty,
 478                     valid,
 479                 );
 480             }
 481             (_, ref tt) => {
 482                 state = NestedMacroState::Empty;
 483                 check_occurrences(sess, node_id, tt, macros, binders, ops, valid);
 484             }
 485         }
 486     }
 487 }
 488
 489 /// Checks the body of nested macro, returns where the check stopped, and sets `valid` to false in
 490 /// case of errors.
 491 ///
 492 /// The token trees are checked as long as they look like a list of (LHS) => {RHS} token trees. This
 493 /// check is a best-effort to detect a macro definition. It returns the position in `tts` where we
 494 /// stopped checking because we detected we were not in a macro definition anymore.
 495 ///
 496 /// Arguments:
 497 /// - `sess` is used to emit diagnostics and lints
 498 /// - `node_id` is used to emit lints
 499 /// - `macro_rules` specifies whether the macro is `macro_rules`
 500 /// - `tts` is checked as a list of (LHS) => {RHS}
 501 /// - `macros` is the stack of outer macros
 502 /// - `valid` is set in case of errors
 503 fn check_nested_macro(
 504     sess: &ParseSess,
 505     node_id: NodeId,
 506     macro_rules: bool,
 507     tts: &[TokenTree],
 508     macros: &Stack<'_, MacroState<'_>>,
 509     valid: &mut bool,
 510 ) -> usize {
 511     let n = tts.len();
 512     let mut i = 0;
 513     let separator = if macro_rules { TokenKind::Semi } else { TokenKind::Comma };
 514     loop {
 515         // We expect 3 token trees: `(LHS) => {RHS}`. The separator is checked after.
 516         if i + 2 >= n
 517             || !tts[i].is_delimited()
 518             || !tts[i + 1].is_token(&TokenKind::FatArrow)
 519             || !tts[i + 2].is_delimited()
 520         {
 521             break;
 522         }
 523         let lhs = &tts[i];
 524         let rhs = &tts[i + 2];
 525         let mut binders = Binders::default();
 526         check_binders(sess, node_id, lhs, macros, &mut binders, &Stack::Empty, valid);
 527         check_occurrences(sess, node_id, rhs, macros, &binders, &Stack::Empty, valid);
 528         // Since the last semicolon is optional for `macro_rules` macros and decl_macro are not terminated,
 529         // we increment our checked position by how many token trees we already checked (the 3
 530         // above) before checking for the separator.
 531         i += 3;
 532         if i == n || !tts[i].is_token(&separator) {
 533             break;
 534         }
 535         // We increment our checked position for the semicolon.
 536         i += 1;
 537     }
 538     i
 539 }
 540
 541 /// Checks that a meta-variable occurrence is valid.
 542 ///
 543 /// Arguments:
 544 /// - `sess` is used to emit diagnostics and lints
 545 /// - `node_id` is used to emit lints
 546 /// - `macros` is the stack of possible outer macros
 547 /// - `binders` contains the binders of the associated LHS
 548 /// - `ops` is the stack of Kleene operators from the RHS
 549 /// - `span` is the span of the meta-variable to check
 550 /// - `name` is the name of the meta-variable to check
 551 fn check_ops_is_prefix(
 552     sess: &ParseSess,
 553     node_id: NodeId,
 554     macros: &Stack<'_, MacroState<'_>>,
 555     binders: &Binders,
 556     ops: &Stack<'_, KleeneToken>,
 557     span: Span,
 558     name: MacroRulesNormalizedIdent,
 559 ) {
 560     let macros = macros.push(MacroState { binders, ops: ops.into() });
 561     // Accumulates the stacks the operators of each state until (and including when) the
 562     // meta-variable is found. The innermost stack is first.
 563     let mut acc: SmallVec<[&SmallVec<[KleeneToken; 1]>; 1]> = SmallVec::new();
 564     for state in &macros {
 565         acc.push(&state.ops);
 566         if let Some(binder) = state.binders.get(&name) {
 567             // This variable concatenates the stack of operators from the RHS of the LHS where the
 568             // meta-variable was defined to where it is used (in possibly nested macros). The
 569             // outermost operator is first.
 570             let mut occurrence_ops: SmallVec<[KleeneToken; 2]> = SmallVec::new();
 571             // We need to iterate from the end to start with outermost stack.
 572             for ops in acc.iter().rev() {
 573                 occurrence_ops.extend_from_slice(ops);
 574             }
 575             ops_is_prefix(sess, node_id, span, name, &binder.ops, &occurrence_ops);
 576             return;
 577         }
 578     }
 579     buffer_lint(sess, span.into(), node_id, &format!("unknown macro variable `{}`", name));
 580 }
 581
 582 /// Returns whether `binder_ops` is a prefix of `occurrence_ops`.
 583 ///
 584 /// The stack of Kleene operators of a meta-variable occurrence just needs to have the stack of
 585 /// Kleene operators of its binder as a prefix.
 586 ///
 587 /// Consider $i in the following example:
 588 ///
 589 ///     ( $( $i:ident = $($j:ident),+ );* ) => { $($( $i += $j; )+)* }
 590 ///
 591 /// It occurs under the Kleene stack ["*", "+"] and is bound under ["*"] only.
 592 ///
 593 /// Arguments:
 594 /// - `sess` is used to emit diagnostics and lints
 595 /// - `node_id` is used to emit lints
 596 /// - `span` is the span of the meta-variable being check
 597 /// - `name` is the name of the meta-variable being check
 598 /// - `binder_ops` is the stack of Kleene operators for the binder
 599 /// - `occurrence_ops` is the stack of Kleene operators for the occurrence
 600 fn ops_is_prefix(
 601     sess: &ParseSess,
 602     node_id: NodeId,
 603     span: Span,
 604     name: MacroRulesNormalizedIdent,
 605     binder_ops: &[KleeneToken],
 606     occurrence_ops: &[KleeneToken],
 607 ) {
 608     for (i, binder) in binder_ops.iter().enumerate() {
 609         if i >= occurrence_ops.len() {
 610             let mut span = MultiSpan::from_span(span);
 611             span.push_span_label(binder.span, "expected repetition".into());
 612             let message = &format!("variable '{}' is still repeating at this depth", name);
 613             buffer_lint(sess, span, node_id, message);
 614             return;
 615         }
 616         let occurrence = &occurrence_ops[i];
 617         if occurrence.op != binder.op {
 618             let mut span = MultiSpan::from_span(span);
 619             span.push_span_label(binder.span, "expected repetition".into());
 620             span.push_span_label(occurrence.span, "conflicting repetition".into());
 621             let message = "meta-variable repeats with different Kleene operator";
 622             buffer_lint(sess, span, node_id, message);
 623             return;
 624         }
 625     }
 626 }
 627
 628 fn buffer_lint(sess: &ParseSess, span: MultiSpan, node_id: NodeId, message: &str) {
 629     // Macros loaded from other crates have dummy node ids.
 630     if node_id != DUMMY_NODE_ID {
 631         sess.buffer_lint(&META_VARIABLE_MISUSE, span, node_id, message);
 632     }
 633 }