1 use super::{Parser, TokenType};
2 use crate::maybe_whole;
3 use rustc_errors::{PResult, Applicability, pluralize};
4 use syntax::ast::{self, QSelf, Path, PathSegment, Ident, ParenthesizedArgs, AngleBracketedArgs};
5 use syntax::ast::{AnonConst, GenericArg, AssocTyConstraint, AssocTyConstraintKind, BlockCheckMode};
7 use syntax::token::{self, Token};
8 use syntax_pos::source_map::{Span, BytePos};
9 use syntax_pos::symbol::{kw, sym};
14 /// Specifies how to parse a path.
15 #[derive(Copy, Clone, PartialEq)]
17 /// In some contexts, notably in expressions, paths with generic arguments are ambiguous
18 /// with something else. For example, in expressions `segment < ....` can be interpreted
19 /// as a comparison and `segment ( ....` can be interpreted as a function call.
20 /// In all such contexts the non-path interpretation is preferred by default for practical
21 /// reasons, but the path interpretation can be forced by the disambiguator `::`, e.g.
22 /// `x<y>` - comparisons, `x::<y>` - unambiguously a path.
24 /// In other contexts, notably in types, no ambiguity exists and paths can be written
25 /// without the disambiguator, e.g., `x<y>` - unambiguously a path.
26 /// Paths with disambiguators are still accepted, `x::<Y>` - unambiguously a path too.
28 /// A path with generic arguments disallowed, e.g., `foo::bar::Baz`, used in imports,
29 /// visibilities or attributes.
30 /// Technically, this variant is unnecessary and e.g., `Expr` can be used instead
31 /// (paths in "mod" contexts have to be checked later for absence of generic arguments
32 /// anyway, due to macros), but it is used to avoid weird suggestions about expected
33 /// tokens when something goes wrong.
38 /// Parses a qualified path.
39 /// Assumes that the leading `<` has been parsed already.
41 /// `qualified_path = <type [as trait_ref]>::path`
46 /// `<T as U>::F::a<S>` (without disambiguator)
47 /// `<T as U>::F::a::<S>` (with disambiguator)
48 pub(super) fn parse_qpath(&mut self, style: PathStyle) -> PResult<'a, (QSelf, Path)> {
49 let lo = self.prev_span;
50 let ty = self.parse_ty()?;
52 // `path` will contain the prefix of the path up to the `>`,
53 // if any (e.g., `U` in the `<T as U>::*` examples
54 // above). `path_span` has the span of that path, or an empty
55 // span in the case of something like `<T>::Bar`.
56 let (mut path, path_span);
57 if self.eat_keyword(kw::As) {
58 let path_lo = self.token.span;
59 path = self.parse_path(PathStyle::Type)?;
60 path_span = path_lo.to(self.prev_span);
62 path_span = self.token.span.to(self.token.span);
63 path = ast::Path { segments: Vec::new(), span: path_span };
66 // See doc comment for `unmatched_angle_bracket_count`.
67 self.expect(&token::Gt)?;
68 if self.unmatched_angle_bracket_count > 0 {
69 self.unmatched_angle_bracket_count -= 1;
70 debug!("parse_qpath: (decrement) count={:?}", self.unmatched_angle_bracket_count);
73 self.expect(&token::ModSep)?;
75 let qself = QSelf { ty, path_span, position: path.segments.len() };
76 self.parse_path_segments(&mut path.segments, style)?;
78 Ok((qself, Path { segments: path.segments, span: lo.to(self.prev_span) }))
81 /// Parses simple paths.
83 /// `path = [::] segment+`
84 /// `segment = ident | ident[::]<args> | ident[::](args) [-> type]`
87 /// `a::b::C<D>` (without disambiguator)
88 /// `a::b::C::<D>` (with disambiguator)
89 /// `Fn(Args)` (without disambiguator)
90 /// `Fn::(Args)` (with disambiguator)
91 pub fn parse_path(&mut self, style: PathStyle) -> PResult<'a, Path> {
92 maybe_whole!(self, NtPath, |path| {
93 if style == PathStyle::Mod &&
94 path.segments.iter().any(|segment| segment.args.is_some()) {
95 self.diagnostic().span_err(path.span, "unexpected generic arguments in path");
100 let lo = self.meta_var_span.unwrap_or(self.token.span);
101 let mut segments = Vec::new();
102 let mod_sep_ctxt = self.token.span.ctxt();
103 if self.eat(&token::ModSep) {
104 segments.push(PathSegment::path_root(lo.shrink_to_lo().with_ctxt(mod_sep_ctxt)));
106 self.parse_path_segments(&mut segments, style)?;
108 Ok(Path { segments, span: lo.to(self.prev_span) })
111 pub(super) fn parse_path_segments(
113 segments: &mut Vec<PathSegment>,
115 ) -> PResult<'a, ()> {
117 let segment = self.parse_path_segment(style)?;
118 if style == PathStyle::Expr {
119 // In order to check for trailing angle brackets, we must have finished
120 // recursing (`parse_path_segment` can indirectly call this function),
121 // that is, the next token must be the highlighted part of the below example:
123 // `Foo::<Bar as Baz<T>>::Qux`
126 // As opposed to the below highlight (if we had only finished the first
129 // `Foo::<Bar as Baz<T>>::Qux`
132 // `PathStyle::Expr` is only provided at the root invocation and never in
133 // `parse_path_segment` to recurse and therefore can be checked to maintain
135 self.check_trailing_angle_brackets(&segment, token::ModSep);
137 segments.push(segment);
139 if self.is_import_coupler() || !self.eat(&token::ModSep) {
145 pub(super) fn parse_path_segment(&mut self, style: PathStyle) -> PResult<'a, PathSegment> {
146 let ident = self.parse_path_segment_ident()?;
148 let is_args_start = |token: &Token| match token.kind {
149 token::Lt | token::BinOp(token::Shl) | token::OpenDelim(token::Paren)
150 | token::LArrow => true,
153 let check_args_start = |this: &mut Self| {
154 this.expected_tokens.extend_from_slice(
155 &[TokenType::Token(token::Lt), TokenType::Token(token::OpenDelim(token::Paren))]
157 is_args_start(&this.token)
160 Ok(if style == PathStyle::Type && check_args_start(self) ||
161 style != PathStyle::Mod && self.check(&token::ModSep)
162 && self.look_ahead(1, |t| is_args_start(t)) {
163 // We use `style == PathStyle::Expr` to check if this is in a recursion or not. If
164 // it isn't, then we reset the unmatched angle bracket count as we're about to start
165 // parsing a new path.
166 if style == PathStyle::Expr {
167 self.unmatched_angle_bracket_count = 0;
168 self.max_angle_bracket_count = 0;
171 // Generic arguments are found - `<`, `(`, `::<` or `::(`.
172 self.eat(&token::ModSep);
173 let lo = self.token.span;
174 let args = if self.eat_lt() {
176 let (args, constraints) =
177 self.parse_generic_args_with_leaning_angle_bracket_recovery(style, lo)?;
179 let span = lo.to(self.prev_span);
180 AngleBracketedArgs { args, constraints, span }.into()
183 let (inputs, _) = self.parse_paren_comma_seq(|p| p.parse_ty())?;
184 let span = ident.span.to(self.prev_span);
185 let output = if self.eat(&token::RArrow) {
186 Some(self.parse_ty_common(false, false, false)?)
190 ParenthesizedArgs { inputs, output, span }.into()
193 PathSegment { ident, args, id: ast::DUMMY_NODE_ID }
195 // Generic arguments are not found.
196 PathSegment::from_ident(ident)
200 pub(super) fn parse_path_segment_ident(&mut self) -> PResult<'a, Ident> {
201 match self.token.kind {
202 token::Ident(name, _) if name.is_path_segment_keyword() => {
203 let span = self.token.span;
205 Ok(Ident::new(name, span))
207 _ => self.parse_ident(),
211 /// Parses generic args (within a path segment) with recovery for extra leading angle brackets.
212 /// For the purposes of understanding the parsing logic of generic arguments, this function
213 /// can be thought of being the same as just calling `self.parse_generic_args()` if the source
214 /// had the correct amount of leading angle brackets.
216 /// ```ignore (diagnostics)
217 /// bar::<<<<T as Foo>::Output>();
218 /// ^^ help: remove extra angle brackets
220 fn parse_generic_args_with_leaning_angle_bracket_recovery(
224 ) -> PResult<'a, (Vec<GenericArg>, Vec<AssocTyConstraint>)> {
225 // We need to detect whether there are extra leading left angle brackets and produce an
226 // appropriate error and suggestion. This cannot be implemented by looking ahead at
227 // upcoming tokens for a matching `>` character - if there are unmatched `<` tokens
228 // then there won't be matching `>` tokens to find.
230 // To explain how this detection works, consider the following example:
232 // ```ignore (diagnostics)
233 // bar::<<<<T as Foo>::Output>();
234 // ^^ help: remove extra angle brackets
237 // Parsing of the left angle brackets starts in this function. We start by parsing the
238 // `<` token (incrementing the counter of unmatched angle brackets on `Parser` via
241 // *Upcoming tokens:* `<<<<T as Foo>::Output>;`
242 // *Unmatched count:* 1
243 // *`parse_path_segment` calls deep:* 0
245 // This has the effect of recursing as this function is called if a `<` character
246 // is found within the expected generic arguments:
248 // *Upcoming tokens:* `<<<T as Foo>::Output>;`
249 // *Unmatched count:* 2
250 // *`parse_path_segment` calls deep:* 1
252 // Eventually we will have recursed until having consumed all of the `<` tokens and
253 // this will be reflected in the count:
255 // *Upcoming tokens:* `T as Foo>::Output>;`
256 // *Unmatched count:* 4
257 // `parse_path_segment` calls deep:* 3
259 // The parser will continue until reaching the first `>` - this will decrement the
260 // unmatched angle bracket count and return to the parent invocation of this function
261 // having succeeded in parsing:
263 // *Upcoming tokens:* `::Output>;`
264 // *Unmatched count:* 3
265 // *`parse_path_segment` calls deep:* 2
267 // This will continue until the next `>` character which will also return successfully
268 // to the parent invocation of this function and decrement the count:
270 // *Upcoming tokens:* `;`
271 // *Unmatched count:* 2
272 // *`parse_path_segment` calls deep:* 1
274 // At this point, this function will expect to find another matching `>` character but
275 // won't be able to and will return an error. This will continue all the way up the
276 // call stack until the first invocation:
278 // *Upcoming tokens:* `;`
279 // *Unmatched count:* 2
280 // *`parse_path_segment` calls deep:* 0
282 // In doing this, we have managed to work out how many unmatched leading left angle
283 // brackets there are, but we cannot recover as the unmatched angle brackets have
284 // already been consumed. To remedy this, we keep a snapshot of the parser state
285 // before we do the above. We can then inspect whether we ended up with a parsing error
286 // and unmatched left angle brackets and if so, restore the parser state before we
287 // consumed any `<` characters to emit an error and consume the erroneous tokens to
288 // recover by attempting to parse again.
290 // In practice, the recursion of this function is indirect and there will be other
291 // locations that consume some `<` characters - as long as we update the count when
292 // this happens, it isn't an issue.
294 let is_first_invocation = style == PathStyle::Expr;
295 // Take a snapshot before attempting to parse - we can restore this later.
296 let snapshot = if is_first_invocation {
302 debug!("parse_generic_args_with_leading_angle_bracket_recovery: (snapshotting)");
303 match self.parse_generic_args() {
304 Ok(value) => Ok(value),
305 Err(ref mut e) if is_first_invocation && self.unmatched_angle_bracket_count > 0 => {
306 // Cancel error from being unable to find `>`. We know the error
307 // must have been this due to a non-zero unmatched angle bracket
311 // Swap `self` with our backup of the parser state before attempting to parse
312 // generic arguments.
313 let snapshot = mem::replace(self, snapshot.unwrap());
316 "parse_generic_args_with_leading_angle_bracket_recovery: (snapshot failure) \
317 snapshot.count={:?}",
318 snapshot.unmatched_angle_bracket_count,
321 // Eat the unmatched angle brackets.
322 for _ in 0..snapshot.unmatched_angle_bracket_count {
326 // Make a span over ${unmatched angle bracket count} characters.
327 let span = lo.with_hi(
328 lo.lo() + BytePos(snapshot.unmatched_angle_bracket_count)
334 "unmatched angle bracket{}",
335 pluralize!(snapshot.unmatched_angle_bracket_count)
341 "remove extra angle bracket{}",
342 pluralize!(snapshot.unmatched_angle_bracket_count)
345 Applicability::MachineApplicable,
349 // Try again without unmatched angle bracket characters.
350 self.parse_generic_args()
356 /// Parses (possibly empty) list of lifetime and type arguments and associated type bindings,
357 /// possibly including trailing comma.
358 fn parse_generic_args(&mut self) -> PResult<'a, (Vec<GenericArg>, Vec<AssocTyConstraint>)> {
359 let mut args = Vec::new();
360 let mut constraints = Vec::new();
361 let mut misplaced_assoc_ty_constraints: Vec<Span> = Vec::new();
362 let mut assoc_ty_constraints: Vec<Span> = Vec::new();
364 let args_lo = self.token.span;
367 if self.check_lifetime() && self.look_ahead(1, |t| !t.is_like_plus()) {
368 // Parse lifetime argument.
369 args.push(GenericArg::Lifetime(self.expect_lifetime()));
370 misplaced_assoc_ty_constraints.append(&mut assoc_ty_constraints);
371 } else if self.check_ident()
372 && self.look_ahead(1, |t| t == &token::Eq || t == &token::Colon)
374 // Parse associated type constraint.
375 let lo = self.token.span;
376 let ident = self.parse_ident()?;
377 let kind = if self.eat(&token::Eq) {
378 AssocTyConstraintKind::Equality {
379 ty: self.parse_ty()?,
381 } else if self.eat(&token::Colon) {
382 AssocTyConstraintKind::Bound {
383 bounds: self.parse_generic_bounds(Some(self.prev_span))?,
389 let span = lo.to(self.prev_span);
391 // Gate associated type bounds, e.g., `Iterator<Item: Ord>`.
392 if let AssocTyConstraintKind::Bound { .. } = kind {
393 self.sess.gated_spans.gate(sym::associated_type_bounds, span);
396 constraints.push(AssocTyConstraint {
397 id: ast::DUMMY_NODE_ID,
402 assoc_ty_constraints.push(span);
403 } else if self.check_const_arg() {
404 // Parse const argument.
405 let expr = if let token::OpenDelim(token::Brace) = self.token.kind {
406 self.parse_block_expr(
407 None, self.token.span, BlockCheckMode::Default, ThinVec::new()
409 } else if self.token.is_ident() {
410 // FIXME(const_generics): to distinguish between idents for types and consts,
411 // we should introduce a GenericArg::Ident in the AST and distinguish when
412 // lowering to the HIR. For now, idents for const args are not permitted.
413 if self.token.is_bool_lit() {
414 self.parse_literal_maybe_minus()?
417 self.fatal("identifiers may currently not be used for const generics")
421 self.parse_literal_maybe_minus()?
423 let value = AnonConst {
424 id: ast::DUMMY_NODE_ID,
427 args.push(GenericArg::Const(value));
428 misplaced_assoc_ty_constraints.append(&mut assoc_ty_constraints);
429 } else if self.check_type() {
430 // Parse type argument.
431 args.push(GenericArg::Type(self.parse_ty()?));
432 misplaced_assoc_ty_constraints.append(&mut assoc_ty_constraints);
437 if !self.eat(&token::Comma) {
442 // FIXME: we would like to report this in ast_validation instead, but we currently do not
443 // preserve ordering of generic parameters with respect to associated type binding, so we
444 // lose that information after parsing.
445 if misplaced_assoc_ty_constraints.len() > 0 {
446 let mut err = self.struct_span_err(
447 args_lo.to(self.prev_span),
448 "associated type bindings must be declared after generic parameters",
450 for span in misplaced_assoc_ty_constraints {
453 "this associated type binding should be moved after the generic parameters",
459 Ok((args, constraints))