1 use super::{Parser, PResult, TokenType};
3 use crate::{maybe_whole, ThinVec};
4 use crate::ast::{self, QSelf, Path, PathSegment, Ident, ParenthesizedArgs, AngleBracketedArgs};
5 use crate::ast::{AnonConst, GenericArg, AssocTyConstraint, AssocTyConstraintKind, BlockCheckMode};
6 use crate::parse::token::{self, Token};
7 use crate::source_map::{Span, BytePos};
12 use errors::{Applicability, pluralise};
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 /// Like `parse_path`, but also supports parsing `Word` meta items into paths for
112 /// backwards-compatibility. This is used when parsing derive macro paths in `#[derive]`
114 fn parse_path_allowing_meta(&mut self, style: PathStyle) -> PResult<'a, Path> {
115 let meta_ident = match self.token.kind {
116 token::Interpolated(ref nt) => match **nt {
117 token::NtMeta(ref item) => match item.tokens.is_empty() {
118 true => Some(item.path.clone()),
125 if let Some(path) = meta_ident {
129 self.parse_path(style)
132 /// Parse a list of paths inside `#[derive(path_0, ..., path_n)]`.
133 crate fn parse_derive_paths(&mut self) -> PResult<'a, Vec<Path>> {
134 self.expect(&token::OpenDelim(token::Paren))?;
135 let mut list = Vec::new();
136 while !self.eat(&token::CloseDelim(token::Paren)) {
137 let path = self.parse_path_allowing_meta(PathStyle::Mod)?;
139 if !self.eat(&token::Comma) {
140 self.expect(&token::CloseDelim(token::Paren))?;
147 pub(super) fn parse_path_segments(
149 segments: &mut Vec<PathSegment>,
151 ) -> PResult<'a, ()> {
153 let segment = self.parse_path_segment(style)?;
154 if style == PathStyle::Expr {
155 // In order to check for trailing angle brackets, we must have finished
156 // recursing (`parse_path_segment` can indirectly call this function),
157 // that is, the next token must be the highlighted part of the below example:
159 // `Foo::<Bar as Baz<T>>::Qux`
162 // As opposed to the below highlight (if we had only finished the first
165 // `Foo::<Bar as Baz<T>>::Qux`
168 // `PathStyle::Expr` is only provided at the root invocation and never in
169 // `parse_path_segment` to recurse and therefore can be checked to maintain
171 self.check_trailing_angle_brackets(&segment, token::ModSep);
173 segments.push(segment);
175 if self.is_import_coupler() || !self.eat(&token::ModSep) {
181 pub(super) fn parse_path_segment(&mut self, style: PathStyle) -> PResult<'a, PathSegment> {
182 let ident = self.parse_path_segment_ident()?;
184 let is_args_start = |token: &Token| match token.kind {
185 token::Lt | token::BinOp(token::Shl) | token::OpenDelim(token::Paren)
186 | token::LArrow => true,
189 let check_args_start = |this: &mut Self| {
190 this.expected_tokens.extend_from_slice(
191 &[TokenType::Token(token::Lt), TokenType::Token(token::OpenDelim(token::Paren))]
193 is_args_start(&this.token)
196 Ok(if style == PathStyle::Type && check_args_start(self) ||
197 style != PathStyle::Mod && self.check(&token::ModSep)
198 && self.look_ahead(1, |t| is_args_start(t)) {
199 // We use `style == PathStyle::Expr` to check if this is in a recursion or not. If
200 // it isn't, then we reset the unmatched angle bracket count as we're about to start
201 // parsing a new path.
202 if style == PathStyle::Expr {
203 self.unmatched_angle_bracket_count = 0;
204 self.max_angle_bracket_count = 0;
207 // Generic arguments are found - `<`, `(`, `::<` or `::(`.
208 self.eat(&token::ModSep);
209 let lo = self.token.span;
210 let args = if self.eat_lt() {
212 let (args, constraints) =
213 self.parse_generic_args_with_leaning_angle_bracket_recovery(style, lo)?;
215 let span = lo.to(self.prev_span);
216 AngleBracketedArgs { args, constraints, span }.into()
219 let (inputs, _) = self.parse_paren_comma_seq(|p| p.parse_ty())?;
220 let span = ident.span.to(self.prev_span);
221 let output = if self.eat(&token::RArrow) {
222 Some(self.parse_ty_common(false, false, false)?)
226 ParenthesizedArgs { inputs, output, span }.into()
229 PathSegment { ident, args, id: ast::DUMMY_NODE_ID }
231 // Generic arguments are not found.
232 PathSegment::from_ident(ident)
236 pub(super) fn parse_path_segment_ident(&mut self) -> PResult<'a, Ident> {
237 match self.token.kind {
238 token::Ident(name, _) if name.is_path_segment_keyword() => {
239 let span = self.token.span;
241 Ok(Ident::new(name, span))
243 _ => self.parse_ident(),
247 /// Parses generic args (within a path segment) with recovery for extra leading angle brackets.
248 /// For the purposes of understanding the parsing logic of generic arguments, this function
249 /// can be thought of being the same as just calling `self.parse_generic_args()` if the source
250 /// had the correct amount of leading angle brackets.
252 /// ```ignore (diagnostics)
253 /// bar::<<<<T as Foo>::Output>();
254 /// ^^ help: remove extra angle brackets
256 fn parse_generic_args_with_leaning_angle_bracket_recovery(
260 ) -> PResult<'a, (Vec<GenericArg>, Vec<AssocTyConstraint>)> {
261 // We need to detect whether there are extra leading left angle brackets and produce an
262 // appropriate error and suggestion. This cannot be implemented by looking ahead at
263 // upcoming tokens for a matching `>` character - if there are unmatched `<` tokens
264 // then there won't be matching `>` tokens to find.
266 // To explain how this detection works, consider the following example:
268 // ```ignore (diagnostics)
269 // bar::<<<<T as Foo>::Output>();
270 // ^^ help: remove extra angle brackets
273 // Parsing of the left angle brackets starts in this function. We start by parsing the
274 // `<` token (incrementing the counter of unmatched angle brackets on `Parser` via
277 // *Upcoming tokens:* `<<<<T as Foo>::Output>;`
278 // *Unmatched count:* 1
279 // *`parse_path_segment` calls deep:* 0
281 // This has the effect of recursing as this function is called if a `<` character
282 // is found within the expected generic arguments:
284 // *Upcoming tokens:* `<<<T as Foo>::Output>;`
285 // *Unmatched count:* 2
286 // *`parse_path_segment` calls deep:* 1
288 // Eventually we will have recursed until having consumed all of the `<` tokens and
289 // this will be reflected in the count:
291 // *Upcoming tokens:* `T as Foo>::Output>;`
292 // *Unmatched count:* 4
293 // `parse_path_segment` calls deep:* 3
295 // The parser will continue until reaching the first `>` - this will decrement the
296 // unmatched angle bracket count and return to the parent invocation of this function
297 // having succeeded in parsing:
299 // *Upcoming tokens:* `::Output>;`
300 // *Unmatched count:* 3
301 // *`parse_path_segment` calls deep:* 2
303 // This will continue until the next `>` character which will also return successfully
304 // to the parent invocation of this function and decrement the count:
306 // *Upcoming tokens:* `;`
307 // *Unmatched count:* 2
308 // *`parse_path_segment` calls deep:* 1
310 // At this point, this function will expect to find another matching `>` character but
311 // won't be able to and will return an error. This will continue all the way up the
312 // call stack until the first invocation:
314 // *Upcoming tokens:* `;`
315 // *Unmatched count:* 2
316 // *`parse_path_segment` calls deep:* 0
318 // In doing this, we have managed to work out how many unmatched leading left angle
319 // brackets there are, but we cannot recover as the unmatched angle brackets have
320 // already been consumed. To remedy this, we keep a snapshot of the parser state
321 // before we do the above. We can then inspect whether we ended up with a parsing error
322 // and unmatched left angle brackets and if so, restore the parser state before we
323 // consumed any `<` characters to emit an error and consume the erroneous tokens to
324 // recover by attempting to parse again.
326 // In practice, the recursion of this function is indirect and there will be other
327 // locations that consume some `<` characters - as long as we update the count when
328 // this happens, it isn't an issue.
330 let is_first_invocation = style == PathStyle::Expr;
331 // Take a snapshot before attempting to parse - we can restore this later.
332 let snapshot = if is_first_invocation {
338 debug!("parse_generic_args_with_leading_angle_bracket_recovery: (snapshotting)");
339 match self.parse_generic_args() {
340 Ok(value) => Ok(value),
341 Err(ref mut e) if is_first_invocation && self.unmatched_angle_bracket_count > 0 => {
342 // Cancel error from being unable to find `>`. We know the error
343 // must have been this due to a non-zero unmatched angle bracket
347 // Swap `self` with our backup of the parser state before attempting to parse
348 // generic arguments.
349 let snapshot = mem::replace(self, snapshot.unwrap());
352 "parse_generic_args_with_leading_angle_bracket_recovery: (snapshot failure) \
353 snapshot.count={:?}",
354 snapshot.unmatched_angle_bracket_count,
357 // Eat the unmatched angle brackets.
358 for _ in 0..snapshot.unmatched_angle_bracket_count {
362 // Make a span over ${unmatched angle bracket count} characters.
363 let span = lo.with_hi(
364 lo.lo() + BytePos(snapshot.unmatched_angle_bracket_count)
370 "unmatched angle bracket{}",
371 pluralise!(snapshot.unmatched_angle_bracket_count)
377 "remove extra angle bracket{}",
378 pluralise!(snapshot.unmatched_angle_bracket_count)
381 Applicability::MachineApplicable,
385 // Try again without unmatched angle bracket characters.
386 self.parse_generic_args()
392 /// Parses (possibly empty) list of lifetime and type arguments and associated type bindings,
393 /// possibly including trailing comma.
394 fn parse_generic_args(&mut self) -> PResult<'a, (Vec<GenericArg>, Vec<AssocTyConstraint>)> {
395 let mut args = Vec::new();
396 let mut constraints = Vec::new();
397 let mut misplaced_assoc_ty_constraints: Vec<Span> = Vec::new();
398 let mut assoc_ty_constraints: Vec<Span> = Vec::new();
400 let args_lo = self.token.span;
403 if self.check_lifetime() && self.look_ahead(1, |t| !t.is_like_plus()) {
404 // Parse lifetime argument.
405 args.push(GenericArg::Lifetime(self.expect_lifetime()));
406 misplaced_assoc_ty_constraints.append(&mut assoc_ty_constraints);
407 } else if self.check_ident() && self.look_ahead(1,
408 |t| t == &token::Eq || t == &token::Colon) {
409 // Parse associated type constraint.
410 let lo = self.token.span;
411 let ident = self.parse_ident()?;
412 let kind = if self.eat(&token::Eq) {
413 AssocTyConstraintKind::Equality {
414 ty: self.parse_ty()?,
416 } else if self.eat(&token::Colon) {
417 AssocTyConstraintKind::Bound {
418 bounds: self.parse_generic_bounds(Some(self.prev_span))?,
423 let span = lo.to(self.prev_span);
424 constraints.push(AssocTyConstraint {
425 id: ast::DUMMY_NODE_ID,
430 assoc_ty_constraints.push(span);
431 } else if self.check_const_arg() {
432 // Parse const argument.
433 let expr = if let token::OpenDelim(token::Brace) = self.token.kind {
434 self.parse_block_expr(
435 None, self.token.span, BlockCheckMode::Default, ThinVec::new()
437 } else if self.token.is_ident() {
438 // FIXME(const_generics): to distinguish between idents for types and consts,
439 // we should introduce a GenericArg::Ident in the AST and distinguish when
440 // lowering to the HIR. For now, idents for const args are not permitted.
441 if self.token.is_bool_lit() {
442 self.parse_literal_maybe_minus()?
445 self.fatal("identifiers may currently not be used for const generics")
449 self.parse_literal_maybe_minus()?
451 let value = AnonConst {
452 id: ast::DUMMY_NODE_ID,
455 args.push(GenericArg::Const(value));
456 misplaced_assoc_ty_constraints.append(&mut assoc_ty_constraints);
457 } else if self.check_type() {
458 // Parse type argument.
459 args.push(GenericArg::Type(self.parse_ty()?));
460 misplaced_assoc_ty_constraints.append(&mut assoc_ty_constraints);
465 if !self.eat(&token::Comma) {
470 // FIXME: we would like to report this in ast_validation instead, but we currently do not
471 // preserve ordering of generic parameters with respect to associated type binding, so we
472 // lose that information after parsing.
473 if misplaced_assoc_ty_constraints.len() > 0 {
474 let mut err = self.struct_span_err(
475 args_lo.to(self.prev_span),
476 "associated type bindings must be declared after generic parameters",
478 for span in misplaced_assoc_ty_constraints {
481 "this associated type binding should be moved after the generic parameters",
487 Ok((args, constraints))