1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
use super::{ForceCollect, Parser, TrailingToken};
use rustc_ast::token;
use rustc_ast::{
self as ast, Attribute, GenericBounds, GenericParam, GenericParamKind, WhereClause,
};
use rustc_errors::PResult;
use rustc_span::symbol::{kw, sym};
impl<'a> Parser<'a> {
/// Parses bounds of a lifetime parameter `BOUND + BOUND + BOUND`, possibly with trailing `+`.
///
/// ```text
/// BOUND = LT_BOUND (e.g., `'a`)
/// ```
fn parse_lt_param_bounds(&mut self) -> GenericBounds {
let mut lifetimes = Vec::new();
while self.check_lifetime() {
lifetimes.push(ast::GenericBound::Outlives(self.expect_lifetime()));
if !self.eat_plus() {
break;
}
}
lifetimes
}
/// Matches `typaram = IDENT (`?` unbound)? optbounds ( EQ ty )?`.
fn parse_ty_param(&mut self, preceding_attrs: Vec<Attribute>) -> PResult<'a, GenericParam> {
let ident = self.parse_ident()?;
// Parse optional colon and param bounds.
let bounds = if self.eat(&token::Colon) {
self.parse_generic_bounds(Some(self.prev_token.span))?
} else {
Vec::new()
};
let default = if self.eat(&token::Eq) { Some(self.parse_ty()?) } else { None };
Ok(GenericParam {
ident,
id: ast::DUMMY_NODE_ID,
attrs: preceding_attrs.into(),
bounds,
kind: GenericParamKind::Type { default },
is_placeholder: false,
})
}
fn parse_const_param(&mut self, preceding_attrs: Vec<Attribute>) -> PResult<'a, GenericParam> {
let const_span = self.token.span;
self.expect_keyword(kw::Const)?;
let ident = self.parse_ident()?;
self.expect(&token::Colon)?;
let ty = self.parse_ty()?;
// Parse optional const generics default value, taking care of feature gating the spans
// with the unstable syntax mechanism.
let default = if self.eat(&token::Eq) {
// The gated span goes from the `=` to the end of the const argument that follows (and
// which could be a block expression).
let start = self.prev_token.span;
let const_arg = self.parse_const_arg()?;
let span = start.to(const_arg.value.span);
self.sess.gated_spans.gate(sym::const_generics_defaults, span);
Some(const_arg)
} else {
None
};
Ok(GenericParam {
ident,
id: ast::DUMMY_NODE_ID,
attrs: preceding_attrs.into(),
bounds: Vec::new(),
kind: GenericParamKind::Const { ty, kw_span: const_span, default },
is_placeholder: false,
})
}
/// Parses a (possibly empty) list of lifetime and type parameters, possibly including
/// a trailing comma and erroneous trailing attributes.
pub(super) fn parse_generic_params(&mut self) -> PResult<'a, Vec<ast::GenericParam>> {
let mut params = Vec::new();
let mut done = false;
while !done {
let attrs = self.parse_outer_attributes()?;
let param =
self.collect_tokens_trailing_token(attrs, ForceCollect::No, |this, attrs| {
let param = if this.check_lifetime() {
let lifetime = this.expect_lifetime();
// Parse lifetime parameter.
let bounds = if this.eat(&token::Colon) {
this.parse_lt_param_bounds()
} else {
Vec::new()
};
Some(ast::GenericParam {
ident: lifetime.ident,
id: lifetime.id,
attrs: attrs.into(),
bounds,
kind: ast::GenericParamKind::Lifetime,
is_placeholder: false,
})
} else if this.check_keyword(kw::Const) {
// Parse const parameter.
Some(this.parse_const_param(attrs)?)
} else if this.check_ident() {
// Parse type parameter.
Some(this.parse_ty_param(attrs)?)
} else if this.token.can_begin_type() {
// Trying to write an associated type bound? (#26271)
let snapshot = this.clone();
match this.parse_ty_where_predicate() {
Ok(where_predicate) => {
this.struct_span_err(
where_predicate.span(),
"bounds on associated types do not belong here",
)
.span_label(where_predicate.span(), "belongs in `where` clause")
.emit();
// FIXME - try to continue parsing other generics?
return Ok((None, TrailingToken::None));
}
Err(mut err) => {
err.cancel();
// FIXME - maybe we should overwrite 'self' outside of `collect_tokens`?
*this = snapshot;
return Ok((None, TrailingToken::None));
}
}
} else {
// Check for trailing attributes and stop parsing.
if !attrs.is_empty() {
if !params.is_empty() {
this.struct_span_err(
attrs[0].span,
"trailing attribute after generic parameter",
)
.span_label(attrs[0].span, "attributes must go before parameters")
.emit();
} else {
this.struct_span_err(
attrs[0].span,
"attribute without generic parameters",
)
.span_label(
attrs[0].span,
"attributes are only permitted when preceding parameters",
)
.emit();
}
}
return Ok((None, TrailingToken::None));
};
if !this.eat(&token::Comma) {
done = true;
}
// We just ate the comma, so no need to use `TrailingToken`
Ok((param, TrailingToken::None))
})?;
if let Some(param) = param {
params.push(param);
} else {
break;
}
}
Ok(params)
}
/// Parses a set of optional generic type parameter declarations. Where
/// clauses are not parsed here, and must be added later via
/// `parse_where_clause()`.
///
/// matches generics = ( ) | ( < > ) | ( < typaramseq ( , )? > ) | ( < lifetimes ( , )? > )
/// | ( < lifetimes , typaramseq ( , )? > )
/// where typaramseq = ( typaram ) | ( typaram , typaramseq )
pub(super) fn parse_generics(&mut self) -> PResult<'a, ast::Generics> {
let span_lo = self.token.span;
let (params, span) = if self.eat_lt() {
let params = self.parse_generic_params()?;
self.expect_gt()?;
(params, span_lo.to(self.prev_token.span))
} else {
(vec![], self.prev_token.span.shrink_to_hi())
};
Ok(ast::Generics {
params,
where_clause: WhereClause {
has_where_token: false,
predicates: Vec::new(),
span: self.prev_token.span.shrink_to_hi(),
},
span,
})
}
/// Parses an optional where-clause and places it in `generics`.
///
/// ```ignore (only-for-syntax-highlight)
/// where T : Trait<U, V> + 'b, 'a : 'b
/// ```
pub(super) fn parse_where_clause(&mut self) -> PResult<'a, WhereClause> {
let mut where_clause = WhereClause {
has_where_token: false,
predicates: Vec::new(),
span: self.prev_token.span.shrink_to_hi(),
};
if !self.eat_keyword(kw::Where) {
return Ok(where_clause);
}
where_clause.has_where_token = true;
let lo = self.prev_token.span;
// We are considering adding generics to the `where` keyword as an alternative higher-rank
// parameter syntax (as in `where<'a>` or `where<T>`. To avoid that being a breaking
// change we parse those generics now, but report an error.
if self.choose_generics_over_qpath(0) {
let generics = self.parse_generics()?;
self.struct_span_err(
generics.span,
"generic parameters on `where` clauses are reserved for future use",
)
.span_label(generics.span, "currently unsupported")
.emit();
}
loop {
let lo = self.token.span;
if self.check_lifetime() && self.look_ahead(1, |t| !t.is_like_plus()) {
let lifetime = self.expect_lifetime();
// Bounds starting with a colon are mandatory, but possibly empty.
self.expect(&token::Colon)?;
let bounds = self.parse_lt_param_bounds();
where_clause.predicates.push(ast::WherePredicate::RegionPredicate(
ast::WhereRegionPredicate {
span: lo.to(self.prev_token.span),
lifetime,
bounds,
},
));
} else if self.check_type() {
where_clause.predicates.push(self.parse_ty_where_predicate()?);
} else {
break;
}
if !self.eat(&token::Comma) {
break;
}
}
where_clause.span = lo.to(self.prev_token.span);
Ok(where_clause)
}
fn parse_ty_where_predicate(&mut self) -> PResult<'a, ast::WherePredicate> {
let lo = self.token.span;
// Parse optional `for<'a, 'b>`.
// This `for` is parsed greedily and applies to the whole predicate,
// the bounded type can have its own `for` applying only to it.
// Examples:
// * `for<'a> Trait1<'a>: Trait2<'a /* ok */>`
// * `(for<'a> Trait1<'a>): Trait2<'a /* not ok */>`
// * `for<'a> for<'b> Trait1<'a, 'b>: Trait2<'a /* ok */, 'b /* not ok */>`
let lifetime_defs = self.parse_late_bound_lifetime_defs()?;
// Parse type with mandatory colon and (possibly empty) bounds,
// or with mandatory equality sign and the second type.
let ty = self.parse_ty_for_where_clause()?;
if self.eat(&token::Colon) {
let bounds = self.parse_generic_bounds(Some(self.prev_token.span))?;
Ok(ast::WherePredicate::BoundPredicate(ast::WhereBoundPredicate {
span: lo.to(self.prev_token.span),
bound_generic_params: lifetime_defs,
bounded_ty: ty,
bounds,
}))
// FIXME: Decide what should be used here, `=` or `==`.
// FIXME: We are just dropping the binders in lifetime_defs on the floor here.
} else if self.eat(&token::Eq) || self.eat(&token::EqEq) {
let rhs_ty = self.parse_ty()?;
Ok(ast::WherePredicate::EqPredicate(ast::WhereEqPredicate {
span: lo.to(self.prev_token.span),
lhs_ty: ty,
rhs_ty,
id: ast::DUMMY_NODE_ID,
}))
} else {
self.unexpected()
}
}
pub(super) fn choose_generics_over_qpath(&self, start: usize) -> bool {
// There's an ambiguity between generic parameters and qualified paths in impls.
// If we see `<` it may start both, so we have to inspect some following tokens.
// The following combinations can only start generics,
// but not qualified paths (with one exception):
// `<` `>` - empty generic parameters
// `<` `#` - generic parameters with attributes
// `<` (LIFETIME|IDENT) `>` - single generic parameter
// `<` (LIFETIME|IDENT) `,` - first generic parameter in a list
// `<` (LIFETIME|IDENT) `:` - generic parameter with bounds
// `<` (LIFETIME|IDENT) `=` - generic parameter with a default
// `<` const - generic const parameter
// The only truly ambiguous case is
// `<` IDENT `>` `::` IDENT ...
// we disambiguate it in favor of generics (`impl<T> ::absolute::Path<T> { ... }`)
// because this is what almost always expected in practice, qualified paths in impls
// (`impl <Type>::AssocTy { ... }`) aren't even allowed by type checker at the moment.
self.look_ahead(start, |t| t == &token::Lt)
&& (self.look_ahead(start + 1, |t| t == &token::Pound || t == &token::Gt)
|| self.look_ahead(start + 1, |t| t.is_lifetime() || t.is_ident())
&& self.look_ahead(start + 2, |t| {
matches!(t.kind, token::Gt | token::Comma | token::Colon | token::Eq)
})
|| self.is_keyword_ahead(start + 1, &[kw::Const]))
}
}