use super::Parser;
use rustc_ast::ast::{self, Attribute, GenericBounds, GenericParam, GenericParamKind, WhereClause};
use rustc_ast::token;
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 `+`.
///
/// ```
/// 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 lo = self.token.span;
self.expect_keyword(kw::Const)?;
let ident = self.parse_ident()?;
self.expect(&token::Colon)?;
let ty = self.parse_ty()?;
self.sess.gated_spans.gate(sym::const_generics, lo.to(self.prev_token.span));
Ok(GenericParam {
ident,
id: ast::DUMMY_NODE_ID,
attrs: preceding_attrs.into(),
bounds: Vec::new(),
kind: GenericParamKind::Const { ty },
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();
loop {
let attrs = self.parse_outer_attributes()?;
if self.check_lifetime() {
let lifetime = self.expect_lifetime();
// Parse lifetime parameter.
let bounds =
if self.eat(&token::Colon) { self.parse_lt_param_bounds() } else { Vec::new() };
params.push(ast::GenericParam {
ident: lifetime.ident,
id: lifetime.id,
attrs: attrs.into(),
bounds,
kind: ast::GenericParamKind::Lifetime,
is_placeholder: false,
});
} else if self.check_keyword(kw::Const) {
// Parse const parameter.
params.push(self.parse_const_param(attrs)?);
} else if self.check_ident() {
// Parse type parameter.
params.push(self.parse_ty_param(attrs)?);
} else if self.token.can_begin_type() {
// Trying to write an associated type bound? (#26271)
let snapshot = self.clone();
match self.parse_ty_where_predicate() {
Ok(where_predicate) => {
self.struct_span_err(
where_predicate.span(),
"bounds on associated types do not belong here",
)
.span_label(where_predicate.span(), "belongs in `where` clause")
.emit();
}
Err(mut err) => {
err.cancel();
std::mem::replace(self, snapshot);
break;
}
}
} else {
// Check for trailing attributes and stop parsing.
if !attrs.is_empty() {
if !params.is_empty() {
self.struct_span_err(
attrs[0].span,
"trailing attribute after generic parameter",
)
.span_label(attrs[0].span, "attributes must go before parameters")
.emit();
} else {
self.struct_span_err(attrs[0].span, "attribute without generic parameters")
.span_label(
attrs[0].span,
"attributes are only permitted when preceding parameters",
)
.emit();
}
}
break;
}
if !self.eat(&token::Comma) {
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 {
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 { predicates: Vec::new(), span: self.prev_token.span.shrink_to_hi() };
if !self.eat_keyword(kw::Where) {
return Ok(where_clause);
}
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() {
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()?;
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) -> 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.token == token::Lt
&& (self.look_ahead(1, |t| t == &token::Pound || t == &token::Gt)
|| self.look_ahead(1, |t| t.is_lifetime() || t.is_ident())
&& self.look_ahead(2, |t| {
t == &token::Gt
|| t == &token::Comma
|| t == &token::Colon
|| t == &token::Eq
})
|| self.is_keyword_ahead(1, &[kw::Const]))
}
}