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Auto merge of #45294 - petrochenkov:prioplus, r=nikomatsakis

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syntax: Lower priority of `+` in `impl Trait`/`dyn Trait`

Now you have to write `Fn() -> (impl A + B)` instead of `Fn() -> impl A + B`, this is consistent with priority of `+` in trait objects (`Fn() -> A + B` means `(Fn() -> A) + B`).

To make this viable I changed the syntax to also permit `+` in return types in function declarations
```
fn f() -> dyn A + B { ... } // OK, don't have to write `-> (dyn A + B)`

// This is acceptable, because `dyn A + B` here is an isolated type and
// not part of a larger type with various operator priorities in play
// like `dyn A + B` in `Fn() -> dyn A + B` despite syntax similarities.
```
but you still have to use `-> (dyn A + B)` in function types and function-like trait object types (see this PR's tests for examples).

This can be a breaking change for code using `impl Trait` on nightly. The thing that is most likely to break is `&impl A + B`, it needs to be rewritten as `&(impl A + B)`.

cc https://github.com/rust-lang/rust/issues/34511 https://github.com/rust-lang/rust/issues/44662 https://github.com/rust-lang/rfcs/pull/438
This commit is contained in:
bors 2018-01-30 08:23:41 +00:00
commit fe7e1a45f3
5 changed files with 154 additions and 12 deletions
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35
src/libsyntax/parse/parser.rs
View file

@ -1364,7 +1364,7 @@ impl<'a> Parser<'a> {
self.expect_keyword(keywords::Fn)?;
let (inputs, variadic) = self.parse_fn_args(false, true)?;
let ret_ty = self.parse_ret_ty()?;
let ret_ty = self.parse_ret_ty(false)?;
let decl = P(FnDecl {
inputs,
output: ret_ty,
@ -1503,9 +1503,9 @@ impl<'a> Parser<'a> {
}
/// Parse optional return type [ -> TY ] in function decl
pub fn parse_ret_ty(&mut self) -> PResult<'a, FunctionRetTy> {
fn parse_ret_ty(&mut self, allow_plus: bool) -> PResult<'a, FunctionRetTy> {
if self.eat(&token::RArrow) {
Ok(FunctionRetTy::Ty(self.parse_ty_no_plus()?))
Ok(FunctionRetTy::Ty(self.parse_ty_common(allow_plus, true)?))
} else {
Ok(FunctionRetTy::Default(self.span.with_hi(self.span.lo())))
}
@ -1530,6 +1530,7 @@ impl<'a> Parser<'a> {
maybe_whole!(self, NtTy, |x| x);
let lo = self.span;
let mut impl_dyn_multi = false;
let node = if self.eat(&token::OpenDelim(token::Paren)) {
// `(TYPE)` is a parenthesized type.
// `(TYPE,)` is a tuple with a single field of type TYPE.
@ -1616,13 +1617,17 @@ impl<'a> Parser<'a> {
self.parse_remaining_bounds(lifetime_defs, path, lo, parse_plus)?
}
} else if self.eat_keyword(keywords::Impl) {
// FIXME: figure out priority of `+` in `impl Trait1 + Trait2` (#34511).
TyKind::ImplTrait(self.parse_ty_param_bounds()?)
// Always parse bounds greedily for better error recovery.
let bounds = self.parse_ty_param_bounds()?;
impl_dyn_multi = bounds.len() > 1 || self.prev_token_kind == PrevTokenKind::Plus;
TyKind::ImplTrait(bounds)
} else if self.check_keyword(keywords::Dyn) &&
self.look_ahead(1, |t| t.can_begin_bound() && !can_continue_type_after_ident(t)) {
// FIXME: figure out priority of `+` in `dyn Trait1 + Trait2` (#34511).
self.bump(); // `dyn`
TyKind::TraitObject(self.parse_ty_param_bounds()?, TraitObjectSyntax::Dyn)
// Always parse bounds greedily for better error recovery.
let bounds = self.parse_ty_param_bounds()?;
impl_dyn_multi = bounds.len() > 1 || self.prev_token_kind == PrevTokenKind::Plus;
TyKind::TraitObject(bounds, TraitObjectSyntax::Dyn)
} else if self.check(&token::Question) ||
self.check_lifetime() && self.look_ahead(1, |t| t == &token::BinOp(token::Plus)) {
// Bound list (trait object type)
@ -1658,6 +1663,7 @@ impl<'a> Parser<'a> {
let ty = Ty { node, span, id: ast::DUMMY_NODE_ID };
// Try to recover from use of `+` with incorrect priority.
self.maybe_report_ambiguous_plus(allow_plus, impl_dyn_multi, &ty);
self.maybe_recover_from_bad_type_plus(allow_plus, &ty)?;
let ty = self.maybe_recover_from_bad_qpath(ty, allow_qpath_recovery)?;
@ -1675,6 +1681,15 @@ impl<'a> Parser<'a> {
Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::None))
}
fn maybe_report_ambiguous_plus(&mut self, allow_plus: bool, impl_dyn_multi: bool, ty: &Ty) {
if !allow_plus && impl_dyn_multi {
let sum_with_parens = format!("({})", pprust::ty_to_string(&ty));
self.struct_span_err(ty.span, "ambiguous `+` in a type")
.span_suggestion(ty.span, "use parentheses to disambiguate", sum_with_parens)
.emit();
}
}
fn maybe_recover_from_bad_type_plus(&mut self, allow_plus: bool, ty: &Ty) -> PResult<'a, ()> {
// Do not add `+` to expected tokens.
if !allow_plus || self.token != token::BinOp(token::Plus) {
@ -4896,7 +4911,7 @@ impl<'a> Parser<'a> {
pub fn parse_fn_decl(&mut self, allow_variadic: bool) -> PResult<'a, P<FnDecl>> {
let (args, variadic) = self.parse_fn_args(true, allow_variadic)?;
let ret_ty = self.parse_ret_ty()?;
let ret_ty = self.parse_ret_ty(true)?;
Ok(P(FnDecl {
inputs: args,
@ -5037,7 +5052,7 @@ impl<'a> Parser<'a> {
self.expect(&token::CloseDelim(token::Paren))?;
Ok(P(FnDecl {
inputs: fn_inputs,
output: self.parse_ret_ty()?,
output: self.parse_ret_ty(true)?,
variadic: false
}))
}
@ -5059,7 +5074,7 @@ impl<'a> Parser<'a> {
args
}
};
let output = self.parse_ret_ty()?;
let output = self.parse_ret_ty(true)?;
Ok(P(FnDecl {
inputs: inputs_captures,