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Auto merge of #45294 - petrochenkov:prioplus, r=nikomatsakis
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 #34511 #44662 rust-lang/rfcs#438
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// Copyright 2017 The Rust Project Developers. See the COPYRIGHT | ||
// file at the top-level directory of this distribution and at | ||
// http://rust-lang.org/COPYRIGHT. | ||
// | ||
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or | ||
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license | ||
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your | ||
// option. This file may not be copied, modified, or distributed | ||
// except according to those terms. | ||
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// compile-flags: -Z parse-only -Z continue-parse-after-error | ||
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fn f() -> impl A + {} // OK | ||
fn f() -> impl A + B {} // OK | ||
fn f() -> dyn A + B {} // OK | ||
fn f() -> A + B {} // OK | ||
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impl S { | ||
fn f(self) -> impl A + { // OK | ||
let _ = |a, b| -> impl A + {}; // OK | ||
} | ||
fn f(self) -> impl A + B { // OK | ||
let _ = |a, b| -> impl A + B {}; // OK | ||
} | ||
fn f(self) -> dyn A + B { // OK | ||
let _ = |a, b| -> dyn A + B {}; // OK | ||
} | ||
fn f(self) -> A + B { // OK | ||
let _ = |a, b| -> A + B {}; // OK | ||
} | ||
} | ||
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type A = fn() -> impl A +; | ||
//~^ ERROR ambiguous `+` in a type | ||
type A = fn() -> impl A + B; | ||
//~^ ERROR ambiguous `+` in a type | ||
type A = fn() -> dyn A + B; | ||
//~^ ERROR ambiguous `+` in a type | ||
type A = fn() -> A + B; | ||
//~^ ERROR expected a path on the left-hand side of `+`, not `fn() -> A` | ||
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type A = Fn() -> impl A +; | ||
//~^ ERROR ambiguous `+` in a type | ||
type A = Fn() -> impl A + B; | ||
//~^ ERROR ambiguous `+` in a type | ||
type A = Fn() -> dyn A + B; | ||
//~^ ERROR ambiguous `+` in a type | ||
type A = Fn() -> A + B; // OK, interpreted as `(Fn() -> A) + B` for compatibility | ||
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type A = &impl A +; | ||
//~^ ERROR ambiguous `+` in a type | ||
type A = &impl A + B; | ||
//~^ ERROR ambiguous `+` in a type | ||
type A = &dyn A + B; | ||
//~^ ERROR ambiguous `+` in a type | ||
type A = &A + B; | ||
//~^ ERROR expected a path on the left-hand side of `+`, not `&A` | ||
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fn main() {} |
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error: ambiguous `+` in a type | ||
--> $DIR/impl-trait-plus-priority.rs:33:18 | ||
| | ||
33 | type A = fn() -> impl A +; | ||
| ^^^^^^^^ help: use parentheses to disambiguate: `(impl A)` | ||
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error: ambiguous `+` in a type | ||
--> $DIR/impl-trait-plus-priority.rs:35:18 | ||
| | ||
35 | type A = fn() -> impl A + B; | ||
| ^^^^^^^^^^ help: use parentheses to disambiguate: `(impl A + B)` | ||
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error: ambiguous `+` in a type | ||
--> $DIR/impl-trait-plus-priority.rs:37:18 | ||
| | ||
37 | type A = fn() -> dyn A + B; | ||
| ^^^^^^^^^ help: use parentheses to disambiguate: `(dyn A + B)` | ||
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error[E0178]: expected a path on the left-hand side of `+`, not `fn() -> A` | ||
--> $DIR/impl-trait-plus-priority.rs:39:10 | ||
| | ||
39 | type A = fn() -> A + B; | ||
| ^^^^^^^^^^^^^ perhaps you forgot parentheses? | ||
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error: ambiguous `+` in a type | ||
--> $DIR/impl-trait-plus-priority.rs:42:18 | ||
| | ||
42 | type A = Fn() -> impl A +; | ||
| ^^^^^^^^ help: use parentheses to disambiguate: `(impl A)` | ||
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error: ambiguous `+` in a type | ||
--> $DIR/impl-trait-plus-priority.rs:44:18 | ||
| | ||
44 | type A = Fn() -> impl A + B; | ||
| ^^^^^^^^^^ help: use parentheses to disambiguate: `(impl A + B)` | ||
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error: ambiguous `+` in a type | ||
--> $DIR/impl-trait-plus-priority.rs:46:18 | ||
| | ||
46 | type A = Fn() -> dyn A + B; | ||
| ^^^^^^^^^ help: use parentheses to disambiguate: `(dyn A + B)` | ||
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error: ambiguous `+` in a type | ||
--> $DIR/impl-trait-plus-priority.rs:50:11 | ||
| | ||
50 | type A = &impl A +; | ||
| ^^^^^^^^ help: use parentheses to disambiguate: `(impl A)` | ||
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error: ambiguous `+` in a type | ||
--> $DIR/impl-trait-plus-priority.rs:52:11 | ||
| | ||
52 | type A = &impl A + B; | ||
| ^^^^^^^^^^ help: use parentheses to disambiguate: `(impl A + B)` | ||
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error: ambiguous `+` in a type | ||
--> $DIR/impl-trait-plus-priority.rs:54:11 | ||
| | ||
54 | type A = &dyn A + B; | ||
| ^^^^^^^^^ help: use parentheses to disambiguate: `(dyn A + B)` | ||
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error[E0178]: expected a path on the left-hand side of `+`, not `&A` | ||
--> $DIR/impl-trait-plus-priority.rs:56:10 | ||
| | ||
56 | type A = &A + B; | ||
| ^^^^^^ help: try adding parentheses: `&(A + B)` | ||
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error: aborting due to 11 previous errors | ||
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