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Auto merge of rust-lang#35777 - nrc:carrier3, r=@nikomatsakis
Carrier trait (third attempt) This adds a `Carrier` trait to operate with `?`. The only public implementation is for `Result`, so effectively the trait does not exist, however, it ensures future compatibility for the `?` operator. This is not intended to be used, nor is it intended to be a long-term solution. Although this exact PR has not been through Crater, I do not expect it to be a breaking change based on putting numerous similar PRs though Crater in the past. cc: * [? tracking issue](rust-lang#31436) * [previous PR](rust-lang#35056) * [RFC issue](rust-lang/rfcs#1718) for discussion of long-term Carrier trait solutions. r? @nikomatsakis
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// Copyright 2016 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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#![feature(question_mark, question_mark_carrier)] | ||
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// Test that type inference fails where there are multiple possible return types | ||
// for the `?` operator. | ||
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fn f(x: &i32) -> Result<i32, ()> { | ||
Ok(*x) | ||
} | ||
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fn g() -> Result<Vec<i32>, ()> { | ||
let l = [1, 2, 3, 4]; | ||
l.iter().map(f).collect()? //~ ERROR type annotations required: cannot resolve | ||
} | ||
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fn main() { | ||
g(); | ||
} |
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