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Suggest using -> impl Trait when returning type parameter -> T where T: Trait #85991
Description
Some newcomers are confused by impl Trait in return position because they have first internalized impl Trait as an alias for type parameters in argument position. Because of this, some people try the following:
trait Trait {}
impl Trait for () {}
// this works
fn foo() -> impl Trait {
()
}
// this doesn't
fn bar<T: Trait>() -> T {
()
}We don't currently point them in the right direction, like we do in other cases.
Took inspiration from https://twitter.com/JaapRood/status/1400782159812628482
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Area: Messages for errors, warnings, and lintsArea: `impl Trait`. Universally / existentially quantified anonymous types with static dispatch.Area: Suggestions generated by the compiler applied by `cargo fix`Diagnostics: Confusing error or lint; hard to understand for new users.Diagnostics: An error or lint that needs small tweaks.Low priorityRelevant to the compiler team, which will review and decide on the PR/issue.
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Noratrieb commentedon Sep 28, 2021
@rustbot claim
Rollup merge of rust-lang#89892 - Nilstrieb:suggest-return-impl-trait…
jackh726 commentedon Feb 19, 2022
Should have been closed by #89892