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Rollup merge of #89710 - sireliah:e0482, r=GuillaumeGomez
Add long explanation for error E0482 This is longer explanation for error E0482 in the #61137. Please take a look and leave some feedback!
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Original file line number | Diff line number | Diff line change |
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A lifetime of a returned value does not outlive the function call. | ||
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Erroneous code example: | ||
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```compile_fail,E0482 | ||
fn prefix<'a>( | ||
words: impl Iterator<Item = &'a str> | ||
) -> impl Iterator<Item = String> { // error! | ||
words.map(|v| format!("foo-{}", v)) | ||
} | ||
``` | ||
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To fix this error, make the lifetime of the returned value explicit: | ||
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``` | ||
fn prefix<'a>( | ||
words: impl Iterator<Item = &'a str> + 'a | ||
) -> impl Iterator<Item = String> + 'a { // ok! | ||
words.map(|v| format!("foo-{}", v)) | ||
} | ||
``` | ||
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The [`impl Trait`] feature in this example uses an implicit `'static` lifetime | ||
restriction in the returned type. However the type implementing the `Iterator` | ||
passed to the function lives just as long as `'a`, which is not long enough. | ||
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The solution involves adding lifetime bound to both function argument and | ||
the return value to make sure that the values inside the iterator | ||
are not dropped when the function goes out of the scope. | ||
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An alternative solution would be to guarantee that the `Item` references | ||
in the iterator are alive for the whole lifetime of the program. | ||
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``` | ||
fn prefix( | ||
words: impl Iterator<Item = &'static str> | ||
) -> impl Iterator<Item = String> { // ok! | ||
words.map(|v| format!("foo-{}", v)) | ||
} | ||
``` | ||
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A similar lifetime problem might arise when returning closures: | ||
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```compile_fail,E0482 | ||
fn foo( | ||
x: &mut Vec<i32> | ||
) -> impl FnMut(&mut Vec<i32>) -> &[i32] { // error! | ||
|y| { | ||
y.append(x); | ||
y | ||
} | ||
} | ||
``` | ||
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Analogically, a solution here is to use explicit return lifetime | ||
and move the ownership of the variable to the closure. | ||
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``` | ||
fn foo<'a>( | ||
x: &'a mut Vec<i32> | ||
) -> impl FnMut(&mut Vec<i32>) -> &[i32] + 'a { // ok! | ||
move |y| { | ||
y.append(x); | ||
y | ||
} | ||
} | ||
``` | ||
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To better understand the lifetime treatment in the [`impl Trait`], | ||
please see the [RFC 1951]. | ||
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[`impl Trait`]: https://doc.rust-lang.org/reference/types/impl-trait.html | ||
[RFC 1951]: https://rust-lang.github.io/rfcs/1951-expand-impl-trait.html |
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