Error code E0261

An undeclared lifetime was used.

Erroneous code example:

// error, use of undeclared lifetime name `'a`
fn foo(x: &'a str) { }

struct Foo {
    // error, use of undeclared lifetime name `'a`
    x: &'a str,
}

Run

These can be fixed by declaring lifetime parameters:

struct Foo<'a> {
    x: &'a str,
}

fn foo<'a>(x: &'a str) {}

Run

Impl blocks declare lifetime parameters separately. You need to add lifetime parameters to an impl block if you’re implementing a type that has a lifetime parameter of its own. For example:

struct Foo<'a> {
    x: &'a str,
}

// error,  use of undeclared lifetime name `'a`
impl Foo<'a> {
    fn foo<'a>(x: &'a str) {}
}

Run

This is fixed by declaring the impl block like this:

struct Foo<'a> {
    x: &'a str,
}

// correct
impl<'a> Foo<'a> {
    fn foo(x: &'a str) {}
}

Run

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