Rust error E0623: lifetime mismatch

What it meansA lifetime didn't match what was expected.

What rustc prints

error[E0623]: lifetime mismatch
  --> main.rs:15:8
   |
15 |     x: Foo<'in_, 'out, T>,
   |        ^^^^^^^^^^^^^^^^^^
   |        |
   |        this parameter and the return type are declared with different lifetimes...
   |        ...but data from `x` is returned here
16 |     sadness: &'in_ T
17 | ) -> &'out T {
   |      -------

Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.

E0623Ownership and borrowing

Activein rustc 1.99.0

8related errors linked below

The compiler team's explanation

A lifetime didn't match what was expected.

Erroneous code example:

struct Foo<'a, 'b, T>(std::marker::PhantomData<(&'a (), &'b (), T)>)
where
    T: Convert<'a, 'b>;

trait Convert<'a, 'b>: Sized {
    fn cast(&'a self) -> &'b Self;
}
impl<'long: 'short, 'short, T> Convert<'long, 'short> for T {
    fn cast(&'long self) -> &'short T {
        self
    }
}
// error
fn badboi<'in_, 'out, T>(
    x: Foo<'in_, 'out, T>,
    sadness: &'in_ T
) -> &'out T {
    sadness.cast()
}

In this example, we tried to set a value with an incompatible lifetime to another one ('in_ is unrelated to 'out). We can solve this issue in two different ways:

Either we make 'in_ live at least as long as 'out:

struct Foo<'a, 'b, T>(std::marker::PhantomData<(&'a (), &'b (), T)>)
where
    T: Convert<'a, 'b>;

trait Convert<'a, 'b>: Sized {
    fn cast(&'a self) -> &'b Self;
}
impl<'long: 'short, 'short, T> Convert<'long, 'short> for T {
    fn cast(&'long self) -> &'short T {
        self
    }
}
fn badboi<'in_: 'out, 'out, T>(
    x: Foo<'in_, 'out, T>,
    sadness: &'in_ T
) -> &'out T {
    sadness.cast()
}

Or we use only one lifetime:

struct Foo<'a, 'b, T>(std::marker::PhantomData<(&'a (), &'b (), T)>)
where
    T: Convert<'a, 'b>;

trait Convert<'a, 'b>: Sized {
    fn cast(&'a self) -> &'b Self;
}
impl<'long: 'short, 'short, T> Convert<'long, 'short> for T {
    fn cast(&'long self) -> &'short T {
        self
    }
}
fn badboi<'out, T>(x: Foo<'out, 'out, T>, sadness: &'out T) -> &'out T {
    sadness.cast()
}

From rustc --explain E0623 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.

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Rust error E0623: lifetime mismatch

What rustc prints

The compiler team's explanation

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