Rust Error E0623: Mismatched Lifetimes

Understanding Rust Error E0623

Rust Error E0623 occurs when lifetimes in a program do not match the expected lifetimes. This error helps prevent issues that could occur due to incorrect borrowing, references, or object lifetimes. In this article, you will learn how to identify and fix this error using comprehensive code examples.

Erroneous Code Example

Consider the following code snippet that throws error E0623:

#![allow(unused)]
fn main() {
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, the lifetime 'in_ is unrelated to 'out, causing a mismatch.

Fixing the Error

There are two ways to fix Rust error E0623 for this example:

Method 1: Make 'in_ live at least as long as 'out

You can solve the error by making the 'in_ lifetime live at least as long as 'out:

#![allow(unused)]
fn main() {
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()
}
}

Method 2: Use only one lifetime

A simpler approach is to use only one lifetime throughout the function:

#![allow(unused)]
fn main() {
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()
}
}

Conclusion

Rust error E0623 occurs when lifetimes within a specific piece of code do not match the expected values. Understanding lifetime relationships is crucial because they help ensure proper memory management and safe referencing in your Rust programs. Using the appropriate lifetime constraints and relationships as shown in the examples above will help you fix this error and improve your code quality.

Recommended Reading

• Rust Lifetime Syntax