Understanding Rust Error E0688

Introduction

Rust Error E0688 occurs when in-band lifetimes are mixed with explicit lifetime binders. In this article, we will delve into the details of this error, explain its cause and provide solutions for correcting it.

Erroneous Code Example

#![feature(in_band_lifetimes)]

fn foo<'a>(x: &'a u32, y: &'b u32) {}   // error!

struct Foo<'a> { x: &'a u32 }

impl Foo<'a> {
    fn bar<'b>(x: &'a u32, y: &'b u32, z: &'c u32) {}   // error!
}

impl<'b> Foo<'a> {  // error!
    fn baz() {}
}

Reason for Error E0688

The Rust compiler generates Error E0688 when in-band lifetime syntax is mixed with explicit lifetime binders. In-band lifetime syntax is an experimental feature used to simplify the lifetime syntax in function signatures. Mixing both the in-band lifetimes and explicit lifetime binders in the same function or implementation block is not allowed.

Solution

To resolve this error, you need to avoid mixing in-band lifetimes with explicit lifetime binders. One approach involves declaring all required lifetimes explicitly using lifetime binders, as shown in the following code example:

#![allow(unused)]

fn foo<'a, 'b, 'c>(x: &'a u32, y: &'b u32, z: &'c u32) {}   // ok!

struct Foo<'a> { x: &'a u32 }

impl<'a> Foo<'a> {
    fn bar<'b, 'c>(x: &'a u32, y: &'b u32, z: &'c u32) {}   // ok!
}

impl<'b> Foo<'a> {  // ok!
    fn baz() {}
}

Conclusion

By ensuring that you do not mix in-band lifetimes with explicit lifetime binders, you can avoid Rust Error E0688. Always use either in-band lifetimes or explicit lifetime binders consistently to maintain adherence to the language's syntax and prevent the occurrence of this error.