Understanding Rust Error E0495

What does Rust Error E0495 mean?

Rust Error E0495 occurs when a lifetime cannot be determined in a given situation. This error is no longer emitted by the compiler, as the related issues are now covered by other error codes, but it may still occasionally pop up when dealing with old Rust codes.

Example of Erroneous Code:

#![allow(unused)]
fn main() {
  fn transmute_lifetime<'a, 'b, T>(t: &'a (T,)) -> &'b T {
    match (&t,) { // error!
      ((u,),) => u,
    }
  }
  let y = Box::new((42,));
  let x = transmute_lifetime(&y);
}

How to Solve Rust Error E0495?

There are two ways to solve this issue: 1. Enforce that 'a lives at least as long as 'b. 2. Use the same lifetime requirement for both input and output values.

Solution 1: Enforce that 'a lives at least as long as 'b

You can enforce that 'a lives at least as long as 'b by replacing 'a with 'a: 'b.

#![allow(unused)]
fn main() {
  fn transmute_lifetime<'a: 'b, 'b, T>(t: &'a (T,)) -> &'b T {
    match (&t,) { // ok!
      ((u,),) => u,
    }
  }
}

Solution 2: Use the same lifetime requirement for both input and output values

This solution can be achieved by simply removing 'b, so both input and output values use 'a.

#![allow(unused)]
fn main() {
  fn transmute_lifetime<'a, T>(t: &'a (T,)) -> &'a T {
    match (&t,) { // ok!
      ((u,),) => u,
    }
  }
}