E0106: missing lifetime specifier Rust Origin: Application & API Last updated 5 Nov 2026

Rust: error[E0106]: missing lifetime specifier — this function's return type contains a borrowed value, but the signature does not say which one of `x`'s 2 lifetimes it is borrowed from

The compiler can't infer which input reference the returned/stored reference is tied to.

Exact message you may see error[E0106]: missing lifetime specifiermissing lifetime specifierexpected named lifetime parameterthis function's return type contains a borrowed value, but the signature does not say which one ofhelp: consider introducing a named lifetime parameter
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Seen on: Rust

Meaning

Add lifetime parameters: fn longest<'a>(x: &'a str, y: &'a str) -> &'a str, or return an owned type (String). Structs holding references need struct S<'a> { s: &'a str }. With one input reference elision works; with several or none it doesn't.

Common causes

  • Function returns a reference with 2+ reference params
  • Struct field with &T and no lifetime
  • impl blocks missing <'a>
  • Returning a reference to a local
  • Trait objects needing + 'a

⚡ Quick fix

  1. Add <'a> and tie inputs/outputs
  2. Return owned data (String, Vec)
  3. Use 'static only for truly static data

Detailed fix by platform

Rust

  1. fn longest<'a>(x: &'a str, y: &'a str) -> &'a str { if x.len() > y.len() { x } else { y } }

How to diagnose

  1. Params — How many refs?
  2. Return — Borrowed from which?
  3. Alternative — Owned?

🔧 Still not fixed?

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Where this error sits

Origin: Application & API. Errors that are often confused with or caused by this one (click to open):

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