E0004: non-exhaustive patterns Rust Origin: Application & API OSI layer 7 · Application Last updated 5 Nov 2026

Rust: error[E0004]: non-exhaustive patterns: `None` not covered

A match doesn't handle every possible value of the type.

Exact message you may see error[E0004]: non-exhaustive patterns: `None` not coverednon-exhaustive patterns: `&_` not coverederror[E0004]: non-exhaustive patterns: `Err(_)` not coveredpatterns `i32::MIN..=-1_i32` and `1_i32..=i32::MAX` not coveredpattern `Foo::C` not covered
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Seen on: Rust

Meaning

Add the missing arms or a catch-all _ => .... Prefer listing enum variants explicitly so new variants cause compile errors. For integers add ranges or _. if let is fine when you care about one case.

Common causes

  • Missing enum variant arm
  • Option/Result arm omitted
  • Integer range not fully covered
  • New variant added to an enum
  • Match on references missing &

⚡ Quick fix

  1. Add arms for the missing variants shown
  2. _ => unreachable!()/default when appropriate
  3. Use if let Some(x) = ... for single-case handling

Detailed fix by platform

Rust

  1. match opt { Some(v) => v, None => return Err(Error::Missing) }

How to diagnose

  1. Missing — Which patterns?
  2. Catch-all — Appropriate?
  3. if let — Enough?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

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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