Rust error E0005: refutable pattern in local binding

Q: What does Rust error E0005 mean?

A let pattern might not match (like let Some(x) = opt;). Use if let, match or let ... else.

Q: How do I fix E0005 (refutable pattern in local binding)?

let else: Handle the miss and bail out.

What it meansA let pattern might not match (like let Some(x) = opt;). Use if let, match or let ... else.

What rustc prints

error[E0005]: refutable pattern in local binding
 --> main.rs:4:5
  |
4 | let Some(y) = x;
  |     ^^^^^^^ pattern `None` not covered
  |
  = note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
  = note: for more information, visit https://doc.rust-lang.org/book/ch19-02-refutability.html
  = note: the matched value is of type `Option<i32>`
help: you might want to use `let...else` to handle the variant that isn't matched
  |
4 | let Some(y) = x else { todo!() };
  |                 ++++++++++++++++

Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.

Why it happens

Destructuring an Option or enum with plain let.

How to fix it

1. let else

Handle the miss and bail out.

let Some(user) = find(id) else {
    return Err("no such user".into());
};

Other wordings of this error

refutable pattern in local binding, pattern `None` not covered, let else

E0005Names, paths and imports

Activein rustc 1.99.0

6related errors linked below

The compiler team's explanation

Patterns used to bind names must be irrefutable, that is, they must guarantee that a name will be extracted in all cases.

Erroneous code example:

let x = Some(1);
let Some(y) = x;
// error: refutable pattern in local binding: `None` not covered

If you encounter this error you probably need to use a match or if let to deal with the possibility of failure. Example:

let x = Some(1);

match x {
    Some(y) => {
        // do something
    },
    None => {}
}

// or:

if let Some(y) = x {
    // do something
}

From rustc --explain E0005 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.

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