RS.CLIPPY.MISSING_CONST_FOR_FN

Lint functions definitions that could be made `const fn`

This checker is a Clippy lint created by The Rust Project Contributors. The documentation shown here is a copy of the original documentation for: missing_const_for_fn. Copyright ©2025 The Rust Team. All rights reserved.

What it does

Suggests the use of const in functions and methods where possible.

Why is this bad?

Not having the function const prevents callers of the function from being const as well.

Known problems

Const functions are currently still being worked on, with some features only being available on nightly. This lint does not consider all edge cases currently and the suggestions may be incorrect if you are using this lint on stable.

Also, the lint only runs one pass over the code. Consider these two non-const functions:

fn a() -> i32 {
    0
}
fn b() -> i32 {
    a()
}

When running Clippy, the lint will only suggest to make a const, because b at this time can't be const as it calls a non-const function. Making a const and running Clippy again, will suggest to make b const, too.

If you are marking a public function with const, removing it again will break API compatibility.

Example

fn new() -> Self {
    Self { random_number: 42 }
}

Could be a const fn:

const fn new() -> Self {
    Self { random_number: 42 }
}

Configuration

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

(default: current version)