Rust error E0015: cannot call non-const fn in constants

Q: What does Rust error E0015 mean?

A const or static initializer calls a function that cannot run at compile time.

Q: How do I fix E0015 (cannot call non-const fn in constants)?

Use LazyLock: Std has lazy statics since Rust 1.80.

What it meansA const or static initializer calls a function that cannot run at compile time.

What rustc prints

error[E0015]: cannot call non-const function `create_some` in constants
 --> main.rs:6:25
  |
6 | const FOO: Option<u8> = create_some();
  |                         ^^^^^^^^^^^^^
  |
note: function `create_some` is not const
 --> main.rs:1:1
  |
1 | fn create_some() -> Option<u8> {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: calls in constants are limited to constant functions, tuple structs and tuple variants

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

static NAMES: Vec<String> = vec![..]; or String::from in a const.
Building a HashMap or Regex in a static.

How to fix it

1. Use LazyLock

Std has lazy statics since Rust 1.80.

use std::sync::LazyLock;
static RE: LazyLock<regex::Regex> = LazyLock::new(|| regex::Regex::new(r"\d+").unwrap());

Other wordings of this error

cannot call non-const fn in constants, calls in constants are limited to constant functions, cannot call non-const

E0015Const, static and compile-time

Activein rustc 1.99.0

2related errors linked below

The compiler team's explanation

A non-const function was called in a const context.

Erroneous code example:

fn create_some() -> Option<u8> {
    Some(1)
}

// error: cannot call non-const function `create_some` in constants
const FOO: Option<u8> = create_some();

All functions used in a const context (constant or static expression) must be marked const.

To fix this error, you can declare create_some as a constant function:

// declared as a `const` function:
const fn create_some() -> Option<u8> {
    Some(1)
}

const FOO: Option<u8> = create_some(); // no error!

From rustc --explain E0015 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.

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