Fix E0015: Cannot Call Non-Const Fn in Const Context

Thu, 23 Jul 2026 19:56:04 +0800

1. Symptoms

When your Rust code triggers error E0015, the compiler produces output similar to the following:

error[E0015]: cannot call non-const fn `some_function` in a constant
 --> src/main.rs:5:5
  |
5  |     some_function();
  |     ^^^^^^^^^^^^^^^
  |
  = note: non-const functions may only be called at runtime, in constant
    expressions in the right-hand side of `let` statements with no patterns

This error typically manifests when you attempt to use a runtime function inside a const context, whether that context is a const fn, a static variable, or a const variable declaration. The compiler explicitly rejects the operation because constant evaluation in Rust occurs entirely at compile time, and certain operations—including function calls that the compiler cannot evaluate—are not permitted in this phase.

E0015 on ErrorVault — Developer Error Code Dictionary

Fix E0015: Cannot Call Non-Const Fn in Const Context

1. Symptoms