rust const_error ai_generated true

error[E0015]: cannot call non-const fn in const context

ID: rust/e0015-non-const-fn

Also available as: JSON · Markdown
88%Fix Rate
90%Confidence
50Evidence
2023-01-01First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
1 active — — —

Root Cause

Calling a runtime function in a const/static context. Not all functions can run at compile time.

generic

Workarounds

  1. 92% success Use lazy_static! or std::sync::LazyLock for runtime-initialized global values 
    use std::sync::LazyLock;
static CACHE: LazyLock<HashMap<String, i32>> = LazyLock::new(|| HashMap::new());

    Sources: https://doc.rust-lang.org/std/sync/struct.LazyLock.html

  2. 85% success Move initialization from const/static to a function that runs at startup 
    use std::sync::OnceLock;

static CONFIG: OnceLock<Config> = OnceLock::new();

fn get_config() -> &'static Config {
    CONFIG.get_or_init(|| {
        Config::load() // non-const initialization at first access
    })
}

    Sources: https://doc.rust-lang.org/reference/const_eval.html

Dead Ends

Common approaches that don't work:

  1. Make every function const 80% fail

    Most functions can't be const — they allocate, do IO, or have side effects

  2. Use unsafe to bypass const restrictions 90% fail

    unsafe doesn't override const restrictions

Error Chain

Frequently confused with: