E0015: cannot call non-const function in constants Rust Origin: Application & API OSI layer 7 · Application Last updated 5 Nov 2026

Rust: error[E0015]: cannot call non-const function `String::from` in constants

A const/static initializer used something that can't run at compile time.

Exact message you may see error[E0015]: cannot call non-const function `String::from` in constantscannot call non-const method in staticscannot call non-const associated function `HashMap::<K, V>::new` in staticserror[E0015]: cannot call non-const fncalls in constants are limited to constant functions
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Seen on: Rust

Meaning

Use const fn equivalents, literals (&str), LazyLock/OnceLock/lazy_static/once_cell for runtime-initialised statics, or compute at startup. String::from, Vec::new() (const OK), HashMap::new() (not const) differ.

Common causes

  • static X: String = String::from(...)
  • HashMap/Mutex in statics (older Rust)
  • Function not marked const fn
  • Heap allocation in const context
  • Trait method calls in const

⚡ Quick fix

  1. static GREETING: &str = "hi";
  2. static CACHE: LazyLock<Mutex<HashMap<..>>> = LazyLock::new(...)
  3. Mark your own functions const fn when possible

Detailed fix by platform

Rust

  1. use std::sync::LazyLock; static CONFIG: LazyLock<Config> = LazyLock::new(|| Config::load());

How to diagnose

  1. Initializer — const-evaluable?
  2. Alternative — LazyLock?
  3. Fn — const fn?

🔧 Still not fixed?

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Where this error sits

Origin: Application & API. Errors that are often confused with or caused by this one (click to open):

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