1. Symptoms
When you encounter error E0669, the Rust compiler produces output similar to the following:
error[E0669]: calls in constants are limited to inherent functions, structs, variants, and trait methods
--> src/main.rs:6:5
|
6 | do_something();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The error message indicates that the function call cannot be evaluated at compile time because the called function lacks the const qualifier. The compiler explicitly states that constant evaluation contexts are restricted to inherent functions, struct constructors, enum variant constructors, and trait methods that have been marked as const.
You may also encounter variants of this error message that mention specific limitations:
error[E0669]: cannot call non-const fn in constant
--> src/lib.rs:12:3
|
12 | let result = some_function(value);
| ^^^^^^^^^^^^^^^^^^^^^^ call requires
| that `T` implements `const Fn`
The error appears during compilation and prevents the binary from being built, even if the logic inside the function would work correctly at runtime. This error commonly surfaces when initializing static variables, defining const values, or attempting to use runtime functions in compile-time contexts such as const blocks or array length expressions.
2. Root Cause
The fundamental issue behind E0669 stems from Rust’s const evaluation system, which performs computations at compile time to initialize constants and static variables. Rust places strict restrictions on what operations can occur during this evaluation phase to ensure deterministic, reproducible compilation and to prevent accessing runtime-only resources like memory allocation, system calls, or thread synchronization primitives.
When the compiler encounters a constant expression, it must evaluate every function call within that expression. However, the compiler cannot guarantee that an arbitrary non-const function will produce the same result across different compilation contexts or that it won’t perform operations incompatible with compile-time execution. Functions without the const modifier may access global state, perform I/O operations, allocate memory dynamically, or rely on runtime type information that doesn’t exist during compilation.
Rust’s const system operates on a whitelist-based model: only explicitly marked const functions and certain built-in operations are permitted in constant evaluation contexts. This design ensures that const evaluation remains predictable, safe, and suitable for initializing global variables. The compiler enforces this limitation because allowing unrestricted function calls in const contexts would compromise Rust’s guarantees about static initialization order and compile-time determinism.
Trait methods present additional complexity because they require dynamic dispatch, which inherently contradicts the static nature of const evaluation. While Rust does allow const trait methods, the trait itself must be marked with const fn compatibility, and only methods that satisfy this requirement can be called in const contexts.
3. Step-by-Step Fix
To resolve E0669, you must modify either the function being called or the context in which it’s being called. The following approaches address the issue based on your specific scenario.
Option 1: Mark the function as const
If you control the function’s definition, add the const keyword to its signature:
Before:
fn do_something(value: i32) -> i32 {
value * 2 + 5
}
const fn use_it() {
do_something(10) // Error E0669
}
After:
const fn do_something(value: i32) -> i32 {
value * 2 + 5
}
const fn use_it() {
do_something(10) // Now valid
}
Option 2: Evaluate the computation at runtime instead of compile time
If the function cannot be made const, move the computation out of the const context:
Before:
fn compute_value(x: i32) -> i32 {
x * x + 1
}
static VALUE: i32 = compute_value(42); // Error E0669
fn main() {
println!("{}", VALUE);
}
After:
fn compute_value(x: i32) -> i32 {
x * x + 1
}
fn main() {
let value = compute_value(42); // Runtime evaluation
println!("{}", value);
}
Option 3: Use lazy initialization for static variables
For static variables that require runtime computation, use lazy evaluation:
Before:
fn get_default_config() -> String {
std::env::var("CONFIG_PATH").unwrap_or_else(|_| String::from("default"))
}
static CONFIG: String = get_default_config(); // Error E0669
After:
use std::sync::LazyLock;
static CONFIG: LazyLock<String, _> = LazyLock::new(|| {
std::env::var("CONFIG_PATH").unwrap_or_else(|_| String::from("default"))
});
fn main() {
println!("{}", *CONFIG);
}
Option 4: Make trait methods const
If you’re implementing a trait method called in a const context, ensure the trait and method are both marked const:
Before:
trait Processor {
fn process(&self, input: i32) -> i32;
}
impl Processor for MyType {
fn process(&self, input: i32) -> i32 {
input + self.value
}
}
const fn use_processor(p: &MyType) -> i32 {
p.process(5) // Error E0669
}
After:
const trait Processor {
const fn process(&self, input: i32) -> i32;
}
impl Processor for MyType {
const fn process(&self, input: i32) -> i32 {
input + self.value
}
}
const fn use_processor(p: &MyType) -> i32 {
p.process(5) // Now valid
}
Option 5: Use const泛型 for type-level constants
When working with generic parameters that need constant values:
Before:
fn get_array_size() -> usize {
64
}
fn create_buffer() -> [u8; get_array_size()] {
[0; 64]
}
After:
const ARRAY_SIZE: usize = 64;
fn create_buffer<const N: usize>() -> [u8; N] {
[0; N]
}
fn main() {
let buffer: [u8; 64] = create_buffer::<64>();
}
4. Verification
After applying the fix, verify that the error has been resolved by compiling your project:
cargo build
A successful build produces no E0669 errors and completes with exit code zero. For more comprehensive validation, run the test suite:
cargo test
You should also verify that the constant values are actually being evaluated at compile time by checking the generated assembly or using the const_evaluations_checked lint to monitor const evaluation behavior. If you’re using const functions, confirm that the compiler is actually performing constant folding by examining whether the computed values appear correctly in the binary.
For cases where you’ve switched from static initialization to lazy initialization, ensure that the runtime behavior remains identical and that there are no observable changes in program startup time that might indicate the computation is occurring at an unexpected time. Adding integration tests that assert on the computed values provides additional confidence that the refactoring preserved correct behavior.
5. Common Pitfalls
When resolving E0669, developers frequently encounter several subtle issues that can cause confusion or introduce new problems.
Attempting to make methods const when the trait itself isn’t const leads to compilation errors, so ensure the entire trait definition includes const before marking individual methods. This is particularly relevant when implementing standard library traits like Display or Debug in const contexts, as these traits have methods that weren’t originally designed for const evaluation.
In Rust 2018 and later, certain operations that were previously allowed in const contexts became restricted. If you’re migrating code from an older edition, you may need to rewrite some constant initializations. Similarly, using unstable features or compiler intrinsics in const contexts may produce E0669 even when the code appears correct, as stability guarantees don’t extend to all const operations.
Be cautious about adding const to functions that contain unsafe operations, as this can create subtle issues with const evaluation of unsafe code. The const evaluator has different rules for unsafe blocks, and some patterns that work in runtime contexts may fail during compile-time evaluation.
When refactoring to avoid const evaluation, ensure that you don’t accidentally create race conditions in multi-threaded code if the value was previously a static. Runtime initialization of values that were previously static can lead to data races if multiple threads access the value before initialization completes.
6. Related Errors
E0015: Cannot perform operations not allowed in const fn
This error appears when attempting operations in a const context that haven’t been approved for compile-time evaluation, such as dynamic memory allocation, thread synchronization, or panicking. It often accompanies E0669 when a const function calls other non-const functions internally.
E0434: can’t capture dynamic environment in a static context
This error occurs when attempting to capture variables from the surrounding scope in a static or const context, which is prohibited because static variables must be initialized without depending on local variables that may not exist at the time of initialization.
E0747: type declaration in const fn body was imported from a crate without feature flag
This error signals that a const function is attempting to use a type from an external crate that hasn’t enabled the necessary features for const evaluation, even though the type would be valid in a runtime context.