1. Symptoms
The Rust compiler produces error E0594 when a program attempts to assign a value to a constant that cannot be evaluated at compile time. The compiler emits a clear diagnostic message that identifies the offending assignment and explains the fundamental restriction.
Shell output from cargo build:
error[E0594]: cannot assign to an immutable constant
--> src/main.rs:6:5
|
6 | X = 5;
| ^ cannot assign to an immutable constant
error: aborting due to 1 previous error
When the constant in question is initialized with a non-constant expression, the error message provides additional context about the nature of the expression that prevents compile-time evaluation:
error[E0594]: cannot assign to an immutable constant
--> src/lib.rs:10:9
|
10 | const Y: i32 = complicated_function(10);
| ^^^^^^^^^^^^^^^^^^^^^ cannot assign value to `Y`
|
= note: assigning a value to a constant here would require the value to be
evaluatable at compile time, but the expression
`complicated_function(10)` cannot be evaluated at compile time
For more information about this error, try `rustc --explain E0594`.
In more complex scenarios involving function calls within the initialization expression, the compiler may also emit an additional note explaining why the expression fails to qualify as a constant:
note: This function call is not a `const fn`
--> src/lib.rs:5:1
|
5 | fn complicated_function(x: i32) -> i32 {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not a `const fn`
The symptoms manifest during the compilation phase, specifically at the type-checking and const-evaluation stage. There are no runtime symptoms because the error prevents the program from compiling entirely.
2. Root Cause
Error E0594 arises from a fundamental distinction in Rust’s type system between constants and variables. Constants in Rust are values that must be known and fully evaluable at compile time. The compiler performs aggressive constant propagation and evaluation, which means that every expression on the right-hand side of a constant declaration must be reducible to a concrete value during compilation.
The root cause typically falls into one of three categories. The first and most common is attempting to assign to a constant after its initial declaration. Rust constants are inherently immutable and cannot be modified after definition. Unlike static variables, which occupy a fixed memory location and can potentially be mutated through unsafe code, regular constants are pure compile-time values that get inlined wherever they are used. There is no memory location to write to after the constant has been defined.
The second category involves initializing a constant with a function call to a function that is not marked as a const fn. Any expression used in a constant’s initializer must be evaluable at compile time. When you call a regular function, the compiler cannot guarantee that the function’s behavior is side-effect free or that it can be fully evaluated without a runtime context. Even if the function appears simple, Rust’s conservative approach requires explicit opt-in via the const fn keyword.
The third category involves attempting to use runtime values or expressions that depend on input data within a constant initializer. Constants cannot reference variables, function parameters, or any value that only exists at runtime. The initializer must be composed entirely of other constants, literal values, and calls to other const fn functions.
Rust’s const system is intentionally restrictive because constants are guaranteed to be inlined and evaluated at compile time. This guarantee is essential for performance-sensitive code and for features like compile-time function evaluation (CTFE). The compiler must be able to fully evaluate the constant expression without executing the program, which imposes strict requirements on what expressions are permitted in constant initializers.
3. Step-by-Step Fix
The fix for E0594 depends on the specific cause of the error. Below are the most common scenarios and their respective solutions.
Scenario 1: Reassigning a Constant Value
If you are trying to change the value of a constant after its initial declaration, you must refactor your code to use a mutable variable instead.
Before:
const MAX_CONNECTIONS: u32 = 100;
fn configure_server() {
MAX_CONNECTIONS = 200; // Error E0594
}
After:
fn configure_server() {
let max_connections: u32 = 100;
// Use the mutable variable within this scope
let configured = process_config(max_connections);
}
If you genuinely need a global mutable value, use a static mut variable inside an unsafe block, though this pattern is strongly discouraged due to data races. Prefer interior mutability types like Mutex or RwLock wrapped in a static:
use std::sync::Mutex;
static MAX_CONNECTIONS: Mutex<u32> = Mutex::new(100);
fn configure_server() {
let mut guard = MAX_CONNECTIONS.lock().unwrap();
*guard = 200;
}
Scenario 2: Using a Non-Const Function in Constant Initializer
If your constant initialization calls a function that is not marked as const fn, you have two options depending on your requirements.
Before:
fn square(x: i32) -> i32 {
x * x
}
const SQUARED: i32 = square(5); // Error E0594
After (Option A): If the function can be made const, add the const keyword to its signature:
const fn square(x: i32) -> i32 {
x * x
}
const SQUARED: i32 = square(5); // Works correctly
After (Option B): If the function cannot be made const (perhaps it contains complex logic, loops, or system calls), compute the value at runtime instead:
fn square(x: i32) -> i32 {
x * x
}
fn main() {
let squared = square(5);
println!("The square is: {}", squared);
}
Scenario 3: Complex Constant Initialization with Multiple Functions
Before:
fn helper(a: i32, b: i32) -> i32 {
a + b
}
const VALUE: i32 = helper(10, 20); // Error E0594
After:
const fn helper(a: i32, b: i32) -> i32 {
a + b
}
const VALUE: i32 = helper(10, 20); // Valid constant expression
Scenario 4: Using Struct or Array Constants
Before:
struct Config {
timeout: u64,
}
const DEFAULT_CONFIG: Config = Config {
timeout: compute_timeout(), // Error E0594 if compute_timeout is not const
};
fn compute_timeout() -> u64 {
5000
}
After:
const fn compute_timeout() -> u64 {
5000
}
struct Config {
timeout: u64,
}
const DEFAULT_CONFIG: Config = Config {
timeout: compute_timeout(),
};
4. Verification
After applying the fix, verify that the error is resolved by recompiling the project. Use cargo build for a standard project or rustc directly for single-file programs.
Verification steps:
cargo build
A successful build produces no error output and exits with code zero:
echo $?
0
For more granular verification, run the tests to ensure the fix does not introduce regressions:
cargo test
If you converted a constant to a runtime variable, verify that the program’s behavior remains correct by checking the output or test results. For const fn conversions, the program’s observable behavior should remain identical because const evaluation produces the same result as runtime evaluation would have.
To confirm that the constant is genuinely being evaluated at compile time, you can inspect the generated assembly or use the rustc flags to dump constant evaluation results:
rustc --emit=llvm-ir src/main.rs 2>&1 | grep -i "constant"
Alternatively, check that cargo check passes without errors, which is faster than a full build:
cargo check
Checking your_crate v0.1.0 (file://...)
Finished dev [unoptimized + debuginfo] target(s))
5. Common Pitfalls
There are several mistakes developers frequently make when encountering E0594 that can lead to frustration or incorrect fixes.
Confusing constants with statics. A common misconception is that const and static behave identically. While both declare values at static scope, const values are inlined at every use site, whereas static variables have a single memory location. Attempting to use patterns appropriate for static variables (like interior mutability) with const declarations will fail with E0594 or similar errors.
Assuming const fn can contain any code. Not all Rust constructs are permitted inside const fn even when the function is marked with the const keyword. As of recent Rust editions, many features are supported in const contexts, but some remain restricted, such as trait bounds with certain complex predicates, panics that depend on runtime conditions, or calls to non-const functions. When using const fn, start with the simplest possible implementation and gradually add complexity while testing at each step.
Overlooking indirect non-const dependencies. A constant initialization might call a function that internally calls another function that is not const. The compiler traces the entire call chain and rejects the constant if any link in the chain is non-const. Audit the entire dependency tree of your constant’s initializer expression, not just the top-level function calls.
Using runtime configuration values in constant initializers. When building systems that read configuration from environment variables or files, developers sometimes attempt to initialize constants with these values. This is fundamentally impossible because constants must be known at compile time. Use runtime initialization patterns with lazy_static, once_cell, or similar crates instead.
Forgetting to mark helper functions as const. When extracting logic into helper functions for use in constant initializers, it is easy to forget to add the const keyword. The compiler will emit E0594 with a note pointing to the missing const fn marker. Always double-check that every function in the constant evaluation chain carries the const modifier.
6. Related Errors
E0080: Evaluation of a Constant Failed
This error occurs when a constant expression is syntactically valid but fails during const evaluation. For example, a constant that divides by zero or performs an invalid bit cast will trigger E0080 rather than E0594. The distinction is that E0594 addresses the structural impossibility of using a non-constant expression, while E0080 addresses runtime logic errors that occur during the compile-time evaluation of a valid-looking constant expression.
E0015: Calls to Non-Const Functions Not Allowed in Statics
When initializing a static variable with a function call to a non-const function, the compiler produces E0015. The error message is similar to E0594 but specifically applies to static variables rather than const declarations. The underlying restriction is the same—both require expressions that can be evaluated at compile time—but the error code differs based on whether the declaration uses the static or const keyword.
E0433: Failed to Resolve
This error occurs when the compiler cannot find a name that is referenced in the code. While not directly related to constant evaluation, E0433 can appear alongside E0594 when a developer attempts to reference a runtime-only construct (like a function from a missing crate) inside a constant context. The combination of these errors typically indicates a missing dependency or an incorrectly imported module combined with an attempt to use that module in a constant context.