1. Symptoms
The Rust compiler produces error E0080 when it encounters a constant expression that cannot be evaluated at compile time. This error manifests in several distinct ways that you should recognize.
Primary Error Message:
error[E0080]: evaluation of constant value failed –> src/main.rs:X:Y | X | const VALUE: Type = expression; | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute value that was never initialized
**Common Variants:**
error[E0080]: evaluation of constant value failed –> src/main.rs:X:Y | X | const VALUE: u32 = 10 / 0; | ^ attempt to divide by zero
error[E0080]: evaluation of constant value failed –> src/main.rs:X:Y | X | static VALUE: SomeType = SomeType::new(); | ^^^^^^^ the type SomeType does not permit being left uninitialized
**Visual Symptoms in IDE:**
- Red squiggly underlines under `const` or `static` declarations
- Error icon in the editor margin
- Type mismatch errors appearing in the same region
- Hover text showing the evaluation failure reason
**When E0080 Typically Appears:**
- After defining constant expressions with invalid arithmetic operations
- When using uninitialized memory in `const` or `static` contexts
- During compilation of const fn that panic or return invalid values
- When trait bounds cannot be satisfied during const evaluation
## 2. Root Cause
Understanding why E0080 occurs requires knowledge of how Rust's compile-time evaluation works. The error originates from the **const evaluator** (also called miri during evaluation), which is the component responsible for determining the value of constant expressions at compile time.
**Core Cause:**
E0080 is triggered when the const evaluator attempts to evaluate an expression and encounters a condition that would cause undefined behavior or an invalid state at runtime. Unlike runtime panics, which can be caught with `Result` types, const evaluation happens before your program executes and cannot be intercepted.
**Specific Scenarios That Trigger E0080:**
**1. Division by Zero in Constant Context**
The const evaluator must determine the exact value of constant expressions. Division operations are evaluated at compile time, and division by zero is undefined behavior that the compiler refuses to permit.
```rust
const DIVIDED: i32 = 10 / 0; // E0080: division by zero
2. Uninitialized Memory
Rust enforces that certain types cannot be left uninitialized. When you declare a static or const of such a type, the compiler must verify initialization, and failure triggers E0080.
use std::mem::MaybeUninit;
const UNIT: MaybeUninit<i32> = MaybeUninit::uninit();
// E0080: MaybeUninit is not valid for const context
3. Const Fn Panics
When a const fn is called during const evaluation and panics, the entire evaluation fails with E0080.
const fn divide(a: i32, b: i32) -> i32 {
if b == 0 {
panic!("cannot divide by zero");
}
a / b
}
const RESULT: i32 = divide(10, 0); // E0080: panicked at const eval
4. Invalid Enum Variants
Creating an enum variant with an invalid discriminant value can cause E0080.
#[repr(u8)]
enum Color {
Red = 0,
Green = 1,
Blue = 2,
}
const INVALID_COLOR: Color = std::mem::transmute(255u8);
// E0080: invalid enum discriminant
5. Unwinding Through FFI Boundaries
When const evaluation would require unwinding across FFI boundaries, which is undefined behavior.
extern "C" {
fn maybe_panic();
}
const VALUE: i32 = {
unsafe { maybe_panic(); } // E0080: unwinding through FFI
42
};
Theoretical Foundation:
The const evaluator is designed to be a partial evaluator that must maintain Rust’s safety guarantees even during compile-time computation. When it encounters operations that would be UB (undefined behavior) at runtime, it halts with E0080 rather than producing potentially incorrect code.
3. Step-by-Step Fix
Resolving E0080 requires identifying the specific const evaluation failure and restructuring your code accordingly. Follow these systematic steps.
Step 1: Locate the Exact Failure Point
Read the full error message carefully. The caret (^) indicates which part of the expression failed.
cargo build 2>&1 | grep -A5 "E0080"
Step 2: Identify the Failure Type
For Division by Zero:
Before:
const SCALE: f64 = 1.0;
const INVERSE_SCALE: f64 = 1.0 / 0.0; // E0080: division by zero
After:
const SCALE: f64 = 1.0;
const INVERSE_SCALE: Option<f64> = if SCALE == 0.0 {
None
} else {
Some(1.0 / SCALE)
};
For Uninitialized Memory:
Before:
use std::mem::MaybeUninit;
const UNINIT: MaybeUninit<i32> = MaybeUninit::uninit();
const VALUE: i32 = unsafe { UNINIT.assume_init() }; // E0080
After:
const VALUE: i32 = 42;
For Const Fn Panics:
Before:
const fn checked_div(a: i32, b: i32) -> i32 {
a / b // Panics if b == 0
}
const RESULT: i32 = checked_div(10, 0); // E0080
After:
const fn safe_div(a: i32, b: i32) -> Option<i32> {
if b == 0 {
None
} else {
Some(a / b)
}
}
const RESULT: Option<i32> = safe_div(10, 0);
For Invalid Enum Discriminants:
Before:
#[repr(u8)]
enum State {
Active = 1,
Inactive = 2,
}
const fn invalid_state() -> State {
unsafe { std::mem::transmute(0u8) } // E0080: invalid discriminant
}
After:
#[repr(u8)]
enum State {
Unknown = 0,
Active = 1,
Inactive = 2,
}
const UNKNOWN_STATE: State = State::Unknown;
Step 3: Use Runtime Initialization When Needed
When const evaluation genuinely cannot work, use lazy initialization:
use std::sync::OnceLock;
static COMPUTED_VALUE: OnceLock<i32> = OnceLock::new();
fn get_computed() -> &'static i32 {
COMPUTED_VALUE.get_or_init(|| {
// Complex runtime logic here
let base = calculate_base();
let result = process_value(base);
result
})
}
Step 4: Apply Type-Specific Solutions
For Cell Types:
Before:
use std::cell::Cell;
const CELL: Cell<i32> = Cell::new(0); // E0080: Cell not allowed in const
After:
const CONSTANT_VALUE: i32 = 42; // Use direct constant instead
For Thread-Local Statics:
use std::thread::LocalKey;
thread_local! {
static COUNTER: Cell<i32> = const { Cell::new(0) }; // Works in nightly
}
// Or use runtime initialization
thread_local! {
static COUNTER: Cell<i32> = Cell::new(0);
}
Step 5: Simplify Complex Constant Expressions
Break down complex const evaluations into simpler steps:
Before:
const RESULT: Vec<i32> = (0..100).filter(|x| x % 2 == 0).collect(); // E0080
After:
const fn generate_evens() -> [i32; 50] {
let mut arr = [0; 50];
let mut idx = 0;
let mut current = 0;
while idx < 50 {
if current % 2 == 0 {
arr[idx] = current;
idx += 1;
}
current += 1;
}
arr
}
const EVENS: [i32; 50] = generate_evens();
4. Verification
After applying your fix, verify that E0080 has been resolved and that the constant is correctly evaluated.
Immediate Verification
Run the build to confirm the error is gone:
cargo build 2>&1 | grep "E0080"
# Should produce no output if fixed
Full Compilation Check
cargo check --all-targets
If the command completes without E0080, the fix is successful.
Runtime Verification for Computed Values
Create a test to verify your constant values are correct:
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_const_values_are_valid() {
// Verify your constants are properly initialized
assert_eq!(RESULT, Some(10));
assert_eq!(EVENS[0], 0);
assert_eq!(EVENS[1], 2);
}
}
Verify Type Correctness
Ensure the constant has the expected type:
cargo build 2>&1 | grep -E "(expected|found|type)"
Integration Test
If the constant is used in other constants or type definitions:
// Verify constants used in type bounds
fn process_value<const N: usize>(arr: [i32; N]) {
// This will only compile if EVENS constant is valid
}
fn main() {
process_value(EVENS);
}
Check for Related Errors
After fixing E0080, you may encounter related errors that were masked:
cargo build 2>&1 | grep -E "^error"
Common follow-up errors:
- E0308: Type mismatch (often revealed after fixing const evaluation)
- E0046: Missing trait implementations
- E0277: Trait bound not satisfied
5. Common Pitfalls
Avoid these frequent mistakes when addressing E0080.
Pitfall 1: Assuming Const Fn Cannot Fail
Many developers believe const fn functions are infallible. However, any panic in a const fn causes E0080.
Incorrect:
const fn index<T>(slice: &[T], idx: usize) -> &T {
&slice[idx] // Panics if idx >= len()
}
const VALUE: i32 = *index(&[1, 2, 3], 10); // E0080 at compile time
Correct:
const fn safe_index<T, const N: usize>(slice: &[T; N], idx: usize) -> Option<&T> {
if idx < N {
Some(&slice[idx])
} else {
None
}
}
const VALUE: Option<i32> = safe_index(&[1, 2, 3], 10).copied();
Pitfall 2: Using Runtime Features in Const Context
Not all Rust features work in const contexts. Always check the stability of const features.
Incorrect:
const NOW: std::time::Instant = std::time::Instant::now(); // E0080
Correct:
use std::time::{Duration, Instant};
const PROGRAM_START: Instant = Instant::now();
const FIVE_SECONDS: Duration = Duration::from_secs(5);
fn elapsed() -> Duration {
PROGRAM_START.elapsed()
}
Pitfall 3: Forgetting About Lazy Statics
When you need runtime-initialized constants, use the proper mechanisms.
Incorrect:
static CONFIG: Config = Config::load(); // E0080: function call not allowed in const
Correct:
use once_cell::sync::Lazy;
use std::fs;
static CONFIG: Lazy<Config> = Lazy::new(|| {
let contents = fs::read_to_string("config.txt").unwrap();
Config::parse(&contents)
});
fn get_config() -> &'static Config {
&CONFIG
}
Pitfall 4: Unwinding in Const Context
Rust’s panic mechanism uses unwinding, which doesn’t work during const evaluation.
Incorrect:
const fn validate(value: i32) -> i32 {
assert!(value >= 0, "negative value");
value
}
const VALID: i32 = validate(-1); // E0080
Correct:
const fn validate(value: i32) -> Option<i32> {
if value >= 0 {
Some(value)
} else {
None
}
}
const VALID: Option<i32> = validate(-1);
Pitfall 5: Mismatched Const and Runtime Semantics
Some operations behave differently at compile time versus runtime.
Incorrect:
const fn get_element() -> i32 {
let vec = vec![1, 2, 3];
vec[5] // Panics at runtime, E0080 at compile time
}
Correct:
const ARRAY: [i32; 3] = [1, 2, 3];
const fn get_element(idx: usize) -> Option<i32> {
if idx < 3 {
Some(ARRAY[idx])
} else {
None
}
}
Pitfall 6: Assuming Cross-Platform Const Behavior
Some operations are undefined on certain platforms, causing E0080 inconsistently.
# May fail on some platforms
rustc --target x86_64-unknown-linux-gnu -Z macro-backtrace src/lib.rs
Always test your const code on all target platforms.
6. Related Errors
E0080 often appears alongside or is confused with these related Rust compiler errors.
E0013: Static and Const Variables
Error Message:
error[E0013]: statics and consts can only be initialized with constant values
Relationship: E0013 occurs when you try to use a non-constant value in a const/static context. E0080 is more specific—it occurs when the const evaluator successfully attempts to evaluate but fails.
Example:
fn get_value() -> i32 { 42 }
const VALUE: i32 = get_value(); // E0013: not const
const COMPUTED: i32 = {
let x = get_value(); // E0013
x + 1
};
E0023: Pattern Matching on Enum
Error Message:
error[E0023]: this pattern has N types, but expected 1 type
Relationship: When E0080 involves enum discriminants, you may also see E0023.
E0084: Fewer Variant Discriminants
Error Message:
error[E0084]: enums with no variants must not have fields
Relationship: Similar to E0080 in that it involves invalid enum construction.
E0093: Undeclared Feature in Constant Evaluation
Error Message:
error[E0093]: decoration of function is not allowed
Relationship: Occurs when using unstable features in const contexts.
E0433: Failed to Compile
Error Message:
error[E0433]: failed to resolve: use of undeclared type
Relationship: When E0080 prevents full compilation, you may see E0433 as a cascading failure.
Comparison Table
| Error Code | Cause | Fix |
|---|---|---|
| E0013 | Non-constant value in const context | Use static with lazy init or remove const |
| E0080 | Const evaluation failure | Fix the invalid const expression |
| E0084 | Invalid enum structure | Correct enum definition |
| E0093 | Unstable feature in const | Use stable alternative or nightly |
| E0433 | Failed resolution (cascading) | Fix root cause |
Prevention Strategies
- Use
const fnsparingly and ensure they handle all cases gracefully - Prefer
staticwithOnceLockorlazy_staticfor complex initialization - Test const code on all platforms before deployment
- Enable strict const evaluation where available with
-Z strict-core-const - Use
const_paniccrate for better error messages in const contexts
Summary: Error E0080 indicates that Rust’s compile-time const evaluator encountered an invalid operation—typically division by zero, uninitialized memory, panic in const fn, or undefined behavior. Fix it by restructuring code to use safe operations, returning Option or Result types from const functions, or moving complex initialization to runtime using lazy statics. Always verify fixes with cargo check and test across all target platforms.