1. Symptoms
When the Rust compiler encounters E0493, it produces an error message indicating that a destructor cannot be invoked during constant evaluation. The compiler provides context about which type’s destructor is problematic and where the invalid call occurs.
Typical compiler output looks like this:
error[E0493]: destructor of `MyStruct` cannot be called in constant function
--> src/main.rs:10:5
|
10 | drop(value);
| ^^^^^^^^^^^ calling the destructor of `MyStruct` in constant function
The error manifests in several common scenarios. When using the drop function on a type that implements Drop inside a const fn, the compiler rejects the code. Similarly, when a constant initialization implicitly requires destructor execution, such as when a const value goes out of scope during const evaluation, the same error appears. Explicit calls to std::mem::drop within constant contexts also trigger this error when the dropped type has a custom destructor.
In more complex cases involving constants that hold types with drop glue, you might encounter the error when attempting to use certain standard library functions that implicitly trigger drop semantics:
error[E0493]: destructor of `Wrapper<T>` cannot be called in constant evaluation
--> src/lib.rs:15:17
|
15 | const VALUE: Wrapper<i32> = Wrapper { inner: 42 };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ calling the destructor
2. Root Cause
The fundamental issue behind E0493 lies in Rust’s const evaluation model and its separation between compile-time and runtime operations. Constants and const functions in Rust are designed to be evaluable at compile time, meaning their results can be embedded directly into the binary without runtime overhead. This compile-time guarantee enables powerful optimizations but imposes significant restrictions on what operations are permitted within constant contexts.
The Drop trait represents a destructor—a function that runs when a value goes out of scope. Destructors are inherently runtime operations because they execute at moments determined by the program’s control flow. A destructor might perform I/O operations, modify global state, release system resources, or execute arbitrary user-defined cleanup logic. These behaviors are fundamentally incompatible with compile-time evaluation, which must be pure, deterministic, and free of side effects.
When the compiler attempts to evaluate a constant, it simulates the runtime behavior of the code within a const evaluation context. However, the compiler cannot execute arbitrary destructor code because doing so would require actual runtime behavior, potentially including system calls, memory allocation, or other operations that have no meaning during compilation. The Rust language specification explicitly prohibits destructor calls during const evaluation for this reason.
The error commonly occurs with the drop function because drop is essentially a wrapper that explicitly invokes the destructor of the passed value. When drop is called on a type that implements Drop within a const fn, the compiler must reject the code since executing that destructor would violate const evaluation guarantees. Even types that have an implicit drop implementation (drop glue) cannot have that glue executed during constant evaluation.
The compiler’s decision to reject these operations stems from Rust’s safety guarantees. Allowing destructors in const evaluation could introduce non-deterministic behavior into compile-time computations, potentially causing surprising bugs where code behaves differently when evaluated at compile time versus runtime. By restricting constant contexts to operations without side effects, Rust ensures that const truly means constant throughout the program’s lifecycle.
3. Step-by-Step Fix
Resolving E0493 requires restructuring your code to avoid destructor invocations during constant evaluation. The appropriate solution depends on your specific use case.
Fix 1: Remove Drop Implementation from Types Used in Const Contexts
If your type only needs cleanup logic for runtime use, consider whether that cleanup is truly necessary for const contexts:
Before:
struct ManagedResource {
id: u32,
}
impl Drop for ManagedResource {
fn drop(&mut self) {
println!("Cleaning up resource {}", self.id);
}
}
const fn create_resource() -> ManagedResource {
ManagedResource { id: 42 }
}
After:
struct ManagedResource {
id: u32,
}
impl Drop for ManagedResource {
fn drop(&mut self) {
println!("Cleaning up resource {}", self.id);
}
}
fn create_resource() -> ManagedResource {
ManagedResource { id: 42 }
}
fn main() {
let resource = create_resource();
// Cleanup happens naturally at end of main
}
Fix 2: Use Types Without Destructors for Const Contexts
For types that must work in constant contexts, ensure they don’t implement Drop:
Before:
use std::sync::Mutex;
struct State {
mutex: Mutex<i32>,
}
impl Drop for State {
fn drop(&mut self) {
println!("State being dropped");
}
}
const STATE: State = State {
mutex: Mutex::new(0),
};
After:
use std::sync::Mutex;
// Use Option for lazy initialization
struct State {
mutex: Option<Mutex<i32>>,
}
impl State {
const fn new() -> Self {
State {
mutex: None,
}
}
fn init(&self) {
if self.mutex.is_none() {
// Initialize at runtime if needed
}
}
}
static STATE: State = State::new();
fn main() {
STATE.init();
}
Fix 3: Restructure Constant Logic to Avoid Implicit Drops
Before:
struct TempFile {
path: String,
}
impl Drop for TempFile {
fn drop(&mut self) {
std::fs::remove_file(&self.path).ok();
}
}
const fn process_file() {
let temp = TempFile { path: String::from("/tmp/data.txt") };
// Error: destructor cannot be called in const fn
}
After:
struct TempFile {
path: String,
}
impl TempFile {
fn new(path: &str) -> Self {
TempFile {
path: path.to_string(),
}
}
fn cleanup(self) {
std::fs::remove_file(&self.path).ok();
}
}
fn process_file() {
let temp = TempFile::new("/tmp/data.txt");
// Process file...
temp.cleanup(); // Explicit cleanup at runtime
}
Fix 4: Use compile_error! for Validation Instead of Panicking Destructors
Before:
struct Config {
value: u32,
}
impl Drop for Config {
fn drop(&mut self) {
if self.value == 0 {
panic!("Config value cannot be zero");
}
}
}
After:
struct Config {
value: u32,
}
impl Config {
const fn new(value: u32) -> Self {
if value == 0 {
panic!("Config value cannot be zero");
}
Config { value }
}
}
const CONFIG: Config = Config::new(42);
4. Verification
After applying a fix for E0493, verify that the code compiles successfully and maintains correct behavior.
First, ensure the code compiles without errors by running:
cargo build
If the fix is correct, you should see no E0493 errors. The compiler should proceed without complaints about destructor invocations in constant contexts.
Next, run any relevant tests to confirm the behavioral correctness of your changes:
cargo test
For cases where you removed const from functions, verify that the runtime behavior remains unchanged. If you previously relied on const evaluation for performance, measure the impact of moving logic to runtime:
// Benchmark if needed
use std::time::Instant;
fn main() {
let start = Instant::now();
// Your function call
let duration = start.elapsed();
println!("Time: {:?}", duration);
}
Check that types without explicit Drop implementations no longer have destructor constraints in const contexts. You can verify this by attempting to use the type in a constant:
#[derive(Debug)]
struct SimpleStruct {
value: i32,
}
// This should compile without error
const TEST: SimpleStruct = SimpleStruct { value: 100 };
fn main() {
println!("{:?}", TEST);
}
For more complex scenarios involving global state or static initialization, ensure that runtime initialization properly handles any setup that was previously deferred to const evaluation:
cargo run --example your_example
If you’re working with library code, run the full test suite and consider adding new test cases that specifically cover the previously-erroneous constant evaluation paths:
#[cfg(test)]
mod tests {
#[test]
fn test_const_initialization() {
// Verify your fix works correctly
}
}
5. Common Pitfalls
When fixing E0493, developers frequently encounter several recurring issues that can complicate the resolution process.
Confusing Const and Static Contexts: One common mistake is conflating const with static. While both relate to compile-time evaluation, static variables have a fixed memory location and can have interior mutability, but they still cannot have types with destructors that would run at constant evaluation time. A static with a destructor will not have that destructor called when the static is initialized, but the static itself cannot be used in const evaluation contexts that would trigger the destructor.
Assuming drop_glue is Different from Drop: Some developers mistakenly believe that types without explicit Drop implementations are safe in const contexts. However, Rust automatically inserts drop glue (implicit destructor calls) for any type that contains fields with destructors. A struct containing a String field has implicit drop semantics even without implementing Drop, and this will trigger E0493 in const contexts.
Overlooking Standard Library Types: The standard library contains many types that implement Drop internally or have drop glue. Types like Box<T>, Vec<T>, String, and most collection types have destructors. Attempting to use these in constant contexts will fail. Always check the type’s Drop implementation status when debugging E0493.
Forgetting About Trait Objects: When a type contains dyn Trait or &dyn Trait fields where the trait has an associated destructor, the containing type may have drop glue that triggers E0493. Be particularly careful with trait objects that wrap resources requiring cleanup.
Ignoring Third-Party Crate Types: Dependencies frequently bring in types with Drop implementations. A seemingly innocent constant that uses a third-party type might fail due to that type’s destructor. Document the types you use in constant contexts and understand their drop semantics.
Attempting Workarounds with std::hint::unreachable_untested: Some developers attempt to bypass E0493 by wrapping destructor calls in unreachable code, thinking the compiler won’t evaluate that path. The compiler performs const evaluation conservatively and will still reject such code because the destructor is potentially callable in the constant context.
6. Related Errors
Understanding E0493 becomes easier when placed in context with similar const evaluation and destructor-related errors.
E0494: Copy and Drop Conflict
The compiler produces E0494 when a type implements both Copy and Drop. Since Copy indicates that a type can be duplicated simply by copying bits (implying trivial resource management), while Drop indicates custom cleanup logic, these traits are mutually exclusive. Rust enforces this invariant because a type that can be copied implicitly would have ambiguous destructor semantics—should copying produce a value that shares resources or duplicates them, and which copy owns the resource for cleanup purposes?
E0509: Cannot Move Out of Non-Copy Types During Drop
Error E0509 occurs when attempting to move a value out of a type during its destructor implementation. Since destructors receive &mut self, not self, moving values out would leave the object in an invalid state. This error commonly appears when developers try to take ownership of self’s fields within drop() implementations, not realizing that self is already being destroyed and cannot be consumed.
E0740: Union Destructors Cannot Be Constexpr Rust 2021 introduced restrictions on union destructors in const contexts with E0740. Unions that contain types with destructors cannot have those destructors evaluated during constant evaluation. This error prevents unsound patterns where a union’s destructor might be called at compile time with inconsistent state, potentially leading to undefined behavior that differs between compile-time and runtime execution.
These related errors collectively illustrate the tension between compile-time evaluation guarantees and runtime-only operations like resource cleanup. Rust’s type system uses these errors to enforce that constant contexts remain pure and deterministic while ensuring that runtime resource management through destructors remains safe and predictable.