1. Symptoms
The Rust compiler emits error E0714 when you attempt to use a pattern involving types with destructors in a const or static item context where the destructor cannot be evaluated at compile time. The error manifests with a clear diagnostic message indicating that destructors are forbidden during constant evaluation.
When you encounter this error, the compiler produces output similar to the following:
error[E0714]: destructors cannot be evaluated at constant expression compilation
--> src/main.rs:6:13
|
6 | const VALUE: Option<Box<i32>> = Some(Box::new(42));
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: this error occurs in the expansion of static item initialization
The error points to the exact line where the problematic pattern appears, and the note clarifies that the issue occurs during static item initialization. In more complex scenarios involving tuple destructuring, you might see additional context about which part of the pattern triggered the evaluation:
error[E0714]: destructors cannot be evaluated at constant expression compilation
--> src/lib.rs:12:5
|
12 | const (a, b): (Box<i32>, i32) = (Box::new(1), 2);
| ^^^^^^^ this pattern requires dropping
The error can also appear in function arguments that are evaluated as constants or when using const generics with types that have non-trivial drop semantics.
2. Root Cause
The fundamental issue behind error E0714 stems from Rust’s const evaluation architecture, which operates entirely at compile time. When the compiler evaluates a const or static item, it must be able to compute the entire value without any runtime participation. This restriction exists because const items are inlined at their use sites, and the compiler needs to know the exact value during compilation.
Rust’s type system distinguishes between types that implement the Copy trait and those that require a destructor. Types like Box<T>, Vec<T>, String, and custom types with Drop implementations have semantics that go beyond simple memory copying. When these types are dropped, Rust must execute their destructor code, which may involve deallocating heap memory, releasing file handles, flushing buffers, or any other cleanup logic defined in the Drop implementation.
The const evaluator in Rust deliberately prohibits destructor execution for several compelling reasons. First, const evaluation must be pure and deterministic—it should produce identical results regardless of how many times or when it runs. Destructors, by their nature, perform side effects like memory deallocation that violate this purity requirement. Second, the const evaluator runs in a restricted environment without access to the global allocator or other runtime services that destructors might depend upon. Third, permitting destructor execution during const evaluation would introduce unsoundness into the type system, as the same const value might be “used” multiple times with different drop semantics each time.
When you write a const item using a type with a destructor, the compiler must determine how to represent that value. For const x: Option<Box<i32>> = Some(Box::new(42)), the compiler would need to allocate heap memory during compilation and then somehow represent that the destructor should run later. This creates a fundamental mismatch between const semantics and drop semantics.
The error commonly occurs with patterns like destructuring assignments in const contexts, using Box::new(), Vec::new(), or String::from() in const initializers, and any pattern that implicitly requires drop ordering. Even when the type itself might conceptually fit in a const, the pattern matching or construction mechanism forces the compiler to consider drop semantics.
3. Step-by-Step Fix
Resolving E0714 requires restructuring your code to avoid destructor requirements in const contexts. The appropriate solution depends on your use case, but the general principle is to either move the value to a static (which has slightly different semantics), use lazy initialization, or redesign your data structures to avoid drop-dependent types in const positions.
Fix 1: Convert Const to Static for Heap-Allocated Types
If you need a global instance of a type with a destructor, use static instead of const. Static items have a fixed memory location and their drop implementation runs exactly once at program termination.
Before:
const CONFIG: Option<Box<i32>> = Some(Box::new(42));
After:
static CONFIG: Option<Box<i32>> = Some(Box::new(42));
fn main() {
println!("{:?}", CONFIG);
}
The static keyword tells the compiler that this value lives for the entire program duration, which makes it acceptable to have a destructor. The compiler will generate appropriate drop code that runs at program exit.
Fix 2: Use Lazy Initialization with OnceLock or Similar
For cases where you need const-compatible construction but also need heap allocation, use the standard library’s lazy initialization primitives.
Before:
const VALUES: Vec<i32> = Vec::from([1, 2, 3, 4, 5]);
After:
use std::sync::OnceLock;
static VALUES: OnceLock<Vec<i32>> = OnceLock::new();
fn get_values() -> &'static Vec<i32> {
VALUES.get_or_init(|| Vec::from([1, 2, 3, 4, 5]))
}
fn main() {
println!("{:?}", get_values());
}
The OnceLock type allows you to define a static location for your data while deferring the actual construction until first access. This pattern is essential when you need const-compatible APIs but must work with heap-allocated types.
Fix 3: Redesign to Use Stack-Allocated Types in Const Contexts
If possible, restructure your data structures to use only stack-allocated, Copy types in const positions.
Before:
const USER: (String, i32) = (String::from("Alice"), 30);
After:
// Use &'static str instead of String in const contexts
const USER: (&'static str, i32) = ("Alice", 30);
// Or if you need owned strings, initialize them at runtime
static USER_NAME: once_cell::sync::Lazy<String> =
once_cell::sync::Lazy::new(|| String::from("Alice"));
Fix 4: Avoid Destructuring Patterns in Static/Const Contexts
When initializing complex types, avoid patterns that require drop ordering.
Before:
const ((a, b), c): ((i32, i32), Box<i32>) = ((1, 2), Box::new(3));
After:
// For static items, this works
static DATA: ((i32, i32), Box<i32>) = ((1, 2), Box::new(3));
// For accessing parts, use destructuring at the use site
fn process() {
let ((a, b), c) = DATA;
println!("{} {} {}", a, b, c);
}
4. Verification
After applying any of the fixes above, you should verify that the error is resolved by recompiling your project. Run the standard Rust compilation command in your project directory:
cargo build
If the fix is successful, the build should complete without any E0714 errors. For more thorough verification, especially when using lazy initialization patterns, run your test suite:
cargo test
Pay particular attention to tests that exercise the const or static values you modified, as these verify that the lazy initialization occurs correctly at runtime. Additionally, consider running the program and checking that the values are initialized as expected:
cargo run
If you’re using OnceLock or similar primitives, add debug output to confirm that initialization happens exactly once:
static COUNTER: OnceLock<u32> = OnceLock::new();
fn get_counter() -> u32 {
*COUNTER.get_or_init(|| {
println!("Initializing counter");
42
})
}
For projects using external crates like once_cell, ensure your Cargo.toml specifies compatible versions:
[dependencies]
once_cell = "1.19"
5. Common Pitfalls
One of the most frequent mistakes developers make when encountering E0714 is immediately reaching for unsafe code to bypass the restriction. While Rust does allow unsafe code for various purposes, using it to force destructor execution in const contexts is not the right solution and will likely lead to undefined behavior or compilation errors.
Another common pitfall involves confusing the semantics of const and static. While both keywords create compile-time-known values, they have fundamentally different drop semantics. A const is inlined wherever it’s used, meaning each use site potentially has its own copy that would need dropping. A static exists exactly once in the binary’s data segment and has a well-defined drop behavior. Understanding this distinction is crucial for choosing the correct fix.
When using lazy initialization patterns, developers sometimes forget that the initializer closure runs at most once per static location, but the resulting value is shared across all threads. Ensure your initialization code is thread-safe and idempotent. The std::sync::OnceLock and once_cell::sync::Lazy types handle this correctly, but if you’re rolling your own initialization using unsafe and raw pointers, you must implement synchronization yourself.
There’s also a subtle distinction between std::sync::OnceLock and once_cell::sync::Lazy regarding panics during initialization. The former will retry initialization if a panic occurs, while the latter caches the panic. Choose based on your requirements, and document your choice if it matters for the correctness of your program.
Finally, be cautious when working with third-party crates that provide alternative lazy initialization solutions. Not all crates have the same safety guarantees or performance characteristics. The once_cell crate is well-maintained and widely used, making it a safe choice for most applications. The lazy_static crate is older but still functional, though once_cell has largely superseded it in modern Rust codebases.
6. Related Errors
E0013: Static Items Cannot Refer to Statics — This error occurs when a static item tries to reference another static item, creating a circular dependency. While related to static initialization, it stems from a different constraint: the linker needs to resolve all static addresses before the program starts, which is impossible with circular references. The fix typically involves using references or restructuring to break the cycle.
E0493: Destructor of Type Cannot Be Evaluated As Part of Constant Evaluation — This error is closely related to E0714 and often appears in similar contexts. E0493 specifically focuses on the destructor aspect, while E0714 is broader and covers any pattern that would require destructor execution during const evaluation. The fixes for both errors converge on the same strategies: avoid types with drop semantics in const contexts, or use lazy initialization.
E0740: Const Polar Types Cannot Have Destructors — This error specifically targets types that are marked as const (via the const_trait feature or similar) but also implement Drop. Rust’s const traits system has restrictions on what can be used in const contexts, and combining const polymorphism with drop semantics creates unsolvable problems for the compiler. The fix requires either removing the const requirement from the type or removing the drop implementation.