Fix E0763: Dereferencing a Raw Pointer in a const fn is Unstable

By Alex Chen
Rust intermediate Linux macOS Windows

1. Symptoms

When attempting to dereference a raw pointer inside a const fn, the Rust compiler produces error E0763 with the following message:

error[E0763]: dereferencing a raw pointer in a `const fn` is unstable
  |
  = note: this error occurs in the following function:
  --> src/main.rs:6:1
    |
  6  | const fn get_value(ptr: *const i32) -> i32 {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The compiler may also emit additional notes about requiring nightly features:

  |
  = note: see Rust RFC #2582 for more information about this feature.

In some cases, you might see additional context about requiring specific feature flags:

help: use `#![feature(const_raw_ptr_methods)]` to use `core::ptr::copy`, `core::ptr::copy_nonoverlapping`, and the dereference operators on raw pointers inside const fns

Common code patterns that trigger this error include pointer arithmetic, pointer dereferencing, and accessing raw pointer values within constant functions. The error manifests during the compilation phase, specifically during MIR construction, indicating that the operation cannot be verified as safe during compile-time evaluation.

2. Root Cause

The underlying cause of E0763 stems from Rust’s const evaluation restrictions. Raw pointer dereferencing operations are inherently unsafe operations that bypass Rust’s normal borrowing rules and ownership system. While these operations are permitted in safe Rust when working with references, const fn evaluation occurs at compile time, which means the compiler must guarantee that the operation produces a deterministic result without invoking undefined behavior.

Rust’s const evaluation mechanism is designed to be sound and predictable. Raw pointer dereferencing in const contexts presents multiple challenges. First, dereferencing a raw pointer can cause undefined behavior if the pointer is null, misaligned, or points to deallocated memory. The compiler cannot perform the runtime safety checks that would normally occur when executing code normally. Second, raw pointer dereferencing might depend on runtime information that is not available during compile-time evaluation, such as the actual memory layout or values that are only known at program execution time.

The stability restrictions exist because dereferencing raw pointers in const contexts requires specific implementation work within the compiler and has undergone several RFC discussions. The feature was gradually stabilized starting in Rust 1.38 with the introduction of const_raw_ptr_methods, but many operations remain unstable on stable Rust. The compiler team has been cautious about stabilizing these operations due to concerns about undefined behavior and soundness in const evaluation contexts.

Additionally, const functions are evaluated in environments where certain runtime assumptions may not hold. For example, memory allocation during const evaluation is not supported, and operations that might trigger dynamic memory access or processor-specific behavior are restricted. Raw pointer dereferencing falls into this category because it effectively performs a memory read that depends on runtime state.

3. Step-by-Step Fix

To resolve E0763, you have several approaches depending on your requirements and whether nightly features are acceptable for your use case.

Approach 1: Use Nightly Rust with Feature Flags

If you require raw pointer dereferencing in const contexts and can use the nightly compiler, enable the necessary feature flag:

#![feature(const_raw_ptr_methods)]

const fn read_value(ptr: *const i32) -> i32 {
    unsafe { *ptr }
}

fn main() {
    let value = 5;
    let result = read_value(&value as *const i32);
    println!("Value: {}", result);
}

Before:

// This fails on stable Rust
const fn get_value(ptr: *const i32) -> i32 {
    unsafe { *ptr }  // E0763
}

After:

// Add feature flag at crate root
#![feature(const_raw_ptr_methods)]

const fn get_value(ptr: *const i32) -> i32 {
    unsafe { *ptr }  // Works on nightly
}

Approach 2: Use const Blocks Instead of const Functions

If you need to perform pointer operations at compile time, use const blocks with runtime evaluation:

fn main() {
    let value = 42;
    const RESULT: i32 = {
        let ptr = &value as *const i32;
        unsafe { *ptr }
    };
    
    println!("Compile-time result: {}", RESULT);
}

Approach 3: Replace Raw Pointer Operations with Safe Alternatives

When possible, refactor your code to avoid raw pointer dereferencing:

// Instead of raw pointers
const fn get_array_value(arr: &[i32; 10], index: usize) -> i32 {
    arr[index]
}

// Use const generics for static indexing
const fn get_static_value<const N: usize>() -> i32 {
    const ARRAY: [i32; N] = [1, 2, 3, 4, 5];
    ARRAY[2]
}

Approach 4: Create Const-Capable Wrapper Functions

Design your API to separate const-compatible operations from those requiring runtime evaluation:

// Provide two versions of the function
const fn get_value_const(data: &[i32]) -> i32 {
    data[0]  // Safe const operation
}

fn get_value_runtime(data: &[i32]) -> i32 {
    // Can use unsafe operations here
    unsafe {
        let ptr = data.as_ptr();
        *ptr
    }
}

Approach 5: Use the ptr::read Function

On nightly Rust with the appropriate feature, use core::ptr::read:

#![feature(const_raw_ptr_methods)]

const fn read_ptr(ptr: *const i32) -> i32 {
    core::ptr::read(ptr)
}

4. Verification

After applying the fix, verify that E0763 is resolved by following these steps:

  1. Check Feature Flags: If using nightly, confirm that the #![feature(...)] attribute is placed at the crate root before any module definitions. The feature flag must appear in the same file where the const function is defined.

  2. Compile the Code: Run cargo build or rustc to verify compilation succeeds. With the feature flag properly enabled on nightly, the raw pointer dereferencing should compile without E0763.

  3. Test Const Evaluation: Verify that the const operation produces the expected result at compile time by checking that the value is actually computed during compilation:

#![feature(const_raw_ptr_methods)]

const fn compute(ptr: *const i32) -> i32 {
    unsafe { *ptr }
}

const VALUE: i32 = {
    let local = 99;
    compute(&local as *const i32)
};

fn main() {
    println!("Compile-time value: {}", VALUE);
}
  1. Run Runtime Tests: Execute the program to ensure that runtime behavior matches expectations and that const evaluation produces correct results:
cargo run
# Should output: Compile-time value: 99
  1. Verify Stable Rust Compatibility: If maintaining stable Rust compatibility is required, ensure that separate code paths exist for stable and nightly compilation. Use conditional compilation with #[cfg(feature)] or separate implementations.

5. Common Pitfalls

When addressing E0763, developers frequently encounter several recurring mistakes that can complicate resolution or introduce subtle bugs.

Forgetting Feature Flags: A very common error is implementing the correct const function syntax but omitting the necessary feature flag. Without #![feature(const_raw_ptr_methods)], the code will fail with E0763 even if the implementation itself is correct.

Assuming Stable Features: Many developers expect raw pointer operations in const fn to work on stable Rust, similar to how unsafe blocks work in regular functions. This assumption leads to confusion when encountering E0763. Remember that const evaluation has stricter requirements than runtime code.

Memory Lifetime Issues: When creating const values involving raw pointers, the pointer must reference data that lives long enough for const evaluation to complete. Using references to stack-local variables in const contexts can lead to subtle bugs:

// This is problematic
const fn bad_example() -> i32 {
    let value = 5;
    let ptr = &value as *const i32;
    unsafe { *ptr }  // value does not live long enough
}

Nightly-to-Stable Porting Oversights: When moving code from nightly to stable Rust for production, raw pointer dereferencing in const functions will break. Always audit const functions for raw pointer operations before stabilizing your dependencies.

Confusing const fn with static: Remember that const fn means the function can be called during const evaluation, but it can also be called at runtime. static variables hold values for the entire program lifetime, which imposes different lifetime constraints than const evaluation contexts.

Incomplete Unsafe Blocks: Raw pointer operations require unsafe blocks, but simply wrapping the dereference in unsafe is insufficientβ€”the const fn itself must be marked const, and the necessary features must be enabled.

Feature Flag Scoping: Feature flags apply per-crate, not per-module. If you have multiple crates in your project, each crate that uses raw pointer dereferencing in const functions must have the appropriate feature flags enabled.

E0013 - Cannot start a const expression with a semicolon

This error occurs when attempting to evaluate a const expression that contains statements terminated by semicolons. While not directly related to raw pointer dereferencing, E0013 often appears when developers attempt to write complex const logic that includes mutable operations or statement sequences. The connection to E0763 exists because both errors frequently occur when attempting to perform low-level operations in const contexts that are not fully supported.

E0658 - Cannot use inline assembly in const fn

Similar to E0763, this error occurs when attempting to use features that are not yet stabilized in const functions. Inline assembly requires nightly features and specific enablement. Both errors stem from the same root cause of const evaluation limitations, and the fix approach is similar: either enable nightly features or refactor to avoid the restricted operation.

E0600 - Cannot borrow as mutable from within a const context

This error relates to Rust’s borrowing restrictions in const contexts. When attempting to mutate data through a mutable reference inside const evaluation, E0600 occurs. Raw pointer operations often involve both dereferencing and potential mutation, which means E0763 and E0600 can appear together. Addressing E0763 requires understanding these interconnected const restrictions.

// Example triggering multiple related errors
const fn modify(ptr: *mut i32) {
    unsafe {
        *ptr = 10;  // E0763 for raw pointer, may also involve borrowing issues
    }
}

The interconnection between these errors reflects the fundamental design of Rust’s const evaluation system, which deliberately restricts operations that could introduce undefined behavior or non-deterministic results during compile-time evaluation. Understanding the relationships between these errors helps developers design more robust const-compatible APIs.

rust e0763 const-fn raw-pointer unstable