1. Symptoms
The Rust compiler emits error E0791 when it encounters raw pointer dereference operations within a const fn context. This error manifests during compilation and prevents the program from building successfully.
When triggered, the compiler produces output similar to the following:
error[E0791]: dereference of raw pointer in const fn
--> src/main.rs:4:5
|
4 | const fn read_value(ptr: *const i32) -> i32 {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ const fn
...
7 | unsafe { *ptr }
| ^^^^^^ raw pointer dereference
|
= note: const ptr arithmetic is not stable yet
= note: see issue #71313 <https://github.com/rust-lang/rust/issues/71313> for more information
error: aborting due to 1 previous error
The error message specifically identifies the raw pointer dereference operation (*ptr) and clarifies that the offending function is a const fn. The compiler also references an upstream tracking issue in the Rust repository, indicating this is a limitation of the current stable compiler rather than a fundamental language design choice.
You may also encounter this error with slightly different wording depending on the exact Rust version:
error[E0791]: dereference of raw pointer in a `const fn`
--> src/lib.rs:12:13
|
12 | let val = *raw_ptr;
| ^^^^^^ raw pointer dereference
The error consistently points to the exact line containing the problematic dereference and includes a note explaining the instability of const pointer operations.
2. Root Cause
The underlying cause of error E0791 stems from the fundamental restrictions Rust imposes on compile-time evaluation. Const functions in Rust are designed to be evaluable at compile time, meaning the Rust compiler must be able to execute the function’s logic during compilation to determine constant values. However, raw pointer dereferencing is inherently unsafe and relies on runtime memory access patterns that the compiler cannot safely verify during static analysis.
Rust’s const evaluation engine operates under strict safety guarantees that differ significantly from runtime execution. When evaluating a const fn, the compiler performs its own internal interpretation of the function body, but it cannot guarantee that a raw pointer still points to valid, initialized memory. Raw pointers (*const T and *mut T) bypass Rust’s ownership and borrowing systems, which means dereferencing them could lead to undefined behavior if the pointer addresses an invalid memory location, uninitialized memory, or memory that has been deallocated.
The Rust language team deliberately chose to restrict raw pointer dereferencing in const contexts because such operations fundamentally require runtime guarantees about memory safety that static compilation cannot provide. Even though you might wrap the dereference in an unsafe block—which would be valid in normal runtime code—the compiler still rejects the operation during const evaluation because the safety contract of unsafe cannot be verified at compile time.
Additionally, const evaluation in Rust follows a staged model where const functions may eventually accept pointers or references that point to data determined at runtime. Allowing raw pointer dereferencing would create ambiguities about what can be evaluated statically versus what must be deferred to runtime, potentially breaking the compiler’s ability to perform optimizations or correctly evaluate constant expressions.
3. Step-by-Step Fix
Resolving error E0791 requires restructuring your code to avoid raw pointer dereference in const contexts. The appropriate solution depends on your specific use case, but several patterns can address this limitation effectively.
Solution 1: Use References Instead of Raw Pointers
The cleanest fix is to replace raw pointers with references where possible. References in Rust carry lifetime and validity information that the const evaluator can reason about.
Before:
const fn read_value(ptr: *const i32) -> i32 {
unsafe { *ptr }
}
const VALUE: i32 = read_value(std::ptr::null());
After:
const fn read_value(reference: &i32) -> i32 {
*reference
}
const VALUE: i32 = read_value(&42);
If you must work with pointer-like semantics in const contexts, consider using references with associated constants or trait objects that the compiler can fully evaluate.
Solution 2: Defer Evaluation to Runtime
When const evaluation is not strictly necessary, remove the const modifier from the function signature to allow normal runtime execution with raw pointers.
Before:
const fn unsafe_read(ptr: *const u8) -> u8 {
unsafe { *ptr }
}
After:
fn unsafe_read(ptr: *const u8) -> u8 {
unsafe { *ptr }
}
fn main() {
let value = unsafe_read(std::ptr::null());
}
Solution 3: Use const trait methods with associated types
For more complex scenarios involving pointer arithmetic or custom memory layouts, consider defining a const trait implementation that the compiler can evaluate directly.
Before:
const fn process_pointer(ptr: *mut u32, offset: usize) -> u32 {
unsafe { *ptr.add(offset) }
}
After:
const ARRAY: [u32; 4] = [10, 20, 30, 40];
const fn process_array(index: usize) -> u32 {
ARRAY[index]
}
const RESULT: u32 = process_array(2);
Solution 4: Use const generics with array indexing
For performance-critical code that needs indexed access, const generics provide a compile-time alternative to pointer arithmetic.
Before:
const fn get_element<T>(ptr: *const T, index: usize) -> &T {
unsafe { &*ptr.add(index) }
}
After:
trait ConstArrayAccess {
const fn get<const I: usize>(&self) -> &<Self as ConstArrayAccess>::Output;
}
impl<T, const N: usize> ConstArrayAccess for [T; N] {
type Output = T;
const fn get<const I: usize>(&self) -> &T {
&self[I]
}
}
4. Verification
After implementing the fix, verify that error E0791 no longer appears during compilation. Run the following command to confirm successful compilation:
cargo build
A successful build produces no error output:
Compiling myproject v0.1.0 (path/to/myproject)
Finished dev [unoptimized + debuginfo] target(s) in 0.45s
For functions that should evaluate at compile time, confirm the value is actually computed during compilation by examining the compiled binary or using std::mem::size_of_val with compile-time known values. You can also inspect the generated assembly to verify that constant propagation occurred:
cargo rustc --release -- --emit asm
Check the assembly output for references to your constant value. If the value appears as an immediate operand rather than a runtime load instruction, the const evaluation succeeded.
For functions converted from const fn to regular functions, verify that removing the const keyword does not break existing code that expects compile-time evaluation:
// This should still work with runtime evaluation
let value = unsafe_read(valid_pointer);
If you previously relied on const evaluation for array lengths or other compile-time checks, ensure those checks still pass at runtime where applicable.
5. Common Pitfalls
When addressing error E0791, developers frequently encounter several recurring mistakes that can complicate the resolution process or introduce new issues.
Forgetting that unsafe blocks do not exempt const evaluation: Many developers assume that wrapping raw pointer operations in unsafe blocks should be sufficient for const functions, since unsafe is required for raw pointer dereference at runtime. This assumption is incorrect. The const evaluator cannot verify the safety of raw pointer dereferencing regardless of the unsafe wrapper, because the safety guarantees that make unsafe code valid depend on runtime invariants the compiler cannot check statically.
Attempting to use std::ptr::read in const functions: Similarly, functions like std::ptr::read, std::ptr::write, and std::ptr::copy cannot be used in const contexts because they involve raw pointer operations that the const evaluator rejects. Attempting to work around E0791 by using these functions produces related errors such as E0010.
Confusing const evaluation with monomorphization: Some developers believe that const functions are evaluated separately for each type instantiation, thinking this would provide runtime guarantees. However, const evaluation occurs before monomorphization, so the restrictions apply uniformly across all instantiations.
Removing const from too many functions: When fixing E0791, it can be tempting to simply remove the const keyword from all affected functions. This approach works but sacrifices potential compile-time evaluation benefits. Audit your codebase to identify which functions genuinely require const evaluation for array generics, const generics bounds, or static initialization, and preserve the const modifier for those while converting others to runtime functions.
Using mem::transmute workarounds: Some developers attempt to bypass raw pointer restrictions using mem::transmute to convert raw pointers to other types. This approach is dangerous and often leads to undefined behavior or subsequent errors like E0512, as transmute operations with raw pointers cannot preserve the validity guarantees the type system requires.
6. Related Errors
Error E0791 frequently appears alongside other const evaluation limitations in Rust. Understanding these related errors provides context for the broader restrictions on compile-time computation.
E0010: Cannot mutate statics through raw pointer dereference in const fn: This error complements E0791 by addressing writes to memory through raw pointers in const contexts. While E0791 covers reading through raw pointers, E0010 addresses the corresponding write operations. Both errors stem from the same fundamental inability of the const evaluator to guarantee memory safety for raw pointer operations.
// Triggers E0010
const fn write_value(ptr: *mut i32, value: i32) {
unsafe { *ptr = value; } // Error: cannot mutate statics
}
E0013: Const functions cannot be called at runtime in certain contexts: This error occurs when attempting to use a const function in a context that requires runtime evaluation before the const evaluation is complete. This can interact with E0791 when refactoring code, as converting a const function to a runtime function may change how the compiler handles the call site.
E0747: Type declarations cannot contain generic parameters: While not directly related to raw pointer operations, this error often appears when developers attempt to create type-level constants that would require generic parameters to handle raw pointer types in a generic const context. Resolving this error typically requires the same kind of redesign as fixing E0791.