Rust error E0133: call to unsafe function `f` is unsafe and requires unsafe function or block

The compiler team's explanation

Unsafe code was used outside of an unsafe block.

Erroneous code example:

unsafe fn f() { return; } // This is the unsafe code

fn main() {
    f(); // error: call to unsafe function requires unsafe function or block
}

Using unsafe functionality is potentially dangerous and disallowed by safety checks. Examples:

• Dereferencing raw pointers
• Calling functions via FFI
• Calling functions marked unsafe

These safety checks can be relaxed for a section of the code by wrapping the unsafe instructions with an unsafe block. For instance:

unsafe fn f() { return; }

fn main() {
    unsafe { f(); } // ok!
}

See the unsafe section of the Book for more details.

Unsafe code in functions

Unsafe code is currently accepted in unsafe functions, but that is being phased out in favor of requiring unsafe blocks here too.

unsafe fn f() { return; }

unsafe fn g() {
    f(); // Is accepted, but no longer recommended
    unsafe { f(); } // Recommended way to write this
}

Linting against this is controlled via the unsafe_op_in_unsafe_fn lint, which is warn by default in the 2024 edition and allow by default in earlier editions.

From rustc --explain E0133 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.