rust/src/librustc_error_codes/error_codes/E0133.md

Unsafe code was used outside of an unsafe function or 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 also https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html