LYR-PAR0067

an 'unsafe' block where a value stands

error · spec 13-abi

unsafe { … } stands where an expression is wanted — after =, after return, as an argument —; it is a statement and gives no value.

Explanation

unsafe { … } is a block statement: the memory operations of std.ffi (alloc, read, write, offset, cast, free) may be called inside it, and nothing else about it differs. Like any block statement it gives no value, so let p = unsafe { alloc<int>(1) }; has nothing to bind.

Declare the variable before the block and assign it inside: var p: Ptr<int>; unsafe { p = alloc<int>(1); }. Or put the work in a function that returns the value from inside its block. Keep the block around the memory operations only — that is where the reader looks for what the compiler cannot check.

Wrong

import std.ffi { alloc, free, Ptr };

fn main(): void {
    let p: Ptr<int> = unsafe { alloc<int>(1) };
    unsafe { free(p); }
}

Right

import std.ffi { alloc, free, Ptr };

fn main(): void {
    var p: Ptr<int>;
    unsafe { p = alloc<int>(1); }
    unsafe { free(p); }
}