Why is this an issue?

When a transmute is placed within an expression that uses eager evaluation (like bool::then_some), it will execute even if a preceding validity check fails. This can result in creating invalid values, potentially leading to undefined behavior.

How to fix it

Use lazy evaluation (for example by replacing then_some with then) and providing a closure that contains the transmute. This ensures the transmute only occurs if the validity check passes.

Code examples

Noncompliant code example

#[repr(u8)]
enum Opcode {
    Add = 0,
    Sub = 1,
    Mul = 2,
    Div = 3
}

fn int_to_opcode(op: u8) -> Option<Opcode> {
    (op < 4).then_some(unsafe { std::mem::transmute(op) })
}

Compliant solution

#[repr(u8)]
enum Opcode {
    Add = 0,
    Sub = 1,
    Mul = 2,
    Div = 3
}

fn int_to_opcode(op: u8) -> Option<Opcode> {
    (op < 4).then(|| unsafe { std::mem::transmute(op) })
}

Resources

Documentation