E030: enum-cast-missing-safety-comment

Metadata

• Code: E030
• Rule: enum-cast-missing-safety-comment
• Status: Stable since v0.0.1
• Quick fix: not available

What it does

Requires a SAFETY comment for non-literal as casts to enum types.

Why is this bad?

Casting arbitrary values to enums can construct invalid enum values. Compiler optimizations may rely on enum invariants, so invalid values can lead to subtle and hard-to-debug behavior.

Example

enum Op { Add = 0, Sub = 1 }

fun parse(v: int): Op {
    return v as Op;
E030: non-literal cast to enum requires safety comment
}

Use instead:

enum Op { Add = 0, Sub = 1 }

fun parse(v: int): Op {
    if (v != 0 && v != 1) {
        throw 7
    }
    // SAFETY: input is validated and guaranteed to be either 0 or 1.
    return v as Op;
}

// or even better, use match

fun parse(v: int): Op {
    return match (v) {
        0 => Op.Add,
        1 => Op.Sub,
        else => throw 7,
    }
}