Struct `const` fields, or a way to `@compileError` on mutation

[deep terrace]

I have an extern struct that must be passed to and from a consumer outside of my control. In C, this struct contains a field that is of type const int32_t, to denote that just this field within the struct should never be modified. However, I have no way to express the equivalent in Zig. Is there some other way I can denote that this field should never be mutated in some compiler-checked way?

[lethal bone]

not really. you can name it with a leading underscore or something and put a doc comment saying to not change it. you could hide it in like enum(i32) {_}. there's no way to mark a field as constant in zig though, only a pointer.

[blissful patrol]

Yeah - in Zig, the mutability of the struct itself controls whether its fields are mutable.

Consider adding a doc-comment (///) to that field that says why you shouldn't mutate it.

Otherwise no bother

[inland pebble]

I am not sure if it will fit your use case @Mocha, but I was trying to do something along these lines today (which is how I found this thread) and had some success using a generic struct :

fn myGrid(comptime num_rows: u8, comptime num_cols: u8) type {
    return struct {
        data: [num_rows * num_cols]f32,
        const Self = @This();

        pub fn getItem(self: *const Self, i: u8, j: u8) f32 {
            // the below would break if num_cols were modified, but it is comptime so that can't happen
            return self.data[i * num_cols + j];
        }
    };
}

[wispy flicker]

that doesn't stop you from modifying data tho