Add @parentFieldPtr

[alexnask]

Given a pointer to a struct, returns the pointer to a field.

@parentFieldPtr(comptime ParentType: type, comptime field_name: []const u8,
    parent_ptr: &ParentType) -> &FieldType

// Test usage
// get the i'th field of a struct
fn get(self: var, comptime i: usize) &(@memberType(@typeOf(self).Child, i)) {
    const Self = @typeOf(self).Child;
    const field_name = @memberName(Self, i);
    return @parentFieldPtr(Self, field_name, self);
}

const Foo = struct {
    a: usize,
    b: bool,
};

test "struct get" {
    var foo = Foo {
        .a = 0,
        .b = false
    };

    *get(foo, 0) = 1;
    
    assert(foo.a == 1);
    assert(!*get(foo, 1));
}

[andrewrk]

this? https://ziglang.org/documentation/master/#fieldParentPtr

[alexnask]

Sorry if I'm misunderstanding or I wasn't clear.
What I want to do is the exact opposite, given a pointer to the struct and the name of the field, I would like to get the pointer to the field.

[andrewrk]

#766