I think the way to say it is that Go requires you to declare every allocation, but allows over-declaration in the case of copying.

> := [...] an error to use it when it doesn't allocate.

> := [...] not allocating if you have a variable on the left that is already allocated,

This appears to be a contradiction.

I suppose you mean something like "error to use it when there's no possible context where that line of code would allocate"; what's an example of that?