[YorickPeterse]

On the diagnostics side of things, the compiler produces these in two places:

1. If you try to define a `mut` field (= one you can normally assign a new value), it will produce an error at the definition site

2. If you try to do something like `some_inline_value.field = value` it also produces an error at the call/assignment site

The actual messages could use some improvements, but that's not a super high priority for the time being as I'm more focusing on the language overall.