Incidentally, the last time this came up on internals, we failed to come up with any use cases where a "partial borrows" feature in the core language would've actually helped. Specifically, it seemed like all the use cases could be worked around with some refactoring, and any core language change that did not require such refactoring would amount to turning a borrow check error into a silent API compatibility hazard, which seemed like a net loss.
So if anybody does know of a compelling use case where that argument doesn't hold, please necro that thread!