Borrow Checking?
None of the desugarings so far mention borrow-checking. And the reason why is that unlike type-checking, borrow-checking really is just a "check". In particular, the result of borrow-checking must not influence runtime behavior.
So there should not need to be a desugaring related to borrow-checking.
Discussion
The question remains of when to run borrow-checking. Ideally we'd run it at the end of all the desugarings; as presented though we lose some information while desugaring, in particular around matches, so borrow-checking here would accept unsound code (see e.g. Bound Checks), and reject code that is accepted today (see the note about slice patterns in Pattern Unnesting).
If we wanted to have accurate borrow-checking, we'd need, at least:
- Fake borrows/fake reads of the places involved in a match (see Match Guard Mutable Bindings);
- Fake borrows/fake reads of the matched place to detect
let x: !; match x {}; - Fake borrows/fake reads around bounds checks to reject
x[0][{x = &[]; 0}](see Bound Checks); - Some false edges I don't recall where (I know MIR has some for loops and match guards but both of these are irrelevant for us);
- Keep
let _ = $place;place mentions I think; - Support tracking some constant slice indices, for the purpose of borrow-checking slice patterns (see Pattern Unnesting).
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