52db6b9d02
`adjust_upvar_deref` and friends are implemented so that they reuse existing region so new region vars don't have to be generated. Since now we don't have to generate region vars in `InferBorrowKind`, creating a new capture info would be cheap and we can just use `determine_capture_info`. syn is updated so that let_else syntax can be used in fn with `#[instrument]` attribute. `restrict_repr_packed_field_ref_capture` is changed to take place by value since cloning is needed anyway.
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.