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From the NLL RFC: “The deep [aka ‘supporting’] prefixes for an place are formed by stripping away fields and derefs, except that we stop when we reach the deref of a shared reference. […] “

“Shallow prefixes are found by stripping away fields, but stop at any dereference. So: writing a path like a is illegal if a.b is borrowed. But: writing a is legal if *a is borrowed, whether or not a is a shared or mutable reference. […] “