Ausborgen - Überprüfung
Rust's borrow checker puts constraints on the ways you can borrow values. We've already seen that a reference cannot outlive the value it borrows:
There's also a second main rule that the borrow checker enforces: The aliasing rule. For a given value, at any time:
- You can have one or more shared references to the value, or
- You can have exactly one exclusive reference to the value.
Speaker Notes
This slide should take about 10 minutes.
- The "outlives" rule was demonstrated previously when we first looked at references. We review it here to show students that the borrow checking is following a few different rules to validate borrowing.
- Note that the requirement is that conflicting references not exist at the same point. It does not matter where the reference is dereferenced.
- The above code does not compile because
a
is borrowed as mutable (throughc
) and as immutable (throughb
) at the same time. - Move the
println!
statement forb
before the scope that introducesc
to make the code compile. - After that change, the compiler realizes that
b
is only ever used before the new mutable borrow ofa
throughc
. This is a feature of the borrow checker called "non-lexical lifetimes". - The exclusive reference constraint is quite strong. Rust uses it to ensure that data races do not occur. Rust also relies on this constraint to optimize code. For example, a value behind a shared reference can be safely cached in a register for the lifetime of that reference.
- The borrow checker is designed to accommodate many common patterns, such as taking exclusive references to different fields in a struct at the same time. But, there are some situations where it doesn't quite "get it" and this often results in "fighting with the borrow checker."