Capabilities

There are four types of capabilites:

Exclusive (E)

Places with this capability can be read from, written to, or mutably borrowed.

We do not differentiate between locals bound with let bindings and let mut bindings: a variable bound with let would still have this capability if it could be written to if it was mutably borrowed.

Read (R)

Places with this capability can be read from. Shared borrows can also be created to this place. Shared references with this capability can be dereferenced.

A place $p$ with capability E is downgraded to R if a shared borrow is created to a place that is a pre- or postfix of $p$.

When a shared reference $p$ is dereferenced, the capability to $p$ is downgraded to R. Any place projecting from a shared references will have capability R.

Write (W)

The place can be written to.

When an exclusive reference $p$ is dereferenced, the capability to $p$ is downgraded to W.

ShallowExclusive (e)

This is used for a rather specific and uncommon situation. When converting a raw pointer *mut T into a Box<T>, there is an intermediate state where the memory for the box is allocated on the heap but the box does not yet hold a value. We use ShallowExclusive to represent this state.

Writing to a Box-typed place p via e.g. *p = 5 requires that p have capability e.