Read-Only Shared-Mutable Handle

This type marks a handle to a shared-mutable type that may be modified through other handles, but cannot be modified through this one. It is used when a BitSlice region has partial ownership of an element and wishes to expose the entire underlying raw element to the user without granting them write permissions.

Under the feature = "atomic" build setting, this uses radium’s best-effort atomic alias; when this feature is disabled, it reverts to Cell.