Select Connection: INPUT[inlineListSuggester(optionQuery(#permanent_note), optionQuery(#literature_note), optionQuery(#fleeting_note)):connections]
Reference Types
“immutable” means “non-exclusive”. Actually non-mutable references can mutate with a process called “interior mutability”, which happens when working with threads.
The better naming would be shared reference and exclusive reference.
Beginners view
A function that takes an argument by immutable reference is allowed to read the data behind the reference:
struct Point {
x: u32,
y: u32,
}
fn print_point(pt: &Point) {
println!("x={} y={}", pt.x, pt.y);
}but it is not allowed to mutate the data:
// gives error
fn embiggen_x(pt: &Point) {
pt.x = pt.x * 2;
}Where it falls apart
&T is not an “immutable reference” or “const reference” to data of type T — it is a “shared reference”. And &mut T is not a “mutable reference” — it is an “exclusive reference”.
Exclusive reference: no other reference to the same value could possibly exist at the same time.
Shared reference: other references to the same value might exist, on other threads or the caller’s stack frame on the current thread.
In some cases, like Interior mutability, shared references (&T) may mutate.
Interior mutability
Interior mutability is the mutation through a shared reference.
The std library UnsafeCell<T> is the only way to hold data that is mutable through a shared reference. This should not be used directly, there are abstractions over this type for different use cases, in particular:
| Mutable? | Description | |
|---|---|---|
| Cell | ✅ | single threaded and cannot obtain a reference to the content |
| RefCell | ✅ | single threaded and dynamically checked borrow rules will prevent attempts to mutate while a reader is holding a reference into the content |
| Mutex | ✅ | only one of the references may operate on the inner type at a time; other accesses will block until the current one has release its lock |
| RwLock | ✅ | only one of the references may be used to mutate the type at a time, and only while no other references are being used for reading |