rust/lifetime-misc.md

% misc

This is just a dumping ground while I work out what to do with this stuff


# PhantomData

When working with unsafe code, we can often end up in a situation where
types or lifetimes are logically associated with a struct, but not actually
part of a field. This most commonly occurs with lifetimes. For instance, the `Iter`
for `&'a [T]` is (approximately) defined as follows:

```rust
pub struct Iter<'a, T: 'a> {
    ptr: *const T,
    end: *const T,
}
```

However because `'a` is unused within the struct's body, it's *unbound*.
Because of the troubles this has historically caused, unbound lifetimes and
types are *illegal* in struct definitions. Therefore we must somehow refer
to these types in the body. Correctly doing this is necessary to have
correct variance and drop checking.

We do this using *PhantomData*, which is a special marker type. PhantomData
consumes no space, but simulates a field of the given type for the purpose of
static analysis. This was deemed to be less error-prone than explicitly telling
the type-system the kind of variance that you want, while also providing other
useful information.

Iter logically contains `&'a T`, so this is exactly what we tell
the PhantomData to simulate:

```
pub struct Iter<'a, T: 'a> {
    ptr: *const T,
    end: *const T,
    _marker: marker::PhantomData<&'a T>,
}
```




# Dropck

When a type is going out of scope, Rust will try to Drop it. Drop executes
arbitrary code, and in fact allows us to "smuggle" arbitrary code execution
into many places. As such additional soundness checks (dropck) are necessary to
ensure that a type T can be safely instantiated and dropped. It turns out that we
*really* don't need to care about dropck in practice, as it often "just works".

However the one exception is with PhantomData. Given a struct like Vec:

```
struct Vec<T> {
    data: *const T, // *const for variance!
    len: usize,
    cap: usize,
}
```

dropck will generously determine that Vec<T> does not own any values of
type T. This will unfortunately allow people to construct unsound Drop
implementations that access data that has already been dropped. In order to
tell dropck that we *do* own values of type T, and may call destructors of that
type, we must add extra PhantomData:

```
struct Vec<T> {
    data: *const T, // *const for covariance!
    len: usize,
    cap: usize,
    _marker: marker::PhantomData<T>,
}
```

Raw pointers that own an allocation is such a pervasive pattern that the
standard library made a utility for itself called `Unique<T>` which:

* wraps a `*const T`,
* includes a `PhantomData<T>`,
* auto-derives Send/Sync as if T was contained
* marks the pointer as NonZero for the null-pointer optimization




# Splitting Lifetimes

The mutual exclusion property of mutable references can be very limiting when
working with a composite structure. The borrow checker understands some basic stuff, but
will fall over pretty easily. It *does* understand structs sufficiently to
know that it's possible to borrow disjoint fields of a struct simultaneously.
So this works today:

```rust
struct Foo {
    a: i32,
    b: i32,
    c: i32,
}

let mut x = Foo {a: 0, b: 0, c: 0};
let a = &mut x.a;
let b = &mut x.b;
let c = &x.c;
*b += 1;
let c2 = &x.c;
*a += 10;
println!("{} {} {} {}", a, b, c, c2);
```

However borrowck doesn't understand arrays or slices in any way, so this doesn't
work:

```rust
let x = [1, 2, 3];
let a = &mut x[0];
let b = &mut x[1];
println!("{} {}", a, b);
```

```text
<anon>:3:18: 3:22 error: cannot borrow immutable indexed content `x[..]` as mutable
<anon>:3     let a = &mut x[0];
                          ^~~~
<anon>:4:18: 4:22 error: cannot borrow immutable indexed content `x[..]` as mutable
<anon>:4     let b = &mut x[1];
                          ^~~~
error: aborting due to 2 previous errors
```

While it was plausible that borrowck could understand this simple case, it's
pretty clearly hopeless for borrowck to understand disjointness in general
container types like a tree, especially if distinct keys actually *do* map
to the same value.

In order to "teach" borrowck that what we're doing is ok, we need to drop down
to unsafe code. For instance, mutable slices expose a `split_at_mut` function that
consumes the slice and returns *two* mutable slices. One for everything to the
left of the index, and one for everything to the right. Intuitively we know this
is safe because the slices don't alias. However the implementation requires some
unsafety:

```rust
fn split_at_mut(&mut self, mid: usize) -> (&mut [T], &mut [T]) {
    unsafe {
        let self2: &mut [T] = mem::transmute_copy(&self);

        (ops::IndexMut::index_mut(self, ops::RangeTo { end: mid } ),
         ops::IndexMut::index_mut(self2, ops::RangeFrom { start: mid } ))
    }
}
```

This is pretty plainly dangerous. We use transmute to duplicate the slice with an
*unbounded* lifetime, so that it can be treated as disjoint from the other until
we unify them when we return.

However more subtle is how iterators that yield mutable references work.
The iterator trait is defined as follows:

```rust
trait Iterator {
    type Item;

    fn next(&mut self) -> Option<Self::Item>;
}
```

Given this definition, Self::Item has *no* connection to `self`. This means
that we can call `next` several times in a row, and hold onto all the results
*concurrently*. This is perfectly fine for by-value iterators, which have exactly
these semantics. It's also actually fine for shared references, as they admit
arbitrarily many references to the same thing (although the
iterator needs to be a separate object from the thing being shared). But mutable
references make this a mess. At first glance, they might seem completely
incompatible with this API, as it would produce multiple mutable references to
the same object!

However it actually *does* work, exactly because iterators are one-shot objects.
Everything an IterMut yields will be yielded *at most* once, so we don't *actually*
ever yield multiple mutable references to the same piece of data.

In general all mutable iterators require *some* unsafe code *somewhere*, though.
Whether it's raw pointers, or safely composing on top of *another* IterMut.

For instance, VecDeque's IterMut:

```rust
pub struct IterMut<'a, T:'a> {
    // The whole backing array. Some of these indices are initialized!
    ring: &'a mut [T],
    tail: usize,
    head: usize,
}

impl<'a, T> Iterator for IterMut<'a, T> {
    type Item = &'a mut T;

    fn next(&mut self) -> Option<&'a mut T> {
        if self.tail == self.head {
            return None;
        }
        let tail = self.tail;
        self.tail = wrap_index(self.tail.wrapping_add(1), self.ring.len());

        unsafe {
            // might as well do unchecked indexing since wrap_index has us
            // in-bounds, and many of the "middle" indices are uninitialized
            // anyway.
            let elem = self.ring.get_unchecked_mut(tail);

            // round-trip through a raw pointer to unbound the lifetime from
            // ourselves
            Some(&mut *(elem as *mut _))
        }
    }
}
```

A very subtle but interesting detail in this design is that it *relies on
privacy to be sound*. Borrowck works on some very simple rules. One of those rules
is that if we have a live &mut Foo and Foo contains an &mut Bar, then that &mut
Bar is *also* live. Since IterMut is always live when `next` can be called, if
`ring` were public then we could mutate `ring` while outstanding mutable borrows
to it exist!