230 lines
7.4 KiB
Markdown
230 lines
7.4 KiB
Markdown
% 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!
|