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nits and realigning

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Alexis Beingessner 9 years ago committed by Manish Goregaokar
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6
README.md
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@ -34,6 +34,6 @@ Due to the nature of advanced Rust programming, we will be spending a lot of tim
talking about *safety* and *guarantees*. In particular, a significant portion of
the book will be dedicated to correctly writing and understanding Unsafe Rust.
[trpl]: https://doc.rust-lang.org/book/
[The stack and heap]: https://doc.rust-lang.org/book/the-stack-and-the-heap.html
[Basic Rust]: https://doc.rust-lang.org/book/syntax-and-semantics.html
[trpl]: ../book/
[The stack and heap]: ../book/the-stack-and-the-heap.html
[Basic Rust]: ../book/syntax-and-semantics.html

2
arc-and-mutex.md
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@ -1,6 +1,6 @@
% Implementing Arc and Mutex
Knowing the theory is all fine and good, but the *best* was to understand
Knowing the theory is all fine and good, but the *best* way to understand
something is to use it. To better understand atomics and interior mutability,
we'll be implementing versions of the standard library's Arc and Mutex types.

161
atomics.md
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@ -2,21 +2,22 @@
Rust pretty blatantly just inherits C11's memory model for atomics. This is not
due this model being particularly excellent or easy to understand. Indeed, this
model is quite complex and known to have [several flaws][C11-busted]. Rather,
it is a pragmatic concession to the fact that *everyone* is pretty bad at modeling
model is quite complex and known to have [several flaws][C11-busted]. Rather, it
is a pragmatic concession to the fact that *everyone* is pretty bad at modeling
atomics. At very least, we can benefit from existing tooling and research around
C.
Trying to fully explain the model in this book is fairly hopeless. It's defined
in terms of madness-inducing causality graphs that require a full book to properly
understand in a practical way. If you want all the nitty-gritty details, you
should check out [C's specification (Section 7.17)][C11-model]. Still, we'll try
to cover the basics and some of the problems Rust developers face.
in terms of madness-inducing causality graphs that require a full book to
properly understand in a practical way. If you want all the nitty-gritty
details, you should check out [C's specification (Section 7.17)][C11-model].
Still, we'll try to cover the basics and some of the problems Rust developers
face.
The C11 memory model is fundamentally about trying to bridge the gap between
the semantics we want, the optimizations compilers want, and the inconsistent
chaos our hardware wants. *We* would like to just write programs and have them
do exactly what we said but, you know, *fast*. Wouldn't that be great?
The C11 memory model is fundamentally about trying to bridge the gap between the
semantics we want, the optimizations compilers want, and the inconsistent chaos
our hardware wants. *We* would like to just write programs and have them do
exactly what we said but, you know, *fast*. Wouldn't that be great?
@ -41,13 +42,14 @@ x = 2;
y = 3;
```
This has inverted the order of events *and* completely eliminated one event. From
a single-threaded perspective this is completely unobservable: after all the
statements have executed we are in exactly the same state. But if our program is
multi-threaded, we may have been relying on `x` to *actually* be assigned to 1 before
`y` was assigned. We would *really* like the compiler to be able to make these kinds
of optimizations, because they can seriously improve performance. On the other hand,
we'd really like to be able to depend on our program *doing the thing we said*.
This has inverted the order of events *and* completely eliminated one event.
From a single-threaded perspective this is completely unobservable: after all
the statements have executed we are in exactly the same state. But if our
program is multi-threaded, we may have been relying on `x` to *actually* be
assigned to 1 before `y` was assigned. We would *really* like the compiler to be
able to make these kinds of optimizations, because they can seriously improve
performance. On the other hand, we'd really like to be able to depend on our
program *doing the thing we said*.
@ -55,19 +57,20 @@ we'd really like to be able to depend on our program *doing the thing we said*.
# Hardware Reordering
On the other hand, even if the compiler totally understood what we wanted and
respected our wishes, our *hardware* might instead get us in trouble. Trouble comes
from CPUs in the form of memory hierarchies. There is indeed a global shared memory
space somewhere in your hardware, but from the perspective of each CPU core it is
*so very far away* and *so very slow*. Each CPU would rather work with its local
cache of the data and only go through all the *anguish* of talking to shared
memory *only* when it doesn't actually have that memory in cache.
respected our wishes, our *hardware* might instead get us in trouble. Trouble
comes from CPUs in the form of memory hierarchies. There is indeed a global
shared memory space somewhere in your hardware, but from the perspective of each
CPU core it is *so very far away* and *so very slow*. Each CPU would rather work
with its local cache of the data and only go through all the *anguish* of
talking to shared memory *only* when it doesn't actually have that memory in
cache.
After all, that's the whole *point* of the cache, right? If every read from the
cache had to run back to shared memory to double check that it hadn't changed,
what would the point be? The end result is that the hardware doesn't guarantee
that events that occur in the same order on *one* thread, occur in the same order
on *another* thread. To guarantee this, we must issue special instructions to
the CPU telling it to be a bit less smart.
that events that occur in the same order on *one* thread, occur in the same
order on *another* thread. To guarantee this, we must issue special instructions
to the CPU telling it to be a bit less smart.
For instance, say we convince the compiler to emit this logic:
@ -82,27 +85,27 @@ x = 1; y *= 2;
Ideally this program has 2 possible final states:
* `y = 3`: (thread 2 did the check before thread 1 completed)
* `y = 6`: (thread 2 did the check after thread 1 completed)
* `y = 3`: (thread 2 did the check before thread 1 completed) y = 6`: (thread 2
* `did the check after thread 1 completed)
However there's a third potential state that the hardware enables:
* `y = 2`: (thread 2 saw `x = 2`, but not `y = 3`, and then overwrote `y = 3`)
It's worth noting that different kinds of CPU provide different guarantees. It
is common to seperate hardware into two categories: strongly-ordered and weakly-
ordered. Most notably x86/64 provides strong ordering guarantees, while ARM and
provides weak ordering guarantees. This has two consequences for
concurrent programming:
is common to separate hardware into two categories: strongly-ordered and weakly-
ordered. Most notably x86/64 provides strong ordering guarantees, while ARM
provides weak ordering guarantees. This has two consequences for concurrent
programming:
* Asking for stronger guarantees on strongly-ordered hardware may be cheap or
even *free* because they already provide strong guarantees unconditionally.
Weaker guarantees may only yield performance wins on weakly-ordered hardware.
* Asking for guarantees that are *too* weak on strongly-ordered hardware
is more likely to *happen* to work, even though your program is strictly
incorrect. If possible, concurrent algorithms should be tested on
weakly-ordered hardware.
* Asking for guarantees that are *too* weak on strongly-ordered hardware is
more likely to *happen* to work, even though your program is strictly
incorrect. If possible, concurrent algorithms should be tested on weakly-
ordered hardware.
@ -110,58 +113,54 @@ concurrent programming:
# Data Accesses
The C11 memory model attempts to bridge the gap by allowing us to talk about
the *causality* of our program. Generally, this is by establishing a
*happens before* relationships between parts of the program and the threads
that are running them. This gives the hardware and compiler room to optimize the
program more aggressively where a strict happens-before relationship isn't
established, but forces them to be more careful where one *is* established.
The way we communicate these relationships are through *data accesses* and
*atomic accesses*.
The C11 memory model attempts to bridge the gap by allowing us to talk about the
*causality* of our program. Generally, this is by establishing a *happens
before* relationships between parts of the program and the threads that are
running them. This gives the hardware and compiler room to optimize the program
more aggressively where a strict happens-before relationship isn't established,
but forces them to be more careful where one *is* established. The way we
communicate these relationships are through *data accesses* and *atomic
accesses*.
Data accesses are the bread-and-butter of the programming world. They are
fundamentally unsynchronized and compilers are free to aggressively optimize
them. In particular, data accesses are free to be reordered by the compiler
on the assumption that the program is single-threaded. The hardware is also free
to propagate the changes made in data accesses to other threads
as lazily and inconsistently as it wants. Mostly critically, data accesses are
how data races happen. Data accesses are very friendly to the hardware and
compiler, but as we've seen they offer *awful* semantics to try to
write synchronized code with. Actually, that's too weak. *It is literally
impossible to write correct synchronized code using only data accesses*.
them. In particular, data accesses are free to be reordered by the compiler on
the assumption that the program is single-threaded. The hardware is also free to
propagate the changes made in data accesses to other threads as lazily and
inconsistently as it wants. Mostly critically, data accesses are how data races
happen. Data accesses are very friendly to the hardware and compiler, but as
we've seen they offer *awful* semantics to try to write synchronized code with.
Actually, that's too weak. *It is literally impossible to write correct
synchronized code using only data accesses*.
Atomic accesses are how we tell the hardware and compiler that our program is
multi-threaded. Each atomic access can be marked with
an *ordering* that specifies what kind of relationship it establishes with
other accesses. In practice, this boils down to telling the compiler and hardware
certain things they *can't* do. For the compiler, this largely revolves
around re-ordering of instructions. For the hardware, this largely revolves
around how writes are propagated to other threads. The set of orderings Rust
exposes are:
* Sequentially Consistent (SeqCst)
* Release
* Acquire
* Relaxed
multi-threaded. Each atomic access can be marked with an *ordering* that
specifies what kind of relationship it establishes with other accesses. In
practice, this boils down to telling the compiler and hardware certain things
they *can't* do. For the compiler, this largely revolves around re-ordering of
instructions. For the hardware, this largely revolves around how writes are
propagated to other threads. The set of orderings Rust exposes are:
* Sequentially Consistent (SeqCst) Release Acquire Relaxed
(Note: We explicitly do not expose the C11 *consume* ordering)
TODO: negative reasoning vs positive reasoning?
TODO: "can't forget to synchronize"
TODO: negative reasoning vs positive reasoning? TODO: "can't forget to
synchronize"
# Sequentially Consistent
Sequentially Consistent is the most powerful of all, implying the restrictions
of all other orderings. Intuitively, a sequentially consistent operation *cannot*
be reordered: all accesses on one thread that happen before and after it *stay*
before and after it. A data-race-free program that uses only sequentially consistent
atomics and data accesses has the very nice property that there is a single global
execution of the program's instructions that all threads agree on. This execution
is also particularly nice to reason about: it's just an interleaving of each thread's
individual executions. This *does not* hold if you start using the weaker atomic
orderings.
of all other orderings. Intuitively, a sequentially consistent operation
*cannot* be reordered: all accesses on one thread that happen before and after a
SeqCst access *stay* before and after it. A data-race-free program that uses
only sequentially consistent atomics and data accesses has the very nice
property that there is a single global execution of the program's instructions
that all threads agree on. This execution is also particularly nice to reason
about: it's just an interleaving of each thread's individual executions. This
*does not* hold if you start using the weaker atomic orderings.
The relative developer-friendliness of sequential consistency doesn't come for
free. Even on strongly-ordered platforms sequential consistency involves
@ -173,26 +172,26 @@ confident about the other memory orders. Having your program run a bit slower
than it needs to is certainly better than it running incorrectly! It's also
*mechanically* trivial to downgrade atomic operations to have a weaker
consistency later on. Just change `SeqCst` to e.g. `Relaxed` and you're done! Of
course, proving that this transformation is *correct* is whole other matter.
course, proving that this transformation is *correct* is a whole other matter.
# Acquire-Release
Acquire and Release are largely intended to be paired. Their names hint at
their use case: they're perfectly suited for acquiring and releasing locks,
and ensuring that critical sections don't overlap.
Acquire and Release are largely intended to be paired. Their names hint at their
use case: they're perfectly suited for acquiring and releasing locks, and
ensuring that critical sections don't overlap.
Intuitively, an acquire access ensures that every access after it *stays* after
it. However operations that occur before an acquire are free to be reordered to
occur after it. Similarly, a release access ensures that every access before it
*stays* before it. However operations that occur after a release are free to
be reordered to occur before it.
*stays* before it. However operations that occur after a release are free to be
reordered to occur before it.
When thread A releases a location in memory and then thread B subsequently
acquires *the same* location in memory, causality is established. Every write
that happened *before* A's release will be observed by B *after* it's release.
that happened *before* A's release will be observed by B *after* its release.
However no causality is established with any other threads. Similarly, no
causality is established if A and B access *different* locations in memory.

27
casts.md
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@ -1,12 +1,13 @@
% Casts
Casts are a superset of coercions: every coercion can be explicitly invoked via a
cast, but some conversions *require* a cast. These "true casts" are generally regarded
as dangerous or problematic actions. True casts revolve around raw pointers and
the primitive numeric types. True casts aren't checked.
Casts are a superset of coercions: every coercion can be explicitly invoked via
a cast, but some conversions *require* a cast. These "true casts" are generally
regarded as dangerous or problematic actions. True casts revolve around raw
pointers and the primitive numeric types. True casts aren't checked.
Here's an exhaustive list of all the true casts. For brevity, we will use `*`
to denote either a `*const` or `*mut`, and `integer` to denote any integral primitive:
to denote either a `*const` or `*mut`, and `integer` to denote any integral
primitive:
* `*T as *U` where `T, U: Sized`
* `*T as *U` TODO: explain unsized situation
@ -37,19 +38,21 @@ expression, `e as U2` is not necessarily so (in fact it will only be valid if
For numeric casts, there are quite a few cases to consider:
* casting between two integers of the same size (e.g. i32 -> u32) is a no-op
* casting from a larger integer to a smaller integer (e.g. u32 -> u8) will truncate
* casting from a larger integer to a smaller integer (e.g. u32 -> u8) will
truncate
* casting from a smaller integer to a larger integer (e.g. u8 -> u32) will
* zero-extend if the source is unsigned
* sign-extend if the source is signed
* casting from a float to an integer will round the float towards zero
* **NOTE: currently this will cause Undefined Behaviour if the rounded
value cannot be represented by the target integer type**. This is a bug
and will be fixed. (TODO: figure out what Inf and NaN do)
* casting from an integer to float will produce the floating point representation
of the integer, rounded if necessary (rounding strategy unspecified).
* casting from an f32 to an f64 is perfect and lossless.
value cannot be represented by the target integer type**. This includes
Inf and NaN. This is a bug and will be fixed.
* casting from an integer to float will produce the floating point
representation of the integer, rounded if necessary (rounding strategy
unspecified)
* casting from an f32 to an f64 is perfect and lossless
* casting from an f64 to an f32 will produce the closest possible value
(rounding strategy unspecified).
(rounding strategy unspecified)
* **NOTE: currently this will cause Undefined Behaviour if the value
is finite but larger or smaller than the largest or smallest finite
value representable by f32**. This is a bug and will be fixed.

6
checked-uninit.md
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@ -1,8 +1,8 @@
% Checked Uninitialized Memory
Like C, all stack variables in Rust are uninitialized until a
value is explicitly assigned to them. Unlike C, Rust statically prevents you
from ever reading them until you do:
Like C, all stack variables in Rust are uninitialized until a value is
explicitly assigned to them. Unlike C, Rust statically prevents you from ever
reading them until you do:
```rust
fn main() {

20
constructors.md
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@ -15,21 +15,21 @@ enum Bar {
Y(bool),
}
struct Empty;
struct Unit;
let foo = Foo { a: 0, b: 1, c: false };
let bar = Bar::X(0);
let empty = Empty;
let empty = Unit;
```
That's it. Every other way you make an instance of a type is just calling a
totally vanilla function that does some stuff and eventually bottoms out to The
One True Constructor.
Unlike C++, Rust does not come with a slew of built in kinds of constructor.
Unlike C++, Rust does not come with a slew of built-in kinds of constructor.
There are no Copy, Default, Assignment, Move, or whatever constructors. The
reasons for this are varied, but it largely boils down to Rust's philosophy
of *being explicit*.
reasons for this are varied, but it largely boils down to Rust's philosophy of
*being explicit*.
Move constructors are meaningless in Rust because we don't enable types to
"care" about their location in memory. Every type must be ready for it to be
@ -37,9 +37,9 @@ blindly memcopied to somewhere else in memory. This means pure on-the-stack-but-
still-movable intrusive linked lists are simply not happening in Rust (safely).
Assignment and copy constructors similarly don't exist because move semantics
are the *only* semantics in Rust. At most `x = y` just moves the bits of y into the x
variable. Rust *does* provide two facilities for providing C++'s copy-oriented
semantics: `Copy` and `Clone`. Clone is our moral equivalent of a copy
are the *only* semantics in Rust. At most `x = y` just moves the bits of y into
the x variable. Rust *does* provide two facilities for providing C++'s copy-
oriented semantics: `Copy` and `Clone`. Clone is our moral equivalent of a copy
constructor, but it's never implicitly invoked. You have to explicitly call
`clone` on an element you want to be cloned. Copy is a special case of Clone
where the implementation is just "copy the bits". Copy types *are* implicitly
@ -53,3 +53,7 @@ only useful for generic programming. In concrete contexts, a type will provide a
static `new` method for any kind of "default" constructor. This has no relation
to `new` in other languages and has no special meaning. It's just a naming
convention.
TODO: talk about "placement new"?
[uninit]: uninitialized.html

16
conversions.md
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@ -1,13 +1,13 @@
% Type Conversions
At the end of the day, everything is just a pile of bits somewhere, and type systems
are just there to help us use those bits right. Needing to reinterpret those piles
of bits as different types is a common problem and Rust consequently gives you
several ways to do that.
At the end of the day, everything is just a pile of bits somewhere, and type
systems are just there to help us use those bits right. Needing to reinterpret
those piles of bits as different types is a common problem and Rust consequently
gives you several ways to do that.
First we'll look at the ways that *Safe Rust* gives you to reinterpret values. The
most trivial way to do this is to just destructure a value into its constituent
parts and then build a new type out of them. e.g.
First we'll look at the ways that *Safe Rust* gives you to reinterpret values.
The most trivial way to do this is to just destructure a value into its
constituent parts and then build a new type out of them. e.g.
```rust
struct Foo {
@ -26,6 +26,6 @@ fn reinterpret(foo: Foo) -> Bar {
}
```
But this is, at best, annoying to do. For common conversions, rust provides
But this is, at best, annoying to do. For common conversions, Rust provides
more ergonomic alternatives.

49
destructors.md
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@ -1,23 +1,24 @@
% Destructors
What the language *does* provide is full-blown automatic destructors through the `Drop` trait,
which provides the following method:
What the language *does* provide is full-blown automatic destructors through the
`Drop` trait, which provides the following method:
```rust
fn drop(&mut self);
```
This method gives the type time to somehow finish what it was doing. **After `drop` is run,
Rust will recursively try to drop all of the fields of `self`**. This is a
convenience feature so that you don't have to write "destructor boilerplate" to drop
children. If a struct has no special logic for being dropped other than dropping its
children, then it means `Drop` doesn't need to be implemented at all!
This method gives the type time to somehow finish what it was doing. **After
`drop` is run, Rust will recursively try to drop all of the fields of `self`**.
This is a convenience feature so that you don't have to write "destructor
boilerplate" to drop children. If a struct has no special logic for being
dropped other than dropping its children, then it means `Drop` doesn't need to
be implemented at all!
**There is no stable way to prevent this behaviour in Rust 1.0**.
**There is no stable way to prevent this behaviour in Rust 1.0.
Note that taking `&mut self` means that even if you *could* suppress recursive Drop,
Rust will prevent you from e.g. moving fields out of self. For most types, this
is totally fine.
Note that taking `&mut self` means that even if you *could* suppress recursive
Drop, Rust will prevent you from e.g. moving fields out of self. For most types,
this is totally fine.
For instance, a custom implementation of `Box` might write `Drop` like this:
@ -34,9 +35,9 @@ impl<T> Drop for Box<T> {
}
```
and this works fine because when Rust goes to drop the `ptr` field it just sees a *mut that
has no actual `Drop` implementation. Similarly nothing can use-after-free the `ptr` because
the Box is immediately marked as uninitialized.
and this works fine because when Rust goes to drop the `ptr` field it just sees
a *mut that has no actual `Drop` implementation. Similarly nothing can use-
after-free the `ptr` because the Box is immediately marked as uninitialized.
However this wouldn't work:
@ -93,11 +94,13 @@ enum Link {
}
```
will have its inner Box field dropped *if and only if* an instance stores the Next variant.
will have its inner Box field dropped *if and only if* an instance stores the
Next variant.
In general this works really nice because you don't need to worry about adding/removing
drops when you refactor your data layout. Still there's certainly many valid usecases for
needing to do trickier things with destructors.
In general this works really nice because you don't need to worry about
adding/removing drops when you refactor your data layout. Still there's
certainly many valid usecases for needing to do trickier things with
destructors.
The classic safe solution to overriding recursive drop and allowing moving out
of Self during `drop` is to use an Option:
@ -128,11 +131,11 @@ impl<T> Drop for SuperBox<T> {
}
```
However this has fairly odd semantics: you're saying that a field that *should* always
be Some may be None, just because that happens in the destructor. Of course this
conversely makes a lot of sense: you can call arbitrary methods on self during
the destructor, and this should prevent you from ever doing so after deinitializing
the field. Not that it will prevent you from producing any other
However this has fairly odd semantics: you're saying that a field that *should*
always be Some may be None, just because that happens in the destructor. Of
course this conversely makes a lot of sense: you can call arbitrary methods on
self during the destructor, and this should prevent you from ever doing so after
deinitializing the field. Not that it will prevent you from producing any other
arbitrarily invalid state in there.
On balance this is an ok choice. Certainly what you should reach for by default.

42
drop-flags.md
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@ -3,42 +3,42 @@
The examples in the previous section introduce an interesting problem for Rust.
We have seen that's possible to conditionally initialize, deinitialize, and
*reinitialize* locations of memory totally safely. For Copy types, this isn't
particularly notable since they're just a random pile of bits. However types with
destructors are a different story: Rust needs to know whether to call a destructor
whenever a variable is assigned to, or a variable goes out of scope. How can it
do this with conditional initialization?
particularly notable since they're just a random pile of bits. However types
with destructors are a different story: Rust needs to know whether to call a
destructor whenever a variable is assigned to, or a variable goes out of scope.
How can it do this with conditional initialization?
It turns out that Rust actually tracks whether a type should be dropped or not *at
runtime*. As a variable becomes initialized and uninitialized, a *drop flag* for
that variable is toggled. When a variable *might* need to be dropped, this flag
is evaluated to determine if it *should* be dropped.
It turns out that Rust actually tracks whether a type should be dropped or not
*at runtime*. As a variable becomes initialized and uninitialized, a *drop flag*
for that variable is toggled. When a variable *might* need to be dropped, this
flag is evaluated to determine if it *should* be dropped.
Of course, it is *often* the case that a value's initialization state can be
*statically* known at every point in the program. If this is the case, then the
compiler can theoretically generate more effecient code! For instance,
straight-line code has such *static drop semantics*:
compiler can theoretically generate more effecient code! For instance, straight-
line code has such *static drop semantics*:
```rust
let mut x = Box::new(0); // x was uninit
let mut y = x; // y was uninit
x = Box::new(0); // x was uninit
y = x; // y was init; Drop y!
let mut x = Box::new(0); // x was uninit; just overwrite.
let mut y = x; // y was uninit; just overwrite and make x uninit.
x = Box::new(0); // x was uninit; just overwrite.
y = x; // y was init; Drop y, overwrite it, and make x uninit!
// y was init; Drop y!
// x was uninit
// x was uninit; do nothing.
```
And even branched code where all branches have the same behaviour with respect
to initialization:
```rust
let mut x = Box::new(0); // x was uninit
let mut x = Box::new(0); // x was uninit; just overwrite.
if condition {
drop(x) // x gets moved out
drop(x) // x gets moved out; make x uninit.
} else {
println!("{}", x);
drop(x) // x gets moved out
drop(x) // x gets moved out; make x uninit.
}
x = Box::new(0); // x was uninit
x = Box::new(0); // x was uninit; just overwrite.
// x was init; Drop x!
```
@ -47,10 +47,10 @@ However code like this *requires* runtime information to correctly Drop:
```rust
let x;
if condition {
x = Box::new(0); // x was uninit
x = Box::new(0); // x was uninit; just overwrite.
println!("{}", x);
}
// x might be uninit; check the flag!
// x *might* be uninit; check the flag!
```
Of course, in this case it's trivial to retrieve static drop semantics:

13
exception-safety.md
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@ -7,7 +7,7 @@ if it overflows. Unless you are very careful and tightly control what code runs,
pretty much everything can unwind, and you need to be ready for it.
Being ready for unwinding is often referred to as *exception safety*
in the broader programming world. In Rust, their are two levels of exception
in the broader programming world. In Rust, there are two levels of exception
safety that one may concern themselves with:
* In unsafe code, we *must* be exception safe to the point of not violating
@ -58,16 +58,17 @@ impl<T: Clone> Vec<T> {
We bypass `push` in order to avoid redundant capacity and `len` checks on the
Vec that we definitely know has capacity. The logic is totally correct, except
there's a subtle problem with our code: it's not exception-safe! `set_len`,
`offset`, and `write` are all fine, but *clone* is the panic bomb we over-looked.
`offset`, and `write` are all fine, but *clone* is the panic bomb we over-
looked.
Clone is completely out of our control, and is totally free to panic. If it does,
our function will exit early with the length of the Vec set too large. If
Clone is completely out of our control, and is totally free to panic. If it
does, our function will exit early with the length of the Vec set too large. If
the Vec is looked at or dropped, uninitialized memory will be read!
The fix in this case is fairly simple. If we want to guarantee that the values
we *did* clone are dropped we can set the len *in* the loop. If we just want to
guarantee that uninitialized memory can't be observed, we can set the len *after*
the loop.
guarantee that uninitialized memory can't be observed, we can set the len
*after* the loop.

48
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@ -9,18 +9,19 @@ is not always the case, however.
# Dynamically Sized Types (DSTs)
Rust also supports types without a statically known size. On the surface,
this is a bit nonsensical: Rust *must* know the size of something in order to
work with it! DSTs are generally produced as views, or through type-erasure
of types that *do* have a known size. Due to their lack of a statically known
size, these types can only exist *behind* some kind of pointer. They consequently
produce a *fat* pointer consisting of the pointer and the information that
*completes* them.
For instance, the slice type, `[T]`, is some statically unknown number of elements
stored contiguously. `&[T]` consequently consists of a `(&T, usize)` pair that specifies
where the slice starts, and how many elements it contains. Similarly, Trait Objects
support interface-oriented type erasure through a `(data_ptr, vtable_ptr)` pair.
Rust also supports types without a statically known size. On the surface, this
is a bit nonsensical: Rust *must* know the size of something in order to work
with it! DSTs are generally produced as views, or through type-erasure of types
that *do* have a known size. Due to their lack of a statically known size, these
types can only exist *behind* some kind of pointer. They consequently produce a
*fat* pointer consisting of the pointer and the information that *completes*
them.
For instance, the slice type, `[T]`, is some statically unknown number of
elements stored contiguously. `&[T]` consequently consists of a `(&T, usize)`
pair that specifies where the slice starts, and how many elements it contains.
Similarly, Trait Objects support interface-oriented type erasure through a
`(data_ptr, vtable_ptr)` pair.
Structs can actually store a single DST directly as their last field, but this
makes them a DST as well:
@ -55,33 +56,34 @@ struct Baz {
}
```
On their own, ZSTs are, for obvious reasons, pretty useless. However
as with many curious layout choices in Rust, their potential is realized in a generic
On their own, ZSTs are, for obvious reasons, pretty useless. However as with
many curious layout choices in Rust, their potential is realized in a generic
context.
Rust largely understands that any operation that produces or stores a ZST
can be reduced to a no-op. For instance, a `HashSet<T>` can be effeciently implemented
as a thin wrapper around `HashMap<T, ()>` because all the operations `HashMap` normally
does to store and retrieve keys will be completely stripped in monomorphization.
Rust largely understands that any operation that produces or stores a ZST can be
reduced to a no-op. For instance, a `HashSet<T>` can be effeciently implemented
as a thin wrapper around `HashMap<T, ()>` because all the operations `HashMap`
normally does to store and retrieve keys will be completely stripped in
monomorphization.
Similarly `Result<(), ()>` and `Option<()>` are effectively just fancy `bool`s.
Safe code need not worry about ZSTs, but *unsafe* code must be careful about the
consequence of types with no size. In particular, pointer offsets are no-ops, and
standard allocators (including jemalloc, the one used by Rust) generally consider
passing in `0` as Undefined Behaviour.
consequence of types with no size. In particular, pointer offsets are no-ops,
and standard allocators (including jemalloc, the one used by Rust) generally
consider passing in `0` as Undefined Behaviour.
# Void Types
# Empty Types
Rust also enables types to be declared that *cannot even be instantiated*. These
types can only be talked about at the type level, and never at the value level.
```rust
enum Foo { } // No variants = VOID
enum Foo { } // No variants = EMPTY
```
TODO: WHY?!

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@ -1,46 +1,46 @@
% Leaking
Ownership based resource management is intended to simplify composition. You
acquire resources when you create the object, and you release the resources
when it gets destroyed. Since destruction is handled for you, it means you
can't forget to release the resources, and it happens as soon as possible!
Surely this is perfect and all of our problems are solved.
Ownership-based resource management is intended to simplify composition. You
acquire resources when you create the object, and you release the resources when
it gets destroyed. Since destruction is handled for you, it means you can't
forget to release the resources, and it happens as soon as possible! Surely this
is perfect and all of our problems are solved.
Everything is terrible and we have new and exotic problems to try to solve.
Many people like to believe that Rust eliminates resource leaks, but this
is absolutely not the case, no matter how you look at it. In the strictest
sense, "leaking" is so abstract as to be unpreventable. It's quite trivial
to initialize a collection at the start of a program, fill it with tons of
objects with destructors, and then enter an infinite event loop that never
refers to it. The collection will sit around uselessly, holding on to its
precious resources until the program terminates (at which point all those
resources would have been reclaimed by the OS anyway).
Many people like to believe that Rust eliminates resource leaks, but this is
absolutely not the case, no matter how you look at it. In the strictest sense,
"leaking" is so abstract as to be unpreventable. It's quite trivial to
initialize a collection at the start of a program, fill it with tons of objects
with destructors, and then enter an infinite event loop that never refers to it.
The collection will sit around uselessly, holding on to its precious resources
until the program terminates (at which point all those resources would have been
reclaimed by the OS anyway).
We may consider a more restricted form of leak: failing to drop a value that
is unreachable. Rust also doesn't prevent this. In fact Rust has a *function
for doing this*: `mem::forget`. This function consumes the value it is passed
*and then doesn't run its destructor*.
We may consider a more restricted form of leak: failing to drop a value that is
unreachable. Rust also doesn't prevent this. In fact Rust has a *function for
doing this*: `mem::forget`. This function consumes the value it is passed *and
then doesn't run its destructor*.
In the past `mem::forget` was marked as unsafe as a sort of lint against using
it, since failing to call a destructor is generally not a well-behaved thing to
do (though useful for some special unsafe code). However this was generally
determined to be an untenable stance to take: there are *many* ways to fail to
call a destructor in safe code. The most famous example is creating a cycle
of reference counted pointers using interior mutability.
call a destructor in safe code. The most famous example is creating a cycle of
reference-counted pointers using interior mutability.
It is reasonable for safe code to assume that destructor leaks do not happen,
as any program that leaks destructors is probably wrong. However *unsafe* code
It is reasonable for safe code to assume that destructor leaks do not happen, as
any program that leaks destructors is probably wrong. However *unsafe* code
cannot rely on destructors to be run to be *safe*. For most types this doesn't
matter: if you leak the destructor then the type is *by definition* inaccessible,
so it doesn't matter, right? For instance, if you leak a `Box<u8>` then you
waste some memory but that's hardly going to violate memory-safety.
matter: if you leak the destructor then the type is *by definition*
inaccessible, so it doesn't matter, right? For instance, if you leak a `Box<u8>`
then you waste some memory but that's hardly going to violate memory-safety.
However where we must be careful with destructor leaks are *proxy* types.
These are types which manage access to a distinct object, but don't actually
own it. Proxy objects are quite rare. Proxy objects you'll need to care about
are even rarer. However we'll focus on three interesting examples in the
standard library:
However where we must be careful with destructor leaks are *proxy* types. These
are types which manage access to a distinct object, but don't actually own it.
Proxy objects are quite rare. Proxy objects you'll need to care about are even
rarer. However we'll focus on three interesting examples in the standard
library:
* `vec::Drain`
* `Rc`
@ -58,7 +58,8 @@ after claiming ownership over all of its contents. It produces an iterator
Now, consider Drain in the middle of iteration: some values have been moved out,
and others haven't. This means that part of the Vec is now full of logically
uninitialized data! We could backshift all the elements in the Vec every time we
remove a value, but this would have pretty catastrophic performance consequences.
remove a value, but this would have pretty catastrophic performance
consequences.
Instead, we would like Drain to *fix* the Vec's backing storage when it is
dropped. It should run itself to completion, backshift any elements that weren't
@ -86,20 +87,20 @@ let mut vec = vec![Box::new(0); 4];
println!("{}", vec[0]);
```
This is pretty clearly Not Good. Unfortunately, we're kind've stuck between
a rock and a hard place: maintaining consistent state at every step has
an enormous cost (and would negate any benefits of the API). Failing to maintain
This is pretty clearly Not Good. Unfortunately, we're kind've stuck between a
rock and a hard place: maintaining consistent state at every step has an
enormous cost (and would negate any benefits of the API). Failing to maintain
consistent state gives us Undefined Behaviour in safe code (making the API
unsound).
So what can we do? Well, we can pick a trivially consistent state: set the Vec's
len to be 0 when we *start* the iteration, and fix it up if necessary in the
destructor. That way, if everything executes like normal we get the desired
behaviour with minimal overhead. But if someone has the *audacity* to mem::forget
us in the middle of the iteration, all that does is *leak even more* (and possibly
leave the Vec in an *unexpected* but consistent state). Since we've
accepted that mem::forget is safe, this is definitely safe. We call leaks causing
more leaks a *leak amplification*.
behaviour with minimal overhead. But if someone has the *audacity* to
mem::forget us in the middle of the iteration, all that does is *leak even more*
(and possibly leave the Vec in an *unexpected* but consistent state). Since
we've accepted that mem::forget is safe, this is definitely safe. We call leaks
causing more leaks a *leak amplification*.
@ -108,8 +109,8 @@ more leaks a *leak amplification*.
Rc is an interesting case because at first glance it doesn't appear to be a
proxy value at all. After all, it manages the data it points to, and dropping
all the Rcs for a value will drop that value. leaking an Rc doesn't seem like
it would be particularly dangerous. It will leave the refcount permanently
all the Rcs for a value will drop that value. Leaking an Rc doesn't seem like it
would be particularly dangerous. It will leave the refcount permanently
incremented and prevent the data from being freed or dropped, but that seems
just like Box, right?

52
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@ -8,19 +8,19 @@ Rust allows you to specify alternative data layout strategies from the default.
# repr(C)
This is the most important `repr`. It has fairly simple intent: do what C does.
The order, size, and alignment of fields is exactly what you would expect from
C or C++. Any type you expect to pass through an FFI boundary should have `repr(C)`,
as C is the lingua-franca of the programming world. This is also necessary
to soundly do more elaborate tricks with data layout such as reintepretting values
as a different type.
However, the interaction with Rust's more exotic data layout features must be kept
in mind. Due to its dual purpose as "for FFI" and "for layout control", `repr(C)`
can be applied to types that will be nonsensical or problematic if passed through
the FFI boundary.
* ZSTs are still zero-sized, even though this is not a standard behaviour
in C, and is explicitly contrary to the behaviour of an empty type in C++, which
The order, size, and alignment of fields is exactly what you would expect from C
or C++. Any type you expect to pass through an FFI boundary should have
`repr(C)`, as C is the lingua-franca of the programming world. This is also
necessary to soundly do more elaborate tricks with data layout such as
reintepretting values as a different type.
However, the interaction with Rust's more exotic data layout features must be
kept in mind. Due to its dual purpose as "for FFI" and "for layout control",
`repr(C)` can be applied to types that will be nonsensical or problematic if
passed through the FFI boundary.
* ZSTs are still zero-sized, even though this is not a standard behaviour in
C, and is explicitly contrary to the behaviour of an empty type in C++, which
still consumes a byte of space.
* DSTs, tuples, and tagged unions are not a concept in C and as such are never
@ -30,8 +30,9 @@ the FFI boundary.
* This is equivalent to one of `repr(u*)` (see the next section) for enums. The
chosen size is the default enum size for the target platform's C ABI. Note that
enum representation in C is undefined, and this may be incorrect when the C
code is compiled with certain flags.
enum representation in C is implementation defined, so this is really a "best
guess". In particular, this may be incorrect when the C code of interest is
compiled with certain flags.
@ -40,10 +41,11 @@ the FFI boundary.
These specify the size to make a C-like enum. If the discriminant overflows the
integer it has to fit in, it will be an error. You can manually ask Rust to
allow this by setting the overflowing element to explicitly be 0. However Rust
will not allow you to create an enum where two variants have the same discriminant.
will not allow you to create an enum where two variants have the same
discriminant.
On non-C-like enums, this will inhibit certain optimizations like the null-pointer
optimization.
On non-C-like enums, this will inhibit certain optimizations like the null-
pointer optimization.
These reprs have no affect on a struct.
@ -53,15 +55,15 @@ These reprs have no affect on a struct.
# repr(packed)
`repr(packed)` forces rust to strip any padding, and only align the type to a
byte. This may improve the memory footprint, but will likely have other
negative side-effects.
byte. This may improve the memory footprint, but will likely have other negative
side-effects.
In particular, most architectures *strongly* prefer values to be aligned. This
may mean the unaligned loads are penalized (x86), or even fault (some ARM chips).
For simple cases like directly loading or storing a packed field, the compiler
might be able to paper over alignment issues with shifts and masks. However if
you take a reference to a packed field, it's unlikely that the compiler will be
able to emit code to avoid an unaligned load.
may mean the unaligned loads are penalized (x86), or even fault (some ARM
chips). For simple cases like directly loading or storing a packed field, the
compiler might be able to paper over alignment issues with shifts and masks.
However if you take a reference to a packed field, it's unlikely that the
compiler will be able to emit code to avoid an unaligned load.
`repr(packed)` is not to be used lightly. Unless you have extreme requirements,
this should not be used.

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@ -2,13 +2,11 @@
There are two kinds of reference:
* Shared reference: `&`
* Mutable reference: `&mut`
* Shared reference: `&` Mutable reference: `&mut`
Which obey the following rules:
* A reference cannot outlive its referent
* A mutable reference cannot be aliased
* A reference cannot outlive its referent A mutable reference cannot be aliased
To define aliasing, we must define the notion of *paths* and *liveness*.
@ -17,60 +15,66 @@ To define aliasing, we must define the notion of *paths* and *liveness*.
# Paths
If all Rust had were values, then every value would be uniquely owned
by a variable or composite structure. From this we naturally derive a *tree*
of ownership. The stack itself is the root of the tree, with every variable
as its direct children. Each variable's direct children would be their fields
(if any), and so on.
If all Rust had were values, then every value would be uniquely owned by a
variable or composite structure. From this we naturally derive a *tree* of
ownership. The stack itself is the root of the tree, with every variable as its
direct children. Each variable's direct children would be their fields (if any),
and so on.
From this view, every value in Rust has a unique *path* in the tree of ownership.
References to a value can subsequently be interpreted as a path in this tree.
Of particular interest are *ancestors* and *descendants*: if `x` owns `y`, then
`x` is an *ancestor* of `y`, and `y` is a *descendant* of `x`. Note that this is
an inclusive relationship: `x` is a descendant and ancestor of itself.
From this view, every value in Rust has a unique *path* in the tree of
ownership. References to a value can subsequently be interpreted as a path in
this tree. Of particular interest are *ancestors* and *descendants*: if `x` owns
`y`, then `x` is an *ancestor* of `y`, and `y` is a *descendant* of `x`. Note
that this is an inclusive relationship: `x` is a descendant and ancestor of
itself.
Tragically, plenty of data doesn't reside on the stack, and we must also accommodate this.
Globals and thread-locals are simple enough to model as residing at the bottom
of the stack (though we must be careful with mutable globals). Data on
the heap poses a different problem.
Tragically, plenty of data doesn't reside on the stack, and we must also
accommodate this. Globals and thread-locals are simple enough to model as
residing at the bottom of the stack (though we must be careful with mutable
globals). Data on the heap poses a different problem.
If all Rust had on the heap was data uniquely owned by a pointer on the stack,
then we can just treat that pointer as a struct that owns the value on
the heap. Box, Vec, String, and HashMap, are examples of types which uniquely
own data on the heap.
then we can just treat that pointer as a struct that owns the value on the heap.
Box, Vec, String, and HashMap, are examples of types which uniquely own data on
the heap.
Unfortunately, data on the heap is not *always* uniquely owned. Rc for instance
introduces a notion of *shared* ownership. Shared ownership means there is no
unique path. A value with no unique path limits what we can do with it. In general, only
shared references can be created to these values. However mechanisms which ensure
mutual exclusion may establish One True Owner temporarily, establishing a unique path
to that value (and therefore all its children).
unique path. A value with no unique path limits what we can do with it. In
general, only shared references can be created to these values. However
mechanisms which ensure mutual exclusion may establish One True Owner
temporarily, establishing a unique path to that value (and therefore all its
children).
The most common way to establish such a path is through *interior mutability*,
in contrast to the *inherited mutability* that everything in Rust normally uses.
Cell, RefCell, Mutex, and RWLock are all examples of interior mutability types. These
types provide exclusive access through runtime restrictions. However it is also
possible to establish unique ownership without interior mutability. For instance,
if an Rc has refcount 1, then it is safe to mutate or move its internals.
Cell, RefCell, Mutex, and RWLock are all examples of interior mutability types.
These types provide exclusive access through runtime restrictions. However it is
also possible to establish unique ownership without interior mutability. For
instance, if an Rc has refcount 1, then it is safe to mutate or move its
internals.
In order to correctly communicate to the type system that a variable or field of
a struct can have interior mutability, it must be wrapped in an UnsafeCell. This
does not in itself make it safe to perform interior mutability operations on that
value. You still must yourself ensure that mutual exclusion is upheld.
does not in itself make it safe to perform interior mutability operations on
that value. You still must yourself ensure that mutual exclusion is upheld.
# Liveness
Note: Liveness is not the same thing as a *lifetime*, which will be explained
in detail in the next section of this chapter.
Roughly, a reference is *live* at some point in a program if it can be
dereferenced. Shared references are always live unless they are literally unreachable
(for instance, they reside in freed or leaked memory). Mutable references can be
reachable but *not* live through the process of *reborrowing*.
dereferenced. Shared references are always live unless they are literally
unreachable (for instance, they reside in freed or leaked memory). Mutable
references can be reachable but *not* live through the process of *reborrowing*.
A mutable reference can be reborrowed to either a shared or mutable reference to
one of its descendants. A reborrowed reference will only be live again once all
reborrows derived from it expire. For instance, a mutable reference can be reborrowed
to point to a field of its referent:
reborrows derived from it expire. For instance, a mutable reference can be
reborrowed to point to a field of its referent:
```rust
let x = &mut (1, 2);
@ -110,18 +114,18 @@ to make such a borrow*, just that Rust isn't as smart as you want.
To simplify things, we can model variables as a fake type of reference: *owned*
references. Owned references have much the same semantics as mutable references:
they can be re-borrowed in a mutable or shared manner, which makes them no longer
live. Live owned references have the unique property that they can be moved
out of (though mutable references *can* be swapped out of). This power is
they can be re-borrowed in a mutable or shared manner, which makes them no
longer live. Live owned references have the unique property that they can be
moved out of (though mutable references *can* be swapped out of). This power is
only given to *live* owned references because moving its referent would of
course invalidate all outstanding references prematurely.
As a local lint against inappropriate mutation, only variables that are marked
as `mut` can be borrowed mutably.
It is interesting to note that Box behaves exactly like an owned
reference. It can be moved out of, and Rust understands it sufficiently to
reason about its paths like a normal variable.
It is interesting to note that Box behaves exactly like an owned reference. It
can be moved out of, and Rust understands it sufficiently to reason about its
paths like a normal variable.
@ -130,21 +134,21 @@ reason about its paths like a normal variable.
With liveness and paths defined, we can now properly define *aliasing*:
**A mutable reference is aliased if there exists another live reference to one of
its ancestors or descendants.**
**A mutable reference is aliased if there exists another live reference to one
of its ancestors or descendants.**
(If you prefer, you may also say the two live references alias *each other*.
This has no semantic consequences, but is probably a more useful notion when
verifying the soundness of a construct.)
That's it. Super simple right? Except for the fact that it took us two pages
to define all of the terms in that definition. You know: Super. Simple.
That's it. Super simple right? Except for the fact that it took us two pages to
define all of the terms in that definition. You know: Super. Simple.
Actually it's a bit more complicated than that. In addition to references,
Rust has *raw pointers*: `*const T` and `*mut T`. Raw pointers have no inherent
ownership or aliasing semantics. As a result, Rust makes absolutely no effort
to track that they are used correctly, and they are wildly unsafe.
Actually it's a bit more complicated than that. In addition to references, Rust
has *raw pointers*: `*const T` and `*mut T`. Raw pointers have no inherent
ownership or aliasing semantics. As a result, Rust makes absolutely no effort to
track that they are used correctly, and they are wildly unsafe.
**It is an open question to what degree raw pointers have alias semantics.
However it is important for these definitions to be sound that the existence
of a raw pointer does not imply some kind of live path.**
However it is important for these definitions to be sound that the existence of
a raw pointer does not imply some kind of live path.**

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@ -1,38 +1,40 @@
% Send and Sync
Not everything obeys inherited mutability, though. Some types allow you to multiply
alias a location in memory while mutating it. Unless these types use synchronization
to manage this access, they are absolutely not thread safe. Rust captures this with
through the `Send` and `Sync` traits.
Not everything obeys inherited mutability, though. Some types allow you to
multiply alias a location in memory while mutating it. Unless these types use
synchronization to manage this access, they are absolutely not thread safe. Rust
captures this with through the `Send` and `Sync` traits.
* A type is Send if it is safe to send it to another thread.
* A type is Sync if it is safe to share between threads (`&T` is Send).
* A type is Send if it is safe to send it to another thread. A type is Sync if
* it is safe to share between threads (`&T` is Send).
Send and Sync are *very* fundamental to Rust's concurrency story. As such, a
substantial amount of special tooling exists to make them work right. First and
foremost, they're *unsafe traits*. This means that they are unsafe *to implement*,
and other unsafe code can *trust* that they are correctly implemented. Since
they're *marker traits* (they have no associated items like methods), correctly
implemented simply means that they have the intrinsic properties an implementor
should have. Incorrectly implementing Send or Sync can cause Undefined Behaviour.
Send and Sync are also what Rust calls *opt-in builtin traits*.
This means that, unlike every other trait, they are *automatically* derived:
if a type is composed entirely of Send or Sync types, then it is Send or Sync.
Almost all primitives are Send and Sync, and as a consequence pretty much
all types you'll ever interact with are Send and Sync.
foremost, they're *unsafe traits*. This means that they are unsafe *to
implement*, and other unsafe code can *trust* that they are correctly
implemented. Since they're *marker traits* (they have no associated items like
methods), correctly implemented simply means that they have the intrinsic
properties an implementor should have. Incorrectly implementing Send or Sync can
cause Undefined Behaviour.
Send and Sync are also what Rust calls *opt-in builtin traits*. This means that,
unlike every other trait, they are *automatically* derived: if a type is
composed entirely of Send or Sync types, then it is Send or Sync. Almost all
primitives are Send and Sync, and as a consequence pretty much all types you'll
ever interact with are Send and Sync.
Major exceptions include:
* raw pointers are neither Send nor Sync (because they have no safety guards)
* `UnsafeCell` isn't Sync (and therefore `Cell` and `RefCell` aren't)
* `Rc` isn't Send or Sync (because the refcount is shared and unsynchronized)
* `UnsafeCell` isn't Sync (and therefore `Cell` and `RefCell` aren't) `Rc` isn't
* Send or Sync (because the refcount is shared and unsynchronized)
`Rc` and `UnsafeCell` are very fundamentally not thread-safe: they enable
unsynchronized shared mutable state. However raw pointers are, strictly speaking,
marked as thread-unsafe as more of a *lint*. Doing anything useful
unsynchronized shared mutable state. However raw pointers are, strictly
speaking, marked as thread-unsafe as more of a *lint*. Doing anything useful
with a raw pointer requires dereferencing it, which is already unsafe. In that
sense, one could argue that it would be "fine" for them to be marked as thread safe.
sense, one could argue that it would be "fine" for them to be marked as thread
safe.
However it's important that they aren't thread safe to prevent types that
*contain them* from being automatically marked as thread safe. These types have
@ -60,17 +62,16 @@ impl !Send for SpecialThreadToken {}
impl !Sync for SpecialThreadToken {}
```
Note that *in and of itself* it is impossible to incorrectly derive Send and Sync.
Only types that are ascribed special meaning by other unsafe code can possible cause
trouble by being incorrectly Send or Sync.
Note that *in and of itself* it is impossible to incorrectly derive Send and
Sync. Only types that are ascribed special meaning by other unsafe code can
possible cause trouble by being incorrectly Send or Sync.
Most uses of raw pointers should be encapsulated behind a sufficient abstraction
that Send and Sync can be derived. For instance all of Rust's standard
collections are Send and Sync (when they contain Send and Sync types)
in spite of their pervasive use raw pointers to
manage allocations and complex ownership. Similarly, most iterators into these
collections are Send and Sync because they largely behave like an `&` or `&mut`
into the collection.
collections are Send and Sync (when they contain Send and Sync types) in spite
of their pervasive use raw pointers to manage allocations and complex ownership.
Similarly, most iterators into these collections are Send and Sync because they
largely behave like an `&` or `&mut` into the collection.
TODO: better explain what can or can't be Send or Sync. Sufficient to appeal
only to data races?
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