Update: Clarified that C# and JavaScript can be understood as similar to a CPS transform, not that they are actually implemented as one.

As you likely know if you’re reading this post Rust has an upcoming async/await feature being tested in nightly. Because of Rust’s unique features and positioning fully understanding the implementation powering this syntax is very different to understanding other well-known implementations (C# and JavaScript’s being the ones I am familiar with). Instead of thinking of a CPS-like transform where an async function is split into a series of continuations that are chained together via a Future::then method, Rust instead uses a generator/coroutine transform to turn the function into a state machine (C# and probably most JavaScript implementations use a similar transform under the hood, but as far as I’m aware because of the garbage collector these are indistinguishable from the naive CPS transform they are normally described as). For more detail on why Rust is taking this approach you should read eRFC 2033: Experimental Coroutines, that lays out the why’s much better than I could here.

What I’m going to try and provide instead, is a look into how this actually works today. What steps the compiler takes to turn an async fn into a normal function returning a state machine that you could write if you wanted to (but you definitely don’t).

The Setup

First, to provide a nice self-contained example I need to get some setup out of the way. In normal async/await in Rust you will bring in the futures library to handle most of the pieces I’m about to mention; but to be able to provide links to running playgrounds of these examples (and so that you can see all the gristle in these here sausages) I’m going to avoid that.

Also, I’m going to assume complete knowledge of soon to be stable Rust 2018 and the basic futures API (including pinning). I will endeavour to explain all the other nightly features being used, but I have been living in nightly-only for far too long now and may forget some.

Nightly Features

There are only three nightly features needed for the basic setup here, these are what I’m assuming complete knowledge of:

async_await to allow using async functions and blocks

to allow using async functions and blocks futures_api to allow using the basic core::future and core::task APIs

to allow using the basic and APIs pin to allow using the basic core::pin APIs

The “Async IO”

To provide a slightly more realistic example I will use the following “trait” for reading in data:

pub struct AsyncRead ( Vec < u8 > ); impl AsyncRead { pub fn new ( data : Vec < u8 > ) -> AsyncRead { AsyncRead ( data ) } pub fn read_to_end ( & mut self ) -> impl Future < Output = Vec < u8 >> + '_ { async move { self . 0 .clone () } } }

This can’t be a real trait because it requires generic associated types to declare the return value of read_to_end , so wherever you see AsyncRead in the later function signatures just imagine impl AsyncRead .

You may also note a lack of errors. This is because adding in the additional error handling paths in the last step is a lot of work and I’m lazy. They don’t really show anything new either, just more matching and more states to implement to handle the different execution paths.

The “Executor”

To run a Future to completion you require an executor to run it on. First there is the most basic API for an executor that can run a single future to completion:

pub fn block_on < F : Future > ( mut future : F ) -> F :: Output ;

Then, this is the simplest implementation of that executor that simply spins polling the future until it completes (there is an equivalent executor that requires less lines of code, and no unsafety, but it does require use of std::sync::Arc and I’m attempting to use the least powerful implementation of everything here):

pub fn block_on < F : Future > ( mut future : F ) -> F :: Output { use core ::{ pin :: Pin , ptr :: NonNull , task ::{ LocalWaker , Poll , UnsafeWake , Waker }, }; struct NoWake ; impl NoWake { fn local_waker () -> LocalWaker { // Safety: all references to NoWake are never // dereferenced unsafe { LocalWaker :: new ( NonNull :: < NoWake > :: dangling ()) } } } unsafe impl UnsafeWake for NoWake { unsafe fn clone_raw ( & self ) -> Waker { NoWake :: local_waker () .into_waker () } unsafe fn drop_raw ( & self ) {} unsafe fn wake ( & self ) {} } let lw = NoWake :: local_waker (); loop { // Safety: `future` is a local variable which is // only ever used in this pinned reference match unsafe { Pin :: new_unchecked ( & mut future ) } .poll ( & lw ) { Poll :: Ready ( value ) => break value , Poll :: Pending => continue , } } }

The async / await! implementation

Now that we have the setup out of the way, here’s the super simple async fn we’re going to be expanding. This function takes in a reference to some async IO, constructs a handle to some “random” one-time-pad, waits for both to complete, then XORs the data and pad together to secure the data. It may seem simple here, but once we get to the final stage you’re going to be glad I chose something so simple.

pub async fn quote_encrypt_unquote ( data : & mut AsyncRead ) -> Vec < u8 > { // one-time-pad chosen by fair dice roll let mut pad = AsyncRead :: new ( vec! [ 4 ; 32 ]); await! ( data .read_to_end ()) .into_iter () .zip ( await! ( pad .read_to_end ())) .map (|( a , b )| a ^ b ) .collect () }

Expanding async / await!

The first step on our journey into madness is to simply expand the async fn into a normal function. This has three parts to it:

Expanding the await! macro Rewriting the function signature Expanding the body

But first

Before doing the actual async / await! expansion I want to slightly rewrite the function from before. This will be a functionally equivalent function, but by pulling out a few temporary variables the control flow between the different transforms will be easier to follow. Mostly, having await! inside other expressions will greatly complicate the upcoming generator transform.

pub async fn quote_encrypt_unquote ( data : & mut AsyncRead ) -> Vec < u8 > { // one-time-pad chosen by fair dice roll let mut pad = AsyncRead :: new ( vec! [ 4 ; 32 ]); let data = await! ( data .read_to_end ()); let pad = await! ( pad .read_to_end ()); data .into_iter () .zip ( pad ) .map (|( a , b )| a ^ b ) .collect () }

Expanding the await! macro

Currently await! is simply a normal macro defined in std . This is unlikely to last, there are some requirements on it that I believe will necessitate it moving into the compiler, but it makes things slightly simpler here for now. We can expand this macro while still leaving the rest of the async fn alone and still get something that compiles. Note the poll_with_tls_waker function introduced here, I’ll come back to it later.

pub async fn quote_encrypt_unquote ( data : & mut AsyncRead ) -> Vec < u8 > { use core ::{ pin :: Pin , task :: Poll }; use std :: future :: poll_with_tls_waker ; // one-time-pad chosen by fair dice roll let mut pad = AsyncRead :: new ( vec! [ 4 ; 32 ]); let data = { let mut pinned = data .read_to_end (); loop { if let Poll :: Ready ( x ) = poll_with_tls_waker ( unsafe { Pin :: new_unchecked ( & mut pinned ) }) { break x ; } yield } }; let pad = { let mut pinned = pad .read_to_end (); loop { if let Poll :: Ready ( x ) = poll_with_tls_waker ( unsafe { Pin :: new_unchecked ( & mut pinned ) }) { break x ; } yield } }; data .into_iter () .zip ( pad ) .map (|( a , b )| a ^ b ) .collect () }

Rewriting the function signature

async fn does some slightly funky things to the function signature. The main thing is just taking the return value ( R ) and wrapping it into impl Future<Output = R> , then the lifetime of the returned future is bound by the lifetimes of all arguments. Currently if you have a function taking multiple references you have to give it a single named lifetime for all those references to use, but I believe the intention is for this to automatically work in the future.

After re-writing the signature the body of the function can be wrapped in an async move { ... } block to keep everything compiling with the exact same semantics as before:

pub fn quote_encrypt_unquote ( data : & mut AsyncRead , ) -> impl Future < Output = Vec < u8 >> + '_ { use core ::{ pin :: Pin , task :: Poll }; use std :: future :: poll_with_tls_waker ; async move { // one-time-pad chosen by fair dice roll let mut pad = AsyncRead :: new ( vec! [ 4 ; 32 ]); let data = { let mut pinned = data .read_to_end (); loop { if let Poll :: Ready ( x ) = poll_with_tls_waker ( unsafe { Pin :: new_unchecked ( & mut pinned ) }) { break x ; } yield } }; let pad = { let mut pinned = pad .read_to_end (); loop { if let Poll :: Ready ( x ) = poll_with_tls_waker ( unsafe { Pin :: new_unchecked ( & mut pinned ) }) { break x ; } yield } }; data .into_iter () .zip ( pad ) .map (|( a , b )| a ^ b ) .collect () } }

Expanding the body

Finally to remove all async syntax from the function we can expand the async move { ... } block into a generator and package this into a wrapper Future with from_generator from std::future .

std::future::from_generator is a counterpart to the poll_with_tls_waker function mentioned earlier, these must be used together, the Future created by from_generator will place the &LocalWaker passed in to Future::poll into thread local storage, poll_with_tls_waker will then retrieve this to pass in to sub-futures that are being await! ed on.

pub fn quote_encrypt_unquote ( data : & mut AsyncRead , ) -> impl Future < Output = Vec < u8 >> + '_ { use core ::{ pin :: Pin , task :: Poll }; use std :: future ::{ from_generator , poll_with_tls_waker }; from_generator ( static move || { // one-time-pad chosen by fair dice roll let mut pad = AsyncRead :: new ( vec! [ 4 ; 32 ]); let data = { let mut pinned = data .read_to_end (); loop { if let Poll :: Ready ( x ) = poll_with_tls_waker ( unsafe { Pin :: new_unchecked ( & mut pinned ) }) { break x ; } yield } }; let pad = { let mut pinned = pad .read_to_end (); loop { if let Poll :: Ready ( x ) = poll_with_tls_waker ( unsafe { Pin :: new_unchecked ( & mut pinned ) }) { break x ; } yield } }; data .into_iter () .zip ( pad ) .map (|( a , b )| a ^ b ) .collect () }) }

Expanding the generator

Now that we have finished the async transform we have the much more complicated generator transform to apply. This transform is implemented in rustc ’s MIR layer, so has much less direct equivalence to anything we can implement manually in Rust’s surface language. I’m going to present a manual implementation of a Generator that does the same thing as the one from the previous snippet, using a sort of similar layout to what rustc would generate, but since this is all very much unstable internals there’s no guarantee that rustc will continue to generate something similar to this.

Also, rather than attempting to transform piece by piece like the last section I’m going to first present the entire transformed generator, then pull out pieces to explain from it. The following code block simply replaces everything inside the from_generator call in the previous block.

existential type ReadToEnd < 'a > : Future < Output = Vec < u8 >> + 'a ; fn read_to_end ( read : & mut AsyncRead ) -> ReadToEnd < '_ > { read .read_to_end () } struct ManualGenerator < 'a > { state : i32 , data_1 : MaybeUninit <& 'a mut AsyncRead > , pad_1 : MaybeUninit < AsyncRead > , pinned_1 : MaybeUninit < ReadToEnd < 'a >> , data_2 : MaybeUninit < Vec < u8 >> , pinned_2 : MaybeUninit < ReadToEnd < 'a >> , } impl < 'a > Generator for ManualGenerator < 'a > { type Yield = (); type Return = Vec < u8 > ; unsafe fn resume ( & mut self ) -> GeneratorState < Self :: Yield , Self :: Return > { match self .state { 0 => { // one-time-pad chosen by fair dice roll self .pad_1 .set ( AsyncRead :: new ( vec! [ 4 ; 32 ])); self .pinned_1 .set ( read_to_end ( & mut * self .data_1 .as_mut_ptr ())); self .state = 1 ; self .resume () } 1 => { self .data_2 .set ( loop { if let Poll :: Ready ( x ) = poll_with_tls_waker ( Pin :: new_unchecked ( self .pinned_1 .get_mut ()), ) { break x ; } return GeneratorState :: Yielded (()); }); self .pinned_2 .set ( read_to_end ( & mut * self .pad_1 .as_mut_ptr ())); ptr :: drop_in_place ( self .pinned_1 .as_mut_ptr ()); self .state = 2 ; self .resume () } 2 => { let pad_2 = loop { if let Poll :: Ready ( x ) = poll_with_tls_waker ( Pin :: new_unchecked ( self .pinned_2 .get_mut ()), ) { break x ; } return GeneratorState :: Yielded (()); }; let result = ptr :: read ( self .data_2 .as_mut_ptr ()) .into_iter () .zip ( pad_2 ) .map (|( a , b )| a ^ b ) .collect (); ptr :: drop_in_place ( self .pinned_2 .as_mut_ptr ()); ptr :: drop_in_place ( self .pad_1 .as_mut_ptr ()); ptr :: drop_in_place ( self .data_1 .as_mut_ptr ()); self .state = - 1 ; GeneratorState :: Complete ( result ) } - 1 => panic! ( "ManualGenerator polled after completion" ), - 2 => panic! ( "ManualGenerator polled after dropped" ), _ => panic! ( "ManualGenerator polled with invalid state" ), } } } impl < 'a > Drop for ManualGenerator < 'a > { fn drop ( & mut self ) { match self .state { 0 => unsafe { ptr :: drop_in_place ( self .data_1 .as_mut_ptr ()); }, 1 => unsafe { ptr :: drop_in_place ( self .pinned_1 .as_mut_ptr ()); ptr :: drop_in_place ( self .pad_1 .as_mut_ptr ()); ptr :: drop_in_place ( self .data_1 .as_mut_ptr ()); }, 2 => unsafe { ptr :: drop_in_place ( self .pinned_2 .as_mut_ptr ()); ptr :: drop_in_place ( self .data_2 .as_mut_ptr ()); ptr :: drop_in_place ( self .pad_1 .as_mut_ptr ()); ptr :: drop_in_place ( self .data_1 .as_mut_ptr ()); }, - 1 => { /* Everything already dropped in resume */ } - 2 => panic! ( "ManualGenerator dropped twice" ), _ => panic! ( "ManualGenerator dropped with invalid state" ), } self .state = - 2 ; } } impl < 'a > ManualGenerator < 'a > { fn new ( data : & 'a mut AsyncRead ) -> Self { ManualGenerator { state : 0 , data_1 : MaybeUninit :: new ( data ), pad_1 : MaybeUninit :: uninitialized (), pinned_1 : MaybeUninit :: uninitialized (), data_2 : MaybeUninit :: uninitialized (), pinned_2 : MaybeUninit :: uninitialized (), } } }

(Unfortunately this playground is too large to be opened via URI, you can find the fully running example for this on GitHub.)

ReadToEnd

existential type ReadToEnd < 'a > : Future < Output = Vec < u8 >> + 'a ; fn read_to_end ( read : & mut AsyncRead ) -> ReadToEnd < '_ > { read .read_to_end () }

The very first part of this code block is a little hack to give a name to an unnameable type. If you look back at AsyncRead::read_to_end you’ll see that the return type was declared as impl Future<Output = Vec<u8>> + '_ . Unfortunately we cannot easily store this type into the fields of a struct, so we have this little hack using the existential_type feature to give ourselves a name for the type.

struct ManualGenerator

struct ManualGenerator < 'a > { state : i32 , data_1 : MaybeUninit <& 'a mut AsyncRead > , pad_1 : MaybeUninit < AsyncRead > , pinned_1 : MaybeUninit < ReadToEnd < 'a >> , data_2 : MaybeUninit < Vec < u8 >> , pinned_2 : MaybeUninit < ReadToEnd < 'a >> , }

Next we have the environment definition for the generator. This includes a state variable to keep track of which yield point we are currently at, along with fields for the upvars that were moved into the environment ( data_1 ) and the variables that live across yield points ( pad_1 , pinned_1 , data_2 and pinned_2 ). Note that any variables that are only alive between yield points are not stored in the environment, these will be normal variables on the stack of the resume function (the final pad result variable, and the temporaries used during polling and the final line).

Each of the fields are stored as MaybeUninit as some of them start uninitialized and others will be dropped before the generator finishes. You might already be able to notice one potential optimization here, pinned_1 and pinned_2 contain the same type but have non-overlapping lifetimes. To keep closer to the real transform I have kept these as separate fields, see rust-lang/rust#52924 for more details.

State 0

0 => { // one-time-pad chosen by fair dice roll self .pad_1 .set ( AsyncRead :: new ( vec! [ 4 ; 32 ])); self .pinned_1 .set ( read_to_end ( & mut * self .data_1 .as_mut_ptr ())); self .state = 1 ; self .resume () }

Now we get into the meat of the generator. First we have the code running from the start of the closure until the beginning of the first loop . We don’t go all the way to the yield point because it is a yield point at the end of a loop, we need a state number representing the start of the loop so we can resume there after yielding. In the real transform this would end with a goto to the start of the loop, but it’s easier in the Rust surface syntax to just recurse into resume again (we can’t stack overflow as we guarantee moving to a new state before recursing and have a limited number of states).

This is a pretty straight-forward transform from the original code, just note that we use MaybeUninit::set to store the variables into the environment instead of = and call our hacky AsyncRead::read_to_end wrapper so that we know the type of the result.

State 1

1 => { self .data_2 .set ( loop { if let Poll :: Ready ( x ) = poll_with_tls_waker ( Pin :: new_unchecked ( self .pinned_1 .get_mut ()), ) { break x ; } return GeneratorState :: Yielded (()); }); self .pinned_2 .set ( read_to_end ( & mut * self .pad_1 .as_mut_ptr ())); ptr :: drop_in_place ( self .pinned_1 .as_mut_ptr ()); self .state = 2 ; self .resume () }

State 1 covers the body of the first loop and the small bit of code between it and the start of the second loop. Here we can see the same sort of straightforward transform as the initial state, with two new things to highlight:

The yield; statement is simply replaced with return GeneratorState::Yielded(()); . Because the yield is at the end of the loop the next statement to execute after resuming is the start of the loop again so this is relatively simple. In a more complicated generator where there is a yield in the middle of a loop you would need to split the body across multiple states and bounce between them until complete. The pinned variable is dropped at the end of its containing block. We have to use drop_in_place here to avoid moving it before dropping, it doesn’t matter for this variable but if we were awaiting some !Unpin futures here then we would cause unsoundness if they were moved before Drop::drop was called.

State 2

2 => { let pad_2 = loop { if let Poll :: Ready ( x ) = poll_with_tls_waker ( Pin :: new_unchecked ( self .pinned_2 .get_mut ()), ) { break x ; } return GeneratorState :: Yielded (()); }; let result = ptr :: read ( self .data_2 .as_mut_ptr ()) .into_iter () .zip ( pad_2 ) .map (|( a , b )| a ^ b ) .collect (); ptr :: drop_in_place ( self .pinned_2 .as_mut_ptr ()); ptr :: drop_in_place ( self .pad_1 .as_mut_ptr ()); ptr :: drop_in_place ( self .data_1 .as_mut_ptr ()); self .state = - 1 ; GeneratorState :: Complete ( result ) }

Another straightforward transform for the body of the second loop, final statement, and implicit drops of all the closure variables. This is the first time we use a variable by value, we have to use ptr::read to move the final data variable out to call into_iter on it, since we have passed ownership of it off to Vec::into_iter we don’t drop this variable, but do drop everything else remaining alive before returning the final result.

Error states

- 1 => panic! ( "ManualGenerator polled after completion" ), - 2 => panic! ( "ManualGenerator polled after dropped" ), _ => panic! ( "ManualGenerator polled with invalid state" ),

We reserve two states for all generators: the generator has completed successfully and the generator has been dropped. A generator should never be polled after either of these states is reached, and should never be able to get into any state we aren’t otherwise handling, so we panic if these are reached.

Drop

impl < 'a > Drop for ManualGenerator < 'a > { fn drop ( & mut self ) { match self .state { 0 => unsafe { ptr :: drop_in_place ( self .data_1 .as_mut_ptr ()); }, 1 => unsafe { ptr :: drop_in_place ( self .pinned_1 .as_mut_ptr ()); ptr :: drop_in_place ( self .pad_1 .as_mut_ptr ()); ptr :: drop_in_place ( self .data_1 .as_mut_ptr ()); }, 2 => unsafe { ptr :: drop_in_place ( self .pinned_2 .as_mut_ptr ()); ptr :: drop_in_place ( self .data_2 .as_mut_ptr ()); ptr :: drop_in_place ( self .pad_1 .as_mut_ptr ()); ptr :: drop_in_place ( self .data_1 .as_mut_ptr ()); }, - 1 => { /* Everything already dropped in resume */ } - 2 => panic! ( "ManualGenerator dropped twice" ), _ => panic! ( "ManualGenerator dropped with invalid state" ), } self .state = - 2 ; } }

Since we are using MaybeUninit for the fields we also need to implement Drop for our generator manually, the compiler generated drop glue will not do anything. We need to then check what state we are in and drop all the variables that are still alive, before finally marking ourselves as dropped.

ManualGenerator::new

impl < 'a > ManualGenerator < 'a > { fn new ( data : & 'a mut AsyncRead ) -> Self { ManualGenerator { state : 0 , data_1 : MaybeUninit :: new ( data ), pad_1 : MaybeUninit :: uninitialized (), pinned_1 : MaybeUninit :: uninitialized (), data_2 : MaybeUninit :: uninitialized (), pinned_2 : MaybeUninit :: uninitialized (), } } }

Finally we need a way to construct the generator from the upvars, this doesn’t really get pulled out to a function in the real transform, but it’s easier to see what’s happening if it is here.