0-sugary-await.rs

use async::fs::File;
use async::io::Async;

fn copy(src: &Path, dst: &Path) -> impl Async<()> {
    let src = try!(File::open(src));
    let dst = try!(File::create(dst));
    
    // Copy a kiB at a time.
    let mut buffer = Vec::with_capacity(1024);
    loop {
        buffer = await!(src.read(buffer));
        if buffer.is_empty() { break; }
        buffer = await!(dst.write(buffer));
        buffer.clear();
        
        let s_meta = await!(src.metadata());
        let d_meta = await!(dst.metadata());
        println!("{}B copied out of {}B", d_meta.size, s_meta.size);
    }
    Ok(())
}

1-expand-await.rs

fn copy(src: &Path, dst: &Path) -> impl Async<()> {
    let src = try!(File::open(src));
    let dst = try!(File::create(dst));
    
    // Copy a kiB at a time.
    let mut buffer = Vec::with_capacity(1024);
    loop {
        buffer = try!(yield+ src.read(buffer));
        if buffer.is_empty() { break; }
        buffer = try!(yield+ dst.write(buffer));
        buffer.clear();
        
        let s_meta = try!(yield+ src.metadata());
        let d_meta = try!(yield+ dst.metadata());
        println!("{}B copied out of {}B", d_meta.size, s_meta.size);
    }
    Ok(())
}

2-expand-multi-yield.rs

fn copy(src: &Path, dst: &Path) -> impl Async<()> {
    let src = try!(File::open(src));
    let dst = try!(File::create(dst));
    
    // Copy a kiB at a time.
    let mut buffer = Vec::with_capacity(1024);
    loop {
        buffer = try!(match src.read(buffer).generate() {
            Yield::Value((mut action, mut gen)) => {
                loop {
                    // NOTE not the case here, but this yield could need
                    // to do polymorphic pass-through to `gen`.
                    match gen.next(yield action) {
                        Yield::Value(next) => action = next,
                        // NOTE cheating via break with value.
                        Yield::Return(res) => break res
                    }
                }
            }
            Yield::Return(res) => res
        });
        if buffer.is_empty() { break; }
        buffer = try!(match dst.write(buffer).generate() {
            Yield::Value((mut action, mut gen)) => {
                loop {
                    match gen.next(yield action) {
                        Yield::Value(next) => action = next,
                        Yield::Return(res) => break res
                    }
                }
            }
            Yield::Return(res) => res
        });
        buffer.clear();
        
        let s_meta = try!(match src.metadata().generate() {
            Yield::Value((mut action, mut gen)) => {
                loop {
                    match gen.next(yield action) {
                        Yield::Value(next) => action = next,
                        Yield::Return(res) => break res
                    }
                }
            }
            Yield::Return(res) => res
        });
        let d_meta = try!(match dst.metadata().generate() {
            Yield::Value((mut action, mut gen)) => {
                loop {
                    match gen.next(yield action) {
                        Yield::Value(next) => action = next,
                        Yield::Return(res) => break res
                    }
                }
            }
            Yield::Return(res) => res
        });
        println!("{}B copied out of {}B", d_meta.size, s_meta.size);
    }
    Ok(())
}

3-expand-try.rs

fn copy(src: &Path, dst: &Path) -> impl Async<()> {
    let src = match File::open(src) {
        Ok(file) => file,
        Err(e) => return Err(e)
    };
    let dst = match File::create(dst) {
        Ok(file) => file,
        Err(e) => return Err(e)
    };
    
    // Copy a kiB at a time.
    let mut buffer = Vec::with_capacity(1024);
    loop {
        let res = match src.read(buffer).generate() {
            Yield::Value((mut action, mut gen)) => {
                loop {
                    // NOTE not the case here, but this yield could need
                    // to do polymorphic pass-through to `gen`.
                    match gen.next(yield action) {
                        Yield::Value(next) => action = next,
                        // NOTE cheating via break with value.
                        Yield::Return(res) => break res
                    }
                }
            }
            Yield::Return(res) => res
        };
        match res {
            Ok(b) => buffer = b,
            Err(e) => return Err(e)
        }
        if buffer.is_empty() { break; }
        let res = match dst.write(buffer).generate() {
            Yield::Value((mut action, mut gen)) => {
                loop {
                    match gen.next(yield action) {
                        Yield::Value(next) => action = next,
                        Yield::Return(res) => break res
                    }
                }
            }
            Yield::Return(res) => res
        };
        match res {
            Ok(b) => buffer = b,
            Err(e) => return Err(e)
        }
        buffer.clear();
        
        let res = match src.metadata().generate() {
            Yield::Value((mut action, mut gen)) => {
                loop {
                    match gen.next(yield action) {
                        Yield::Value(next) => action = next,
                        Yield::Return(res) => break res
                    }
                }
            }
            Yield::Return(res) => res
        };
        let s_meta = match res {
            Ok(x) => x,
            Err(e) => return Err(e)
        };
        let res = match dst.metadata().generate() {
            Yield::Value((mut action, mut gen)) => {
                loop {
                    match gen.next(yield action) {
                        Yield::Value(next) => action = next,
                        Yield::Return(res) => break res
                    }
                }
            }
            Yield::Return(res) => res
        };
        let d_meta = match res {
            Ok(x) => x,
            Err(e) => return Err(e)
        };
        println!("{}B copied out of {}B", d_meta.size, s_meta.size);
    }
    Ok(())
}

4-expand-return-and-yield.rs

fn copy(src: &Path, dst: &Path) -> impl Async<()> {
    let src = match File::open(src) {
        Ok(file) => file,
        Err(e) => return Yield::Return(Err(e))
    };
    let dst = match File::create(dst) {
        Ok(file) => file,
        Err(e) => return Yield::Return(Err(e))
    };
    
    // Copy a kiB at a time.
    let mut buffer = Vec::with_capacity(1024);
    loop {
        let res = match src.read(buffer).generate() {
            Yield::Value((mut action, mut gen)) => {
                loop {
                    return Yield::Value(action);
                    // NOTE input into the generator is denoted with in(T).
                    match gen.next(in(io::Result<Vec<u8>>)) {
                        Yield::Value(next) => action = next,
                        // NOTE cheating via break with value.
                        Yield::Return(res) => break res
                    }
                }
            }
            Yield::Return(res) => res
        };
        match res {
            Ok(b) => buffer = b,
            Err(e) => return Yield::Return(Err(e))
        }
        if buffer.is_empty() { return Yield::Return(Ok(())); }
        let res = match dst.write(buffer).generate() {
            Yield::Value((mut action, mut gen)) => {
                loop {
                    return Yield::Value(action);
                    match gen.next(in(io::Result<Vec<u8>>)) {
                        Yield::Value(next) => action = next,
                        Yield::Return(res) => break res
                    }
                }
            }
            Yield::Return(res) => res
        };
        match res {
            Ok(b) => buffer = b,
            Err(e) => return Yield::Return(Err(e))
        }
        buffer.clear();
        
        let res = match src.metadata().generate() {
            Yield::Value((mut action, mut gen)) => {
                loop {
                    return Yield::Value(action);
                    match gen.next(in(io::Result<fs::Metadata>)) {
                        Yield::Value(next) => action = next,
                        Yield::Return(res) => break res
                    }
                }
            }
            Yield::Return(res) => res
        };
        let s_meta = match res {
            Ok(x) => x,
            Err(e) => return Yield::Return(Err(e))
        };
        let res = match dst.metadata().generate() {
            Yield::Value((mut action, mut gen)) => {
                loop {
                    return Yield::Value(action);
                    match gen.next(in(io::Result<fs::Metadata>)) {
                        Yield::Value(next) => action = next,
                        Yield::Return(res) => break res
                    }
                }
            }
            Yield::Return(res) => res
        };
        let d_meta = match res {
            Ok(x) => x,
            Err(e) => return Yield::Return(Err(e))
        };
        println!("{}B copied out of {}B", d_meta.size, s_meta.size);
    }
}

5-final.rs

use gen::{Generate, Generator, GeneratorBase, Yield, Mirror};
use fs::File;
use io::Async;

use std::mem;
use std::path::Path;

struct Copy<'a, 'b> { src: &'a Path, dst: &'b Path }
fn copy<'a, 'b>(src: &'a Path, dst: &'b Path) -> Copy<'a, 'b> {
    Copy { src: src, dst: dst }
}
impl<'a, 'b> Generate for Copy<'a, 'b> {
    type G = S;
    
    // NOTE due to how generators can return immediatelly,
    // it's theoretically possible to have `copy` finish
    // without yielding any io::Action (e.g. a sync mock-up).
    // However, inlining the src.read(buffer).generate() call
    // from S::advance should kill the rest of that loop here.
    fn generate(self) -> Yield<(io::Action, S), io::Result<()>> {
        let Copy { src, dst } = self;
        let src = match File::open(src) {
            Ok(file) => file,
            Err(e) => return Yield::Return(Err(e))
        };
        let dst = match File::create(dst) {
            Ok(file) => file,
            Err(e) => return Yield::Return(Err(e))
        };
        
        // Copy a kiB at a time.
        let buffer = Vec::with_capacity(1024);
        let mut gen = S::Read { src: src, dst: dst, buffer: buffer };
        match gen.advance() {
            Yield::Value(x) => Yield::Value((x, gen)),
            Yield::Return(r) => Yield::Return(r)
        }
    }
}

enum S {
    Read  { src: File, dst: File, buffer: Vec<u8> },
    ReadY { src: File, dst: File, gen: Mirror<io::Action, io::Result<Vec<u8>>> },
    Write { src: File, dst: File, buffer: Vec<u8> },
    WriteY{ src: File, dst: File, gen: Mirror<io::Action, io::Result<Vec<u8>>> },
    SMeta { src: File, dst: File, buffer: Vec<u8> },
    SMetaY{ src: File, dst: File, buffer: Vec<u8>, gen: Mirror<io::Action, io::Result<fs::Metadata>> },
    DMeta { src: File, dst: File, buffer: Vec<u8>, s_meta: fs::Metadata },
    DMetaY{ src: File, dst: File, buffer: Vec<u8>, s_meta: fs::Metadata, gen: Mirror<io::Action, io::Result<fs::Metadata>> },
    Print { src: File, dst: File, buffer: Vec<u8>, s_meta: fs::Metadata, d_meta: fs::Metadata },
    Completed
}

impl GeneratorBase for S {
    type Value = io::Action;
    type Return = io::Result<()>;
}

impl S {
    // Continue execution until the next yield or return.
    fn advance(&mut self) -> Yield<io::Action, io::Result<()>> {
        loop {
            match mem::replace(self, S::Completed) {
                S::Read { src, dst, buffer } => {
                    match src.read(buffer).generate() {
                        Yield::Value((action, gen)) => {
                            *self = S::ReadY { src: src, dst: dst, gen: gen };
                            return Yield::Value(action);
                        }
                        Yield::Return(Err(e)) => return Yield::Return(Err(e)),
                        Yield::Return(Ok(res)) => {
                            *self = S::Write { src: src, dst: dst, buffer: res };
                        }
                    }
                }
                S::Write { src, dst, buffer } => {
                    if buffer.is_empty() { return Yield::Return(Ok(())); }
                    match dst.write(buffer).generate() {
                        Yield::Value((action, gen)) => {
                            *self = S::WriteY { src: src, dst: dst, gen: gen };
                            return Yield::Value(action);
                        }
                        Yield::Return(Err(e)) => return Yield::Return(Err(e)),
                        Yield::Return(Ok(res)) => {
                            *self = S::SMeta { src: src, dst: dst, buffer: res };
                        }
                    }
                }
                S::SMeta { src, dst, mut buffer } => {
                    buffer.clear();
                    match src.metadata().generate() {
                        Yield::Value((action, gen)) => {
                            *self = S::SMetaY { src: src, dst: dst, buffer: buffer, gen: gen };
                            return Yield::Value(action);
                        }
                        Yield::Return(Err(e)) => return Yield::Return(Err(e)),
                        Yield::Return(Ok(res)) => {
                            *self = S::DMeta { src: src, dst: dst, buffer: buffer, s_meta: res };
                        }
                    }
                }
                S::DMeta { src, dst, buffer, s_meta } => {
                    match dst.metadata().generate() {
                        Yield::Value((action, gen)) => {
                            *self = S::DMetaY { src: src, dst: dst, buffer: buffer, s_meta: s_meta, gen: gen };
                            return Yield::Value(action);
                        }
                        Yield::Return(Err(e)) => return Yield::Return(Err(e)),
                        Yield::Return(Ok(res)) => {
                            *self = S::Print { src: src, dst: dst, buffer: buffer, s_meta: s_meta, d_meta: res };
                        }
                    }
                }
                S::Print { src, dst, buffer, s_meta, d_meta } => {
                    println!("{}B copied out of {}B", d_meta.len(), s_meta.len());
                    *self = S::Read { src: src, dst: dst, buffer: buffer };
                }
                S::ReadY {..} | S::WriteY {..} | S::SMetaY {..} | S::DMetaY {..} => {
                    unreachable!("generator needs input at yield point")
                }
                S::Completed => unreachable!("generator has already completed")
            }
        }
    }
}

impl Generator<io::Result<Vec<u8>>> for S {
    fn next(&mut self, input: io::Result<Vec<u8>>) -> Yield<io::Action, io::Result<()>> {
        match mem::replace(self, S::Completed) {
            S::ReadY { src, dst, mut gen } => {
                match gen.next(input) {
                    Yield::Value(next) => {
                        *self = S::ReadY { src: src, dst: dst, gen: gen };
                        return Yield::Value(next);
                    }
                    Yield::Return(Err(e)) => return Yield::Return(Err(e)),
                    Yield::Return(Ok(res)) => {
                        *self = S::Write { src: src, dst: dst, buffer: res };
                    }
                }
            }
            S::WriteY { src, dst, mut gen } => {
                match gen.next(input) {
                    Yield::Value(next) => {
                        *self = S::WriteY { src: src, dst: dst, gen: gen };
                        return Yield::Value(next);
                    }
                    Yield::Return(Err(e)) => return Yield::Return(Err(e)),
                    Yield::Return(Ok(res)) => {
                        *self = S::SMeta { src: src, dst: dst, buffer: res };
                    }
                }
            }
            _ => unreachable!("generator state does not accept Vec<u8> as input")
        }
        self.advance()
    }
}

impl Generator<io::Result<fs::Metadata>> for S {
    fn next(&mut self, input: io::Result<fs::Metadata>) -> Yield<io::Action, io::Result<()>> {
        match mem::replace(self, S::Completed) {
            S::SMetaY { src, dst, buffer, mut gen } => {
                match gen.next(input) {
                    Yield::Value(next) => {
                        *self = S::SMetaY { src: src, dst: dst, buffer: buffer, gen: gen };
                        return Yield::Value(next);
                    }
                    Yield::Return(Err(e)) => return Yield::Return(Err(e)),
                    Yield::Return(Ok(res)) => {
                        *self = S::DMeta { src: src, dst: dst, buffer: buffer, s_meta: res };
                    }
                }
            }
            S::DMetaY { src, dst, buffer, s_meta, mut gen } => {
                match gen.next(input) {
                    Yield::Value(next) => {
                        *self = S::DMetaY { src: src, dst: dst, buffer: buffer, s_meta: s_meta, gen: gen };
                        return Yield::Value(next);
                    }
                    Yield::Return(Err(e)) => return Yield::Return(Err(e)),
                    Yield::Return(Ok(res)) => {
                        *self = S::Print { src: src, dst: dst, buffer: buffer, s_meta: s_meta, d_meta: res };
                    }
                }
            }
            _ => unreachable!("generator state does not accept fs::Metadata as input")
        }
        self.advance()
    }
}

9-infrastructure.rs

pub mod gen {
    use std::marker::PhantomData;
    
    #[cfg_attr(std, lang = "yield")]
    pub enum Yield<T, R> {
        Value(T),
        Return(R)
    }
    
    #[cfg_attr(std, lang = "generator_base")]
    pub trait GeneratorBase {
        type Value;
        type Return;
    }
    
    fn type_name<T: ?Sized>() -> &'static str {
        "<unknown>"
        /*unsafe {
            ::std::intrinsics::type_name::<T>()
        }*/
    }
    
    #[cfg_attr(std, lang = "generator")]
    pub trait Generator<Input>: GeneratorBase {
        fn next(&mut self, _: Input) -> Yield<Self::Value, Self::Return> {
            panic!("generator `{}` does not take `{}` as input",
                   type_name::<Self>(), type_name::<Input>())
        }
    }
    
    #[cfg(std)]
    impl<G: Generator<(), Return=()>> Iterator for G {
        type Item = G::Value;
        fn next(&mut self) -> Option<G::Value> {
            match self.next(()) {
                Yield::Value(x) => Some(x),
                Yield::Return(_) => None
            }
        }
    }
    
    pub trait Generate {
        type G: GeneratorBase;
        
        /// Run up to the first yield, return that and the generator.
        fn generate(self) -> Yield<(<Self::G as GeneratorBase>::Value, Self::G),
                                    <Self::G as GeneratorBase>::Return>;
    }
    
    pub struct YieldAnd<V, G>(pub V, pub G);
    
    impl<V, G: GeneratorBase<Value = V>> Generate for YieldAnd<V, G> {
        type G = G;
        fn generate(self) -> Yield<(V, G), G::Return> {
            Yield::Value((self.0, self.1))
        }
    }
    
    pub struct Mirror<V, R> {
        _data: PhantomData<(V, R)>
    }
    
    impl<V, R> Mirror<V, R> {
        pub fn new() -> Mirror<V, R> {
            Mirror {
                _data: PhantomData
            }
        }
    }
    
    impl<V, R> GeneratorBase for Mirror<V, R> {
        type Value = V;
        type Return = R;
    }
    
    impl<V, R> Generator<R> for Mirror<V, R> {
        fn next(&mut self, value: R) -> Yield<V, R> {
            Yield::Return(value)
        }
    }
    
    pub type YieldAndMirror<V, R> = YieldAnd<V, Mirror<V, R>>;
}

pub mod io {
    use gen::{Generate, GeneratorBase, Yield};
    use fs;
    pub use std::io::{Error, Result};
    
    pub enum Action {
        File(fs::File, fs::Action)
    }
    
    pub trait Async<R>: Generate
    where Self::G: GeneratorBase<Value=Action, Return=Result<R>>
                 + fs::Async {
        fn run_sync(self) -> Result<R> where Self: Sized {
            match self.generate() {
                Yield::Value((mut action, mut gen)) => {
                    loop {
                        match Self::do_io(&mut gen, action) {
                            Yield::Value(next) => action = next,
                            Yield::Return(res) => return res
                        };
                    }
                }
                Yield::Return(res) => res
            }
        }
        
        fn do_io(gen: &mut Self::G, action: Action) -> Yield<Action, Result<R>> {
            match action {
                Action::File(file, action) => fs::Async::do_file(gen, file, action)
            }
        }
    }
    
    impl<T: Generate, R> Async<R> for T
    where T::G: GeneratorBase<Value=Action, Return=Result<R>>
              + fs::Async {}
   
    macro_rules! await {
        ($x:expr) => (try!(yield+ $x))
    }
}

pub mod fs {
    use gen::{Generator, Mirror, Yield, YieldAnd, YieldAndMirror};
    use io;
    use std::fs as sync;
    use std::io::prelude::*;
    use std::os::unix::prelude::*;
    use std::path::Path;
    
    pub struct File { sync: sync::File }
    pub use std::fs::Metadata;
    
    impl File {
        pub fn open<P: AsRef<Path>>(path: P) -> io::Result<File> {
            sync::File::open(path).map(|file| File { sync: file })
        }
        pub fn create<P: AsRef<Path>>(path: P) -> io::Result<File> {
            sync::File::create(path).map(|file| File { sync: file })
        }
    }
    
    impl Clone for File {
        fn clone(&self) -> File {
            extern "C" {
                fn dup(fd: i32) -> i32;
            }
            File {
                sync: unsafe {
                    sync::File::from_raw_fd(dup(self.sync.as_raw_fd()))
                }
            }
        }
    }
    
    pub enum Action {
        Read(Vec<u8>),
        Write(Vec<u8>),
        GetMetadata
    }
    
    pub trait Async: Generator<io::Result<Vec<u8>>>
                   + Generator<io::Result<sync::Metadata>> {
        fn do_file(&mut self, mut file: File, action: Action)
                   -> Yield<Self::Value, Self::Return> {
            match action {
                Action::Read(mut buffer) => {
                    // Nasty zero-fill because read takes &mut [u8].
                    let extra = buffer.capacity() - buffer.len();
                    buffer.extend((0..extra).map(|_| 0));
                    self.next(file.sync.read(&mut buffer).map(|bytes| {
                        buffer.truncate(bytes);
                        buffer
                    }))
                }
                Action::Write(buffer) => {
                    self.next(file.sync.write(&buffer).map(|_| buffer))
                }
                Action::GetMetadata => {
                    self.next(file.sync.metadata())
                }
            }
        }
    }
    
    impl<G> Async for G
    where G: Generator<io::Result<Vec<u8>>>
           + Generator<io::Result<Metadata>> {}
    
    macro_rules! action {
        ($fun:ident => $act:ident -> $ret:ty) => {
            pub fn $fun(&self) -> YieldAndMirror<io::Action, io::Result<$ret>> {
                YieldAnd(io::Action::File(self.clone(), Action::$act), Mirror::new())
            }
        };
        ($fun:ident => $act:ident($($arg:ident: $ty:ty),+) -> $ret:ty) => {
            pub fn $fun(&self, $($arg: $ty),+) -> YieldAndMirror<io::Action, io::Result<$ret>> {
                YieldAnd(io::Action::File(self.clone(), Action::$act($($arg),+)), Mirror::new())
            }
        }
    }
    impl File {
        action!(read => Read(buffer: Vec<u8>) -> Vec<u8>);
        action!(write => Write(buffer: Vec<u8>) -> Vec<u8>);
        action!(metadata => GetMetadata -> Metadata);
    }
}

https://play.rust-lang.org/?gist=a5c2d92666cd3b39dac9&run=1