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//! The `select!` macro.
/// A helper macro for `select!` to hide the long list of macro patterns from the documentation.
///
/// The macro consists of two stages:
/// 1. Parsing
/// 2. Code generation
///
/// The parsing stage consists of these subparts:
/// 1. `@list`: Turns a list of tokens into a list of cases.
/// 2. `@list_errorN`: Diagnoses the syntax error.
/// 3. `@case`: Parses a single case and verifies its argument list.
///
/// The codegen stage consists of these subparts:
/// 1. `@init`: Attempts to optimize `select!` away and initializes the list of handles.
/// 1. `@count`: Counts the listed cases.
/// 3. `@add`: Adds send/receive operations to the list of handles and starts selection.
/// 4. `@complete`: Completes the selected send/receive operation.
///
/// If the parsing stage encounters a syntax error or the codegen stage ends up with too many
/// cases to process, the macro fails with a compile-time error.
#[doc(hidden)]
#[macro_export]
macro_rules! crossbeam_channel_internal {
// The list is empty. Now check the arguments of each processed case.
(@list
()
($($head:tt)*)
) => {
$crate::crossbeam_channel_internal!(
@case
($($head)*)
()
()
)
};
// If necessary, insert an empty argument list after `default`.
(@list
(default => $($tail:tt)*)
($($head:tt)*)
) => {
$crate::crossbeam_channel_internal!(
@list
(default() => $($tail)*)
($($head)*)
)
};
// But print an error if `default` is followed by a `->`.
(@list
(default -> $($tail:tt)*)
($($head:tt)*)
) => {
compile_error!(
"expected `=>` after `default` case, found `->`"
)
};
// Print an error if there's an `->` after the argument list in the default case.
(@list
(default $args:tt -> $($tail:tt)*)
($($head:tt)*)
) => {
compile_error!(
"expected `=>` after `default` case, found `->`"
)
};
// Print an error if there is a missing result in a recv case.
(@list
(recv($($args:tt)*) => $($tail:tt)*)
($($head:tt)*)
) => {
compile_error!(
"expected `->` after `recv` case, found `=>`"
)
};
// Print an error if there is a missing result in a send case.
(@list
(send($($args:tt)*) => $($tail:tt)*)
($($head:tt)*)
) => {
compile_error!(
"expected `->` after `send` operation, found `=>`"
)
};
// Make sure the arrow and the result are not repeated.
(@list
($case:ident $args:tt -> $res:tt -> $($tail:tt)*)
($($head:tt)*)
) => {
compile_error!("expected `=>`, found `->`")
};
// Print an error if there is a semicolon after the block.
(@list
($case:ident $args:tt $(-> $res:pat)* => $body:block; $($tail:tt)*)
($($head:tt)*)
) => {
compile_error!(
"did you mean to put a comma instead of the semicolon after `}`?"
)
};
// The first case is separated by a comma.
(@list
($case:ident ($($args:tt)*) $(-> $res:pat)* => $body:expr, $($tail:tt)*)
($($head:tt)*)
) => {
$crate::crossbeam_channel_internal!(
@list
($($tail)*)
($($head)* $case ($($args)*) $(-> $res)* => { $body },)
)
};
// Don't require a comma after the case if it has a proper block.
(@list
($case:ident ($($args:tt)*) $(-> $res:pat)* => $body:block $($tail:tt)*)
($($head:tt)*)
) => {
$crate::crossbeam_channel_internal!(
@list
($($tail)*)
($($head)* $case ($($args)*) $(-> $res)* => { $body },)
)
};
// Only one case remains.
(@list
($case:ident ($($args:tt)*) $(-> $res:pat)* => $body:expr $(,)?)
($($head:tt)*)
) => {
$crate::crossbeam_channel_internal!(
@list
()
($($head)* $case ($($args)*) $(-> $res)* => { $body },)
)
};
// Diagnose and print an error.
(@list
($($tail:tt)*)
($($head:tt)*)
) => {
$crate::crossbeam_channel_internal!(@list_error1 $($tail)*)
};
// Stage 1: check the case type.
(@list_error1 recv $($tail:tt)*) => {
$crate::crossbeam_channel_internal!(@list_error2 recv $($tail)*)
};
(@list_error1 send $($tail:tt)*) => {
$crate::crossbeam_channel_internal!(@list_error2 send $($tail)*)
};
(@list_error1 default $($tail:tt)*) => {
$crate::crossbeam_channel_internal!(@list_error2 default $($tail)*)
};
(@list_error1 $t:tt $($tail:tt)*) => {
compile_error!(
concat!(
"expected one of `recv`, `send`, or `default`, found `",
stringify!($t),
"`",
)
)
};
(@list_error1 $($tail:tt)*) => {
$crate::crossbeam_channel_internal!(@list_error2 $($tail)*);
};
// Stage 2: check the argument list.
(@list_error2 $case:ident) => {
compile_error!(
concat!(
"missing argument list after `",
stringify!($case),
"`",
)
)
};
(@list_error2 $case:ident => $($tail:tt)*) => {
compile_error!(
concat!(
"missing argument list after `",
stringify!($case),
"`",
)
)
};
(@list_error2 $($tail:tt)*) => {
$crate::crossbeam_channel_internal!(@list_error3 $($tail)*)
};
// Stage 3: check the `=>` and what comes after it.
(@list_error3 $case:ident($($args:tt)*) $(-> $r:pat)*) => {
compile_error!(
concat!(
"missing `=>` after `",
stringify!($case),
"` case",
)
)
};
(@list_error3 $case:ident($($args:tt)*) $(-> $r:pat)* =>) => {
compile_error!(
"expected expression after `=>`"
)
};
(@list_error3 $case:ident($($args:tt)*) $(-> $r:pat)* => $body:expr; $($tail:tt)*) => {
compile_error!(
concat!(
"did you mean to put a comma instead of the semicolon after `",
stringify!($body),
"`?",
)
)
};
(@list_error3 $case:ident($($args:tt)*) $(-> $r:pat)* => recv($($a:tt)*) $($tail:tt)*) => {
compile_error!(
"expected an expression after `=>`"
)
};
(@list_error3 $case:ident($($args:tt)*) $(-> $r:pat)* => send($($a:tt)*) $($tail:tt)*) => {
compile_error!(
"expected an expression after `=>`"
)
};
(@list_error3 $case:ident($($args:tt)*) $(-> $r:pat)* => default($($a:tt)*) $($tail:tt)*) => {
compile_error!(
"expected an expression after `=>`"
)
};
(@list_error3 $case:ident($($args:tt)*) $(-> $r:pat)* => $f:ident($($a:tt)*) $($tail:tt)*) => {
compile_error!(
concat!(
"did you mean to put a comma after `",
stringify!($f),
"(",
stringify!($($a)*),
")`?",
)
)
};
(@list_error3 $case:ident($($args:tt)*) $(-> $r:pat)* => $f:ident!($($a:tt)*) $($tail:tt)*) => {
compile_error!(
concat!(
"did you mean to put a comma after `",
stringify!($f),
"!(",
stringify!($($a)*),
")`?",
)
)
};
(@list_error3 $case:ident($($args:tt)*) $(-> $r:pat)* => $f:ident![$($a:tt)*] $($tail:tt)*) => {
compile_error!(
concat!(
"did you mean to put a comma after `",
stringify!($f),
"![",
stringify!($($a)*),
"]`?",
)
)
};
(@list_error3 $case:ident($($args:tt)*) $(-> $r:pat)* => $f:ident!{$($a:tt)*} $($tail:tt)*) => {
compile_error!(
concat!(
"did you mean to put a comma after `",
stringify!($f),
"!{",
stringify!($($a)*),
"}`?",
)
)
};
(@list_error3 $case:ident($($args:tt)*) $(-> $r:pat)* => $body:tt $($tail:tt)*) => {
compile_error!(
concat!(
"did you mean to put a comma after `",
stringify!($body),
"`?",
)
)
};
(@list_error3 $case:ident($($args:tt)*) -> => $($tail:tt)*) => {
compile_error!("missing pattern after `->`")
};
(@list_error3 $case:ident($($args:tt)*) $t:tt $(-> $r:pat)* => $($tail:tt)*) => {
compile_error!(
concat!(
"expected `->`, found `",
stringify!($t),
"`",
)
)
};
(@list_error3 $case:ident($($args:tt)*) -> $t:tt $($tail:tt)*) => {
compile_error!(
concat!(
"expected a pattern, found `",
stringify!($t),
"`",
)
)
};
(@list_error3 recv($($args:tt)*) $t:tt $($tail:tt)*) => {
compile_error!(
concat!(
"expected `->`, found `",
stringify!($t),
"`",
)
)
};
(@list_error3 send($($args:tt)*) $t:tt $($tail:tt)*) => {
compile_error!(
concat!(
"expected `->`, found `",
stringify!($t),
"`",
)
)
};
(@list_error3 recv $args:tt $($tail:tt)*) => {
compile_error!(
concat!(
"expected an argument list after `recv`, found `",
stringify!($args),
"`",
)
)
};
(@list_error3 send $args:tt $($tail:tt)*) => {
compile_error!(
concat!(
"expected an argument list after `send`, found `",
stringify!($args),
"`",
)
)
};
(@list_error3 default $args:tt $($tail:tt)*) => {
compile_error!(
concat!(
"expected an argument list or `=>` after `default`, found `",
stringify!($args),
"`",
)
)
};
(@list_error3 $($tail:tt)*) => {
$crate::crossbeam_channel_internal!(@list_error4 $($tail)*)
};
// Stage 4: fail with a generic error message.
(@list_error4 $($tail:tt)*) => {
compile_error!("invalid syntax")
};
// Success! All cases were parsed.
(@case
()
$cases:tt
$default:tt
) => {
$crate::crossbeam_channel_internal!(
@init
$cases
$default
)
};
// Check the format of a recv case.
(@case
(recv($r:expr $(,)?) -> $res:pat => $body:tt, $($tail:tt)*)
($($cases:tt)*)
$default:tt
) => {
$crate::crossbeam_channel_internal!(
@case
($($tail)*)
($($cases)* recv($r) -> $res => $body,)
$default
)
};
// Print an error if the argument list is invalid.
(@case
(recv($($args:tt)*) -> $res:pat => $body:tt, $($tail:tt)*)
($($cases:tt)*)
$default:tt
) => {
compile_error!(
concat!(
"invalid argument list in `recv(",
stringify!($($args)*),
")`",
)
)
};
// Print an error if there is no argument list.
(@case
(recv $t:tt $($tail:tt)*)
($($cases:tt)*)
$default:tt
) => {
compile_error!(
concat!(
"expected an argument list after `recv`, found `",
stringify!($t),
"`",
)
)
};
// Check the format of a send case.
(@case
(send($s:expr, $m:expr $(,)?) -> $res:pat => $body:tt, $($tail:tt)*)
($($cases:tt)*)
$default:tt
) => {
$crate::crossbeam_channel_internal!(
@case
($($tail)*)
($($cases)* send($s, $m) -> $res => $body,)
$default
)
};
// Print an error if the argument list is invalid.
(@case
(send($($args:tt)*) -> $res:pat => $body:tt, $($tail:tt)*)
($($cases:tt)*)
$default:tt
) => {
compile_error!(
concat!(
"invalid argument list in `send(",
stringify!($($args)*),
")`",
)
)
};
// Print an error if there is no argument list.
(@case
(send $t:tt $($tail:tt)*)
($($cases:tt)*)
$default:tt
) => {
compile_error!(
concat!(
"expected an argument list after `send`, found `",
stringify!($t),
"`",
)
)
};
// Check the format of a default case.
(@case
(default() => $body:tt, $($tail:tt)*)
$cases:tt
()
) => {
$crate::crossbeam_channel_internal!(
@case
($($tail)*)
$cases
(default() => $body,)
)
};
// Check the format of a default case with timeout.
(@case
(default($timeout:expr $(,)?) => $body:tt, $($tail:tt)*)
$cases:tt
()
) => {
$crate::crossbeam_channel_internal!(
@case
($($tail)*)
$cases
(default($timeout) => $body,)
)
};
// Check for duplicate default cases...
(@case
(default $($tail:tt)*)
$cases:tt
($($def:tt)+)
) => {
compile_error!(
"there can be only one `default` case in a `select!` block"
)
};
// Print an error if the argument list is invalid.
(@case
(default($($args:tt)*) => $body:tt, $($tail:tt)*)
$cases:tt
$default:tt
) => {
compile_error!(
concat!(
"invalid argument list in `default(",
stringify!($($args)*),
")`",
)
)
};
// Print an error if there is an unexpected token after `default`.
(@case
(default $t:tt $($tail:tt)*)
$cases:tt
$default:tt
) => {
compile_error!(
concat!(
"expected an argument list or `=>` after `default`, found `",
stringify!($t),
"`",
)
)
};
// The case was not consumed, therefore it must be invalid.
(@case
($case:ident $($tail:tt)*)
$cases:tt
$default:tt
) => {
compile_error!(
concat!(
"expected one of `recv`, `send`, or `default`, found `",
stringify!($case),
"`",
)
)
};
// Optimize `select!` into `try_recv()`.
(@init
(recv($r:expr) -> $res:pat => $recv_body:tt,)
(default() => $default_body:tt,)
) => {{
match $r {
ref _r => {
let _r: &$crate::Receiver<_> = _r;
match _r.try_recv() {
::std::result::Result::Err($crate::TryRecvError::Empty) => {
$default_body
}
_res => {
let _res = _res.map_err(|_| $crate::RecvError);
let $res = _res;
$recv_body
}
}
}
}
}};
// Optimize `select!` into `recv()`.
(@init
(recv($r:expr) -> $res:pat => $body:tt,)
()
) => {{
match $r {
ref _r => {
let _r: &$crate::Receiver<_> = _r;
let _res = _r.recv();
let $res = _res;
$body
}
}
}};
// Optimize `select!` into `recv_timeout()`.
(@init
(recv($r:expr) -> $res:pat => $recv_body:tt,)
(default($timeout:expr) => $default_body:tt,)
) => {{
match $r {
ref _r => {
let _r: &$crate::Receiver<_> = _r;
match _r.recv_timeout($timeout) {
::std::result::Result::Err($crate::RecvTimeoutError::Timeout) => {
$default_body
}
_res => {
let _res = _res.map_err(|_| $crate::RecvError);
let $res = _res;
$recv_body
}
}
}
}
}};
// // Optimize the non-blocking case with two receive operations.
// (@init
// (recv($r1:expr) -> $res1:pat => $recv_body1:tt,)
// (recv($r2:expr) -> $res2:pat => $recv_body2:tt,)
// (default() => $default_body:tt,)
// ) => {{
// match $r1 {
// ref _r1 => {
// let _r1: &$crate::Receiver<_> = _r1;
//
// match $r2 {
// ref _r2 => {
// let _r2: &$crate::Receiver<_> = _r2;
//
// // TODO(stjepang): Implement this optimization.
// }
// }
// }
// }
// }};
// // Optimize the blocking case with two receive operations.
// (@init
// (recv($r1:expr) -> $res1:pat => $body1:tt,)
// (recv($r2:expr) -> $res2:pat => $body2:tt,)
// ()
// ) => {{
// match $r1 {
// ref _r1 => {
// let _r1: &$crate::Receiver<_> = _r1;
//
// match $r2 {
// ref _r2 => {
// let _r2: &$crate::Receiver<_> = _r2;
//
// // TODO(stjepang): Implement this optimization.
// }
// }
// }
// }
// }};
// // Optimize the case with two receive operations and a timeout.
// (@init
// (recv($r1:expr) -> $res1:pat => $recv_body1:tt,)
// (recv($r2:expr) -> $res2:pat => $recv_body2:tt,)
// (default($timeout:expr) => $default_body:tt,)
// ) => {{
// match $r1 {
// ref _r1 => {
// let _r1: &$crate::Receiver<_> = _r1;
//
// match $r2 {
// ref _r2 => {
// let _r2: &$crate::Receiver<_> = _r2;
//
// // TODO(stjepang): Implement this optimization.
// }
// }
// }
// }
// }};
// // Optimize `select!` into `try_send()`.
// (@init
// (send($s:expr, $m:expr) -> $res:pat => $send_body:tt,)
// (default() => $default_body:tt,)
// ) => {{
// match $s {
// ref _s => {
// let _s: &$crate::Sender<_> = _s;
// // TODO(stjepang): Implement this optimization.
// }
// }
// }};
// // Optimize `select!` into `send()`.
// (@init
// (send($s:expr, $m:expr) -> $res:pat => $body:tt,)
// ()
// ) => {{
// match $s {
// ref _s => {
// let _s: &$crate::Sender<_> = _s;
// // TODO(stjepang): Implement this optimization.
// }
// }
// }};
// // Optimize `select!` into `send_timeout()`.
// (@init
// (send($s:expr, $m:expr) -> $res:pat => $body:tt,)
// (default($timeout:expr) => $body:tt,)
// ) => {{
// match $s {
// ref _s => {
// let _s: &$crate::Sender<_> = _s;
// // TODO(stjepang): Implement this optimization.
// }
// }
// }};
// Create the list of handles and add operations to it.
(@init
($($cases:tt)*)
$default:tt
) => {{
const _LEN: usize = $crate::crossbeam_channel_internal!(@count ($($cases)*));
let _handle: &$crate::internal::SelectHandle = &$crate::never::<()>();
#[allow(unused_mut)]
let mut _sel = [(_handle, 0, ::std::ptr::null()); _LEN];
$crate::crossbeam_channel_internal!(
@add
_sel
($($cases)*)
$default
(
(0usize _oper0)
(1usize _oper1)
(2usize _oper2)
(3usize _oper3)
(4usize _oper4)
(5usize _oper5)
(6usize _oper6)
(7usize _oper7)
(8usize _oper8)
(9usize _oper9)
(10usize _oper10)
(11usize _oper11)
(12usize _oper12)
(13usize _oper13)
(14usize _oper14)
(15usize _oper15)
(16usize _oper16)
(17usize _oper17)
(18usize _oper18)
(19usize _oper19)
(20usize _oper20)
(21usize _oper21)
(22usize _oper22)
(23usize _oper23)
(24usize _oper24)
(25usize _oper25)
(26usize _oper26)
(27usize _oper27)
(28usize _oper28)
(29usize _oper29)
(30usize _oper30)
(31usize _oper31)
)
()
)
}};
// Count the listed cases.
(@count ()) => {
0
};
(@count ($oper:ident $args:tt -> $res:pat => $body:tt, $($cases:tt)*)) => {
1 + $crate::crossbeam_channel_internal!(@count ($($cases)*))
};
// Run blocking selection.
(@add
$sel:ident
()
()
$labels:tt
$cases:tt
) => {{
let _oper: $crate::SelectedOperation<'_> = {
let _oper = $crate::internal::select(&mut $sel);
// Erase the lifetime so that `sel` can be dropped early even without NLL.
unsafe { ::std::mem::transmute(_oper) }
};
$crate::crossbeam_channel_internal! {
@complete
$sel
_oper
$cases
}
}};
// Run non-blocking selection.
(@add
$sel:ident
()
(default() => $body:tt,)
$labels:tt
$cases:tt
) => {{
let _oper: ::std::option::Option<$crate::SelectedOperation<'_>> = {
let _oper = $crate::internal::try_select(&mut $sel);
// Erase the lifetime so that `sel` can be dropped early even without NLL.
unsafe { ::std::mem::transmute(_oper) }
};
match _oper {
None => {
{ $sel };
$body
}
Some(_oper) => {
$crate::crossbeam_channel_internal! {
@complete
$sel
_oper
$cases
}
}
}
}};
// Run selection with a timeout.
(@add
$sel:ident
()
(default($timeout:expr) => $body:tt,)
$labels:tt
$cases:tt
) => {{
let _oper: ::std::option::Option<$crate::SelectedOperation<'_>> = {
let _oper = $crate::internal::select_timeout(&mut $sel, $timeout);
// Erase the lifetime so that `sel` can be dropped early even without NLL.
unsafe { ::std::mem::transmute(_oper) }
};
match _oper {
::std::option::Option::None => {
{ $sel };
$body
}
::std::option::Option::Some(_oper) => {
$crate::crossbeam_channel_internal! {
@complete
$sel
_oper
$cases
}
}
}
}};
// Have we used up all labels?
(@add
$sel:ident
$input:tt
$default:tt
()
$cases:tt
) => {
compile_error!("too many operations in a `select!` block")
};
// Add a receive operation to `sel`.
(@add
$sel:ident
(recv($r:expr) -> $res:pat => $body:tt, $($tail:tt)*)
$default:tt
(($i:tt $var:ident) $($labels:tt)*)
($($cases:tt)*)
) => {{
match $r {
ref _r => {
let $var: &$crate::Receiver<_> = unsafe {
let _r: &$crate::Receiver<_> = _r;
// Erase the lifetime so that `sel` can be dropped early even without NLL.
unsafe fn unbind<'a, T>(x: &T) -> &'a T {
::std::mem::transmute(x)
}
unbind(_r)
};
$sel[$i] = ($var, $i, $var as *const $crate::Receiver<_> as *const u8);
$crate::crossbeam_channel_internal!(
@add
$sel
($($tail)*)
$default
($($labels)*)
($($cases)* [$i] recv($var) -> $res => $body,)
)
}
}
}};
// Add a send operation to `sel`.
(@add
$sel:ident
(send($s:expr, $m:expr) -> $res:pat => $body:tt, $($tail:tt)*)
$default:tt
(($i:tt $var:ident) $($labels:tt)*)
($($cases:tt)*)
) => {{
match $s {
ref _s => {
let $var: &$crate::Sender<_> = unsafe {
let _s: &$crate::Sender<_> = _s;
// Erase the lifetime so that `sel` can be dropped early even without NLL.
unsafe fn unbind<'a, T>(x: &T) -> &'a T {
::std::mem::transmute(x)
}
unbind(_s)
};
$sel[$i] = ($var, $i, $var as *const $crate::Sender<_> as *const u8);
$crate::crossbeam_channel_internal!(
@add
$sel
($($tail)*)
$default
($($labels)*)
($($cases)* [$i] send($var, $m) -> $res => $body,)
)
}
}
}};
// Complete a receive operation.
(@complete
$sel:ident
$oper:ident
([$i:tt] recv($r:ident) -> $res:pat => $body:tt, $($tail:tt)*)
) => {{
if $oper.index() == $i {
let _res = $oper.recv($r);
{ $sel };
let $res = _res;
$body
} else {
$crate::crossbeam_channel_internal! {
@complete
$sel
$oper
($($tail)*)
}
}
}};
// Complete a send operation.
(@complete
$sel:ident
$oper:ident
([$i:tt] send($s:ident, $m:expr) -> $res:pat => $body:tt, $($tail:tt)*)
) => {{
if $oper.index() == $i {
let _res = $oper.send($s, $m);
{ $sel };
let $res = _res;
$body
} else {
$crate::crossbeam_channel_internal! {
@complete
$sel
$oper
($($tail)*)
}
}
}};
// Panic if we don't identify the selected case, but this should never happen.
(@complete
$sel:ident
$oper:ident
()
) => {{
unreachable!(
"internal error in crossbeam-channel: invalid case"
)
}};
// Catches a bug within this macro (should not happen).
(@$($tokens:tt)*) => {
compile_error!(
concat!(
"internal error in crossbeam-channel: ",
stringify!(@$($tokens)*),
)
)
};
// The entry points.
() => {
compile_error!("empty `select!` block")
};
($($case:ident $(($($args:tt)*))* => $body:expr $(,)*)*) => {
$crate::crossbeam_channel_internal!(
@list
($($case $(($($args)*))* => { $body },)*)
()
)
};
($($tokens:tt)*) => {
$crate::crossbeam_channel_internal!(
@list
($($tokens)*)
()
)
};
}
/// Selects from a set of channel operations.
///
/// This macro allows you to define a set of channel operations, wait until any one of them becomes
/// ready, and finally execute it. If multiple operations are ready at the same time, a random one
/// among them is selected.
///
/// It is also possible to define a `default` case that gets executed if none of the operations are
/// ready, either right away or for a certain duration of time.
///
/// An operation is considered to be ready if it doesn't have to block. Note that it is ready even
/// when it will simply return an error because the channel is disconnected.
///
/// The `select!` macro is a convenience wrapper around [`Select`]. However, it cannot select over a
/// dynamically created list of channel operations.
///
/// [`Select`]: super::Select
///
/// # Examples
///
/// Block until a send or a receive operation is selected:
///
/// ```
/// use crossbeam_channel::{select, unbounded};
///
/// let (s1, r1) = unbounded();
/// let (s2, r2) = unbounded();
/// s1.send(10).unwrap();
///
/// // Since both operations are initially ready, a random one will be executed.
/// select! {
/// recv(r1) -> msg => assert_eq!(msg, Ok(10)),
/// send(s2, 20) -> res => {
/// assert_eq!(res, Ok(()));
/// assert_eq!(r2.recv(), Ok(20));
/// }
/// }
/// ```
///
/// Select from a set of operations without blocking:
///
/// ```
/// use std::thread;
/// use std::time::Duration;
/// use crossbeam_channel::{select, unbounded};
///
/// let (s1, r1) = unbounded();
/// let (s2, r2) = unbounded();
///
/// thread::spawn(move || {
/// thread::sleep(Duration::from_secs(1));
/// s1.send(10).unwrap();
/// });
/// thread::spawn(move || {
/// thread::sleep(Duration::from_millis(500));
/// s2.send(20).unwrap();
/// });
///
/// // None of the operations are initially ready.
/// select! {
/// recv(r1) -> msg => panic!(),
/// recv(r2) -> msg => panic!(),
/// default => println!("not ready"),
/// }
/// ```
///
/// Select over a set of operations with a timeout:
///
/// ```
/// use std::thread;
/// use std::time::Duration;
/// use crossbeam_channel::{select, unbounded};
///
/// let (s1, r1) = unbounded();
/// let (s2, r2) = unbounded();
///
/// thread::spawn(move || {
/// thread::sleep(Duration::from_secs(1));
/// s1.send(10).unwrap();
/// });
/// thread::spawn(move || {
/// thread::sleep(Duration::from_millis(500));
/// s2.send(20).unwrap();
/// });
///
/// // None of the two operations will become ready within 100 milliseconds.
/// select! {
/// recv(r1) -> msg => panic!(),
/// recv(r2) -> msg => panic!(),
/// default(Duration::from_millis(100)) => println!("timed out"),
/// }
/// ```
///
/// Optionally add a receive operation to `select!` using [`never`]:
///
/// ```
/// use std::thread;
/// use std::time::Duration;
/// use crossbeam_channel::{select, never, unbounded};
///
/// let (s1, r1) = unbounded();
/// let (s2, r2) = unbounded();
///
/// thread::spawn(move || {
/// thread::sleep(Duration::from_secs(1));
/// s1.send(10).unwrap();
/// });
/// thread::spawn(move || {
/// thread::sleep(Duration::from_millis(500));
/// s2.send(20).unwrap();
/// });
///
/// // This receiver can be a `Some` or a `None`.
/// let r2 = Some(&r2);
///
/// // None of the two operations will become ready within 100 milliseconds.
/// select! {
/// recv(r1) -> msg => panic!(),
/// recv(r2.unwrap_or(&never())) -> msg => assert_eq!(msg, Ok(20)),
/// }
/// ```
///
/// To optionally add a timeout to `select!`, see the [example] for [`never`].
///
/// [`never`]: super::never
/// [example]: super::never#examples
#[macro_export]
macro_rules! select {
($($tokens:tt)*) => {
$crate::crossbeam_channel_internal!(
$($tokens)*
)
};
}