variadic_from 0.56.0

Variadic from.
Documentation 
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/// Trait for converting from one argument.
pub trait From1< T1 >
where
  Self: Sized,
{
  /// Converts from one argument.
  fn from1(a1: T1) -> Self;
}

/// Trait for converting from two arguments.
pub trait From2< T1, T2 >
where
  Self: Sized,
{
  /// Converts from two arguments.
  fn from2(a1: T1, a2: T2) -> Self;
}

/// Trait for converting from three arguments.
pub trait From3< T1, T2, T3 >
where
  Self: Sized,
{
  /// Converts from three arguments.
  fn from3(a1: T1, a2: T2, a3: T3) -> Self;
}

/// Macro to construct a struct from variadic arguments.
#[ macro_export ]
macro_rules! from {
  () =>
  {
  core ::default ::Default ::default()
 };
  ( $a1: expr ) =>
  {
  ::variadic_from ::variadic ::From1 ::from1($a1)
 };
  ( $a1: expr, $a2: expr ) =>
  {
  ::variadic_from ::variadic ::From2 ::from2($a1, $a2)
 };
  ( $a1: expr, $a2: expr, $a3: expr ) =>
  {
  ::variadic_from ::variadic ::From3 ::from3($a1, $a2, $a3)
 };
  ( $( $rest: expr ),* ) =>
  {
  compile_error!("Too many arguments");
 };
}