创建类型的通用变体的枚举

Question

我的结构具有常规参数 var 。该参数用于方法中，提高了运行时速度，因此可以进行大量比较，以编译时间。 在任何地方存储和传递通用类型是相当困难的。我有固定数量的变体，我可以在枚举 squirrelvariant 中列出，我可以更轻松地传递，并使用 match 分支到通用功能。 从 spirrel＆lt; var＆gt; 创建枚举非常困难，生锈似乎不允许编译时间匹配或类似的东西。

struct Squirrel<const VAR: u8>
{
}


enum SquirrelVariant
{
    V1(Squirrel<1>),
    V2(Squirrel<2>),
    V3(Squirrel<3>),
}

// failed attempt 1)
fn make_squirrel_variant<const VAR: u8>(s: Squirrel<VAR>) -> SquirrelVariant
{
    match VAR {
        1 => SquirrelVariant::V1(s), // expected `1_u8`, found `VAR`
        2 => SquirrelVariant::V2(s), // expected `2_u8`, found `VAR`
        3 => SquirrelVariant::V3(s), // expected `3_u8`, found `VAR`
    }
}


// failed attempt 2)
impl From<Squirrel<1>> for SquirrelVariant
{
    fn from(s: Squirrel<1>) -> Self
    {
        SquirrelVariant::V1(s)
    }
}

impl From<Squirrel<2>> for SquirrelVariant
{
    fn from(s: Squirrel<2>) -> Self
    {
        SquirrelVariant::V2(s)
    }
}

impl From<Squirrel<3>> for SquirrelVariant
{
    fn from(s: Squirrel<3>) -> Self
    {
        SquirrelVariant::V3(s)
    }
}

fn make_squirrel_variant2<const VAR: u8>(s: Squirrel<VAR>) -> SquirrelVariant
{
    s.into() // the trait `From<Squirrel<VAR>>` is not implemented for `SquirrelVariant`
    // = help: the following implementations were found:
    //     <SquirrelVariant as From<Squirrel<1_u8>>>
    //     <SquirrelVariant as From<Squirrel<2_u8>>>
    //     <SquirrelVariant as From<Squirrel<3_u8>>>
}

Answer 1

您可以使用以下技巧来解决类型错误：

struct Squirrel<const VAR: u8> {}

impl<const VAR: u8> Squirrel<VAR> {
    fn transmute_squirrel_type<const VAR2: u8>(self) -> Squirrel<VAR2> {
        if VAR == VAR2 {
            Squirrel { }
        } else {
            unreachable!()
        }
    }
}


enum SquirrelVariant
{
    V1(Squirrel<1>),
    V2(Squirrel<2>),
    V3(Squirrel<3>),
}

impl<const VAR: u8> Squirrel<VAR> {
    fn to_variant(self) -> SquirrelVariant {
        match VAR {
            1 => SquirrelVariant::V1(self.transmute_squirrel_type()),
            2 => SquirrelVariant::V2(self.transmute_squirrel_type()),
            3 => SquirrelVariant::V3(self.transmute_squirrel_type()),
            _ => unreachable!()
        }
    }
}