New upstream version 1.22.1+dfsg1

[rustc.git] / src / vendor / conv / src / macros.rs

/*!\r
This module provides convenience macros to help with implementing the conversion traits.\r
\r
# `TryFrom!`\r
\r
```ignore\r
macro_rules! TryFrom {\r
    (($target:ty) $enum:item) => { ... };\r
}\r
```\r
\r
This macro attempts to derive an implementation of the [`TryFrom`](../trait.TryFrom.html) trait.  Specifically, it supports `enum`s consisting entirely of unitary variants, with or without explicit values.  The source type can be any integer type which the variants of the enumeration can be explicitly cast to (*i.e.* using `as`).\r
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If a conversion fails (due to there being no matching variant for the specified integer value `src`), then the conversion returns `Err(Unrepresentable(src))` (see [`Unrepresentable`](../errors/struct.Unrepresentable.html)).\r
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It is compatible with the [`custom_derive!`](https://crates.io/crates/custom_derive) macro.\r
\r
## Example\r
\r
Using `custom_derive!`:\r
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```\r
#[macro_use] extern crate conv;\r
#[macro_use] extern crate custom_derive;\r
\r
custom_derive! {\r
    #[derive(Debug, PartialEq, TryFrom(i32))]\r
    enum Colours {\r
        Red = 0,\r
        Green = 5,\r
        Blue\r
    }\r
}\r
\r
fn main() {\r
    use conv::{TryFrom, Unrepresentable};\r
\r
    assert_eq!(Colours::try_from(0), Ok(Colours::Red));\r
    assert_eq!(Colours::try_from(1), Err(Unrepresentable(1)));\r
    assert_eq!(Colours::try_from(5), Ok(Colours::Green));\r
    assert_eq!(Colours::try_from(6), Ok(Colours::Blue));\r
    assert_eq!(Colours::try_from(7), Err(Unrepresentable(7)));\r
}\r
```\r
\r
The above is equivalent to the following:\r
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```\r
#[macro_use] extern crate conv;\r
\r
#[derive(Debug, PartialEq)]\r
enum Colours {\r
    Red = 0,\r
    Green = 5,\r
    Blue\r
}\r
\r
TryFrom! { (i32) enum Colours {\r
    Red = 0,\r
    Green = 5,\r
    Blue\r
} }\r
# fn main() {}\r
```\r
*/\r
\r
/**\r
See the documentation for the [`macros`](./macros/index.html#tryfrom!) module for details.\r
*/\r
#[macro_export]\r
macro_rules! TryFrom {\r
    (($prim:ty) $(pub)* enum $name:ident { $($body:tt)* }) => {\r
        TryFrom! {\r
            @collect_variants ($name, $prim),\r
            ($($body)*,) -> ()\r
        }\r
    };\r
\r
    (\r
        @collect_variants ($name:ident, $prim:ty),\r
        ($(,)*) -> ($($var_names:ident,)*)\r
    ) => {\r
        impl $crate::TryFrom<$prim> for $name {\r
            type Err = $crate::errors::Unrepresentable<$prim>;\r
            fn try_from(src: $prim) -> Result<$name, Self::Err> {\r
                $(\r
                    if src == $name::$var_names as $prim {\r
                        return Ok($name::$var_names);\r
                    }\r
                )*\r
                Err($crate::errors::Unrepresentable(src))\r
            }\r
        }\r
    };\r
\r
    (\r
        @collect_variants $fixed:tt,\r
        (#[$_attr:meta] $($tail:tt)*) -> $var_names:tt\r
    ) => {\r
        TryFrom! {\r
            @skip_meta $fixed,\r
            ($($tail)*) -> $var_names\r
        }\r
    };\r
\r
    (\r
        @collect_variants $fixed:tt,\r
        ($var:ident $(= $_val:expr)*, $($tail:tt)*) -> ($($var_names:tt)*)\r
    ) => {\r
        TryFrom! {\r
            @collect_variants $fixed,\r
            ($($tail)*) -> ($($var_names)* $var,)\r
        }\r
    };\r
\r
    (\r
        @collect_variants ($name:ident),\r
        ($var:ident $_struct:tt, $($tail:tt)*) -> ($($var_names:tt)*)\r
    ) => {\r
        const _error: () = concat!(\r
            "cannot derive TryFrom for ",\r
            stringify!($name),\r
            ", due to non-unitary variant ",\r
            stringify!($var),\r
            "."\r
        );\r
    };\r
    \r
    (\r
        @skip_meta $fixed:tt,\r
        (#[$_attr:meta] $($tail:tt)*) -> $var_names:tt\r
    ) => {\r
        TryFrom! {\r
            @skip_meta $fixed,\r
            ($($tail)*) -> $var_names\r
        }\r
    };\r
\r
    (\r
        @skip_meta $fixed:tt,\r
        ($var:ident $($tail:tt)*) -> $var_names:tt\r
    ) => {\r
        TryFrom! {\r
            @collect_variants $fixed,\r
            ($var $($tail)*) -> $var_names\r
        }\r
    };\r
}\r