Rust code shared from the playground

playground.rs

// Did you ever want to have a compile-time evaluated string and didn't
// want to use so called "procedural macros" because it's too easy to
// have that with those? Well, now you can.
//
// ... Oh, right, there is a small catch. The computation has to be in
// Brainfuck. But I mean, it's Turing complete, so you can do any
// computation you like, so it's not a big deal, is it?
//
// Scroll down to the end to see usage, but essentially this file
// defines `bf!` macro to which you can pass whatever Brainfuck code
// you like. Its output will be returned as a constant string you can
// use for whatever purpose you like, including using in `const` and as
// a constant for macros like `concat!` and `compile_error!`.
//
// If you actually want to use this for some crazy reason (...), then
// have a license.
//
// --------------------------------------------------------------------
//
// whyiamdoingthis - Brainfuck compile-time interpreter implemented
//                   using Rust macros
//
// Written in 2017 by Konrad Borowski <konrad@borowski.pw>
//
// To the extent possible under law, the author(s) have dedicated all
// copyright and related and neighboring rights to this software to the
// public domain worldwide. This software is distributed without any
// warranty.
//
// You should have received a copy of the CC0 Public Domain Dedication
// along with this software. If not, see
// <http://creativecommons.org/publicdomain/zero/1.0/>.

#![recursion_limit = "2048"]

macro_rules! bf {
    ($($code:tt)*) => {
        b!(
            [[$($code)*]]
            [] [] []
            []
        )
    }
}

macro_rules! b {
    // End condition
    (
        [[]]
        $left:tt $current:tt $right:tt
        [$([$($out:tt)*])*]
    ) => {
        concat!($(to_ascii!($($out)*)),*)
    };

    // End of current block,
    (
        [[] $($codes:tt)*]
        $left:tt $current:tt $right:tt
        $out:tt
    ) => {
        b!(
            [$($codes)*]
            $left $current $right
            $out
        )
    };

    // Start of a block, skipped
    (
        [[[$($new_block:tt)*] $($rest:tt)*] $($codes:tt)*]
        $left:tt [] $right:tt
        $out:tt
    ) => {
        b!(
            [[$($rest)*] $($codes)*]
            $left [] $right
            $out
        )
    };

    // Start of a block, entering
    (
        [[[$($new_block:tt)*] $($rest:tt)*] $($codes:tt)*]
        $left:tt $current:tt $right:tt
        $out:tt
    ) => {
        b!(
            [[$($new_block)*] [[$($new_block)*] $($rest)*] $($codes)*]
            $left $current $right
            $out
        )
    };

    // Increment
    (
        [[+ $($rest:tt)*] $($codes:tt)*]
        $left:tt [$($current:tt)*] $right:tt
        $out:tt
    ) => {
        b!(
            [[$($rest)*] $($codes)*]
            $left [+ $($current)*] $right
            $out
        )
    };

    // Decrement
    (
        [[- $($rest:tt)*] $($codes:tt)*]
        $left:tt [+ $($current:tt)*] $right:tt
        $out:tt
    ) => {
        b!(
            [[$($rest)*] $($codes)*]
            $left [$($current)*] $right
            $out
        )
    };

    // Move tape left
    (
        [[< $($rest:tt)*] $($codes:tt)*]
        [$next:tt $($left:tt)*] $current:tt [$($right:tt)*]
        $out:tt
    ) => {
        b!(
            [[$($rest)*] $($codes)*]
            [$($left)*] $next [$current $($right)*]
            $out
        )
    };

    // Move tape right, empty right tape
    (
        [[> $($rest:tt)*] $($codes:tt)*]
        [$($left:tt)*] $current:tt []
        $out:tt
    ) => {
        b!(
            [[$($rest)*] $($codes)*]
            [$current $($left)*] [] []
            $out
        )
    };

    // Move tape right
    (
        [[> $($rest:tt)*] $($codes:tt)*]
        [$($left:tt)*] $current:tt [$next:tt $($right:tt)*]
        $out:tt
    ) => {
        b!(
            [[$($rest)*] $($codes)*]
            [$current $($left)*] $next [$($right)*]
            $out
        )
    };

    // Output
    (
        [[. $($rest:tt)*] $($codes:tt)*]
        $left:tt $current:tt $right:tt
        [$($out:tt)*]
    ) => {
        b!(
            [[$($rest)*] $($codes)*]
            $left $current $right
            [$($out)* $current]
        )
    };

    // Special handling for << token
    (
        [[<< $($rest:tt)*] $($codes:tt)*]
        $left:tt $current:tt $right:tt
        $out:tt
    ) => {
        b!(
            [[< < $($rest)*] $($codes)*]
            $left $current $right
            $out
        )
    };

    // >> token
    (
        [[>> $($rest:tt)*] $($codes:tt)*]
        $left:tt $current:tt $right:tt
        $out:tt
    ) => {
        b!(
            [[> > $($rest)*] $($codes)*]
            $left $current $right
            $out
        )
    };

    // -> token
    (
        [[-> $($rest:tt)*] $($codes:tt)*]
        $left:tt $current:tt $right:tt
        $out:tt
    ) => {
        b!(
            [[- > $($rest)*] $($codes)*]
            $left $current $right
            $out
        )
    };

    // <- token
    (
        [[<- $($rest:tt)*] $($codes:tt)*]
        $left:tt $current:tt $right:tt
        $out:tt
    ) => {
        b!(
            [[< - $($rest)*] $($codes)*]
            $left $current $right
            $out
        )
    };

    // .. token
    (
        [[.. $($rest:tt)*] $($codes:tt)*]
        $left:tt $current:tt $right:tt
        $out:tt
    ) => {
        b!(
            [[. . $($rest)*] $($codes)*]
            $left $current $right
            $out
        )
    };
}

macro_rules! to_ascii {
    (++++++++++) => {"\n"};
    (++++++++++++++++++++++++++++++++) => {" "};
    (+++++++++++++++++++++++++++++++++) => {"!"};
    (++++++++++++++++++++++++++++++++++) => {"\""};
    (+++++++++++++++++++++++++++++++++++) => {"#"};
    (++++++++++++++++++++++++++++++++++++) => {"$"};
    (+++++++++++++++++++++++++++++++++++++) => {"%"};
    (++++++++++++++++++++++++++++++++++++++) => {"&"};
    (+++++++++++++++++++++++++++++++++++++++) => {"'"};
    (++++++++++++++++++++++++++++++++++++++++) => {"("};
    (+++++++++++++++++++++++++++++++++++++++++) => {")"};
    (++++++++++++++++++++++++++++++++++++++++++) => {"*"};
    (+++++++++++++++++++++++++++++++++++++++++++) => {"+"};
    (++++++++++++++++++++++++++++++++++++++++++++) => {","};
    (+++++++++++++++++++++++++++++++++++++++++++++) => {"-"};
    (++++++++++++++++++++++++++++++++++++++++++++++) => {"."};
    (+++++++++++++++++++++++++++++++++++++++++++++++) => {"/"};
    (++++++++++++++++++++++++++++++++++++++++++++++++) => {"0"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++) => {"1"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++) => {"2"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++) => {"3"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"4"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"5"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"6"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"7"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"8"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"9"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {":"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {";"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"<"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"="};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {">"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"?"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"@"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"A"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"B"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"C"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"D"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"E"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"F"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"G"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"H"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"I"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"J"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"K"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"L"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"M"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"N"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"O"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"P"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"Q"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"R"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"S"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"T"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"U"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"V"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"W"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"X"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"Y"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"Z"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"["};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"\\"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"]"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"^"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"_"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"`"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"a"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"b"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"c"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"d"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"e"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"f"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"g"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"h"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"i"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"j"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"k"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"l"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"m"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"n"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"o"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"p"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"q"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"r"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"s"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"t"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"u"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"v"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"w"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"x"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"y"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"z"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"{"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"|"};
    (+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"}"};
    (++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++) => {"~"};
}

const OUTPUT: &str = bf!(
    ++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.+++.------.--------.>>+.>++.
);

fn main() {
    print!("{}", OUTPUT);
}