Generate a compile time array of data using "include_bytes!" [duplicate]

Question

I'm trying to load a folder of animation frames at compile time. The frames are named like this: frame1.png, frame2.png, ... frameN.png. Unfortunately after trying multiple methods for a while, I have come to the conclusion that this might not be possible. Am I correct?

Essentially the final data should look like this:

static DATA: &[&[u8]] &[
    [frame1 bytes],
    [frame2 bytes],
...
    [frameN bytes],
];

Here's what I've been trying to do:

macro_rules! import_img_anim {
    ($name:ident, $path:expr, $n:literal) => {
        static $name: &[&[u8]] = &[
            seq_macro::seq!(N in 1..=$n {
                include_bytes!(concat!($path, "/frame", stringify!(N), ".png")),
            })
        ];
    };
}

But the compiler gives this cryptic error message at include_bytes!: "macro expansion ignores token include_bytes and any following the usage of seq_macro::seq! is likely invalid in expression context"

Surely there's a way to do this? Rust's macros are widely regarded as immensely powerful, yet I can't even write a for loop in a comp-time context... (at least without including exernal crate)

"regarded as immensely powerful," that mostly applies to proc macros, declarative macros like you used are still quite powerful, but quite a bit less so than proc macros. — cafce25
– cafce25, Commented Feb 3 at 20:58
You can come up with a macro to do it (I did). The problem is that include_bytes! creates a sized array of bytes. So if you have a fixed frame size, you will have to specify it: play.rust-lang.org/… — Eugene Sh.
– Eugene Sh., Commented Feb 3 at 21:15

Aatos Tapper · Accepted Answer · 2025-02-03 21:36:18Z

0

Okay I finally found the answer thanks to Eugene Sh.'s code:

use seq_macro::seq;

macro_rules! include_each {
    ($n:literal) => {
        seq!(N in 0..$n {
            [#(include_bytes!(stringify!(file~N)),)*]
        })
    };
}

const FRAME_SIZE: usize = 8;
static DATA: [&[u8; FRAME_SIZE]; 2] = include_each!(2);

Which I modified into this:

#[macro_export]
macro_rules! import_img_anim {
    ($path:literal, $n:literal, $extension:literal) => {
        seq_macro::seq!(N in 1..=$n {
            [#(include_bytes!(concat!($path, "/frame", stringify!(N), $extension)),)*]
        })
    };
}

The problem seemed to be this part of the code:

seq_macro::seq!(N in 1..=$n {
    include_bytes!(concat!($path, "/frame", stringify!(N), ".png")),
})

Which just put the include_bytes! macro there without anything else.

Thanks again Eugenne Sh.!

answered Feb 3 at 21:36

Aatos Tapper

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3 Comments

Eugene Sh. Feb 3 at 21:40

I find it very helpful to have a key binding in VSCode (if you are using it) to expand macros.

Aatos Tapper Feb 3 at 21:44

Oh, that seems like a good idea. How can I do that? (Sorry if this isn't a relevant question in stack overflow...)

Eugene Sh. Feb 3 at 21:49

Ctrl+Shift+P and type/lookup "rust-analyzer: Expand macro recursively at caret". Then there is a cogwheel symbol next to it, which you click and configure the keyboard shortcut.

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