[Feature Request] Allow the folder attribute to be any const identifier about rust-embed HOT 7 CLOSED

Yeah the interpolate-folder-path feature totally solved this for me. Maybe I should have completely read the docs before submitting and issue here!

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Yeah, I definitely see that now, I've written some proc macros myself and had not taken into account how they actually work. In particular, I imagine that a const value could be used in the debug case where the directory is searched at runtime since presumably the macro produces code in the host program that does this and thus would have access to a const identifier. However, in the release case I see that this would not be possible since the directory is read at compile time instead.

I did a quick test and verified that you are correct in that neither include_bytes nor include_str work with const identifiers, and explicitly say that they require a string literal.

I tried another approach described in a reddit post I just made here, but it also didn’t work. I’ll also note that the env macro approach does work for include_bytes and include_str. If you’re as stumped as I am, we’ll see if anyone in my reddit post has any ideas for workarounds.

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The value needs to be used during the proc macro execution, which doesn't have access to the rest of your program. Proc macros receive the input as tokens, so we can't resolve the value of a variable, even if it's const.

Unfortunately this means this feature is impossible.

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Thinking a little more on this, it might work if include_bytes supports it, but I don't think it does.

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You can actually include whatever you want in attributes, it's just that right now the library expects a string. Whatever you write there will still just be unevaluated tokens to the macro though. The proc macro does read some metadata about the file (ex. hash, last modified time), so it does want the real path during its execution, but besides that it would technically be possible to accept an expression that resolves at compile time (ex. env macro) to a string that include_bytes would accept.

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Yeah, based on my experience, I assumed that the proc macro itself is specifically looking for a string literal. I'm guessing that the syntax tree for the attribute could just be the macro expression, which does exist in syn for example. However, unless there's a a way inside the proc macro to actually evaluate the env macro I don't think it's possible to do this. It sounds like maybe this is just not possible.

Although, given that include_bytes does the same kind of things at compile time, it must be possible unless it's using some special black magic. I might look into how this is accomplished.

EDIT: Ok I quickly looked into this and include_bytes is a macro_rules! macro that basically does nothing with a comment that this is "compiler built-in", so evidently it is actually a compiler feature. This is exactly what I meant when I referred to special black magic.

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If you use the interpolate-folder-path feature you can use environment variables in the folder string, but that doesn't help much since now you'd need to set an env variable in addition to the const.

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