Conditional default enum value

help
1

There is a simple syntactical solution I can't figure out. For example, I have enum with a default value as below:

pub enum Compression {
    #[cfg(not(feature = "deflate"))]
    #[default]
    Store,
    Shrink,
    Reduction1,
    Reduction2,
    Reduction3,
    Reduction4,
    Implode,
    #[cfg(feature = "deflate")]
    #[default]
    Deflate,
    Deflat64,
    BZIP2,
    LZMA, 
    PPMd ,
}

I want to make a default value in dependency that a certain feature is set. However

#[cfg

directive get applied to the next enum value and ignores #[default]. How can I make the default conditional?

2

I tried this and it worked, so perhaps you have some other issue. When the feature is on, Deflate is the default, and when it is off, Store is the default. This does remove the non-default variant entirely, though. If you want still include the variants and just change which one is default, you can write them twice.

#[derive(Debug, Default)]
pub enum Compression {
    #[cfg(not(feature = "deflate"))]
    #[default]
    Store,
    #[cfg(feature = "deflate")]
    Store,
    Shrink,
    Reduction1,
    Reduction2,
    Reduction3,
    Reduction4,
    Implode,
    #[cfg(feature = "deflate")]
    #[default]
    Deflate,
    #[cfg(not(feature = "deflate"))]
    Deflate,
    Deflat64,
    BZIP2,
    LZMA,
    PPMd,
}

It's kind of ugly, though. I'd probably just impl Default manually at this point.

3

You should just manually impl it:

impl Default for Compression {
    fn default() -> Self {
        if cfg!(feature = "deflate") {
            Compression::Deflate
        } else {
            Compression::Store
        }
    }
}

(something like this, I'm not super familiar w/ cfg stuff)

4

Use cfg_attr to apply attribute itself conditionally.

7 Likes
5

Oh, that's a better idea.

#[derive(Debug, Default)]
pub enum Compression {
    #[cfg_attr(not(feature = "deflate"), default)]
    Store,
    Shrink,
    Reduction1,
    Reduction2,
    Reduction3,
    Reduction4,
    Implode,
    #[cfg_attr(feature = "deflate", default)]
    Deflate,
    Deflat64,
    BZIP2,
    LZMA,
    PPMd,
}
1 Like
6

Awesome, it worked as planned. Thanks to all for the help.