Allow constructing an empty GlobSet in const

[dtolnay]

My use case: I have a struct that contains a GlobSet, among other information:

pub struct Thing {
    pub extra_srcs: GlobSet,
    pub flags: Vec<String>,
    ...
}

and some instances of this struct held in a different data structure — imagine this is akin to Cargo.toml's [dependencies] and [target.'cfg(…)'.dependencies]:

pub struct ConfigFile {
    common: Option<Thing>,
    platform_specific: BTreeMap<Platform, Thing>,
}

and a function where it would be really convenient to be able to use a const to get a sufficiently long-lived reference to a static Thing containing an empty globset:

impl ConfigFile {
    pub fn things(&self) -> impl Iterator<Item = &Thing> {
        self.common
            .iter()
            .chain(self.platform_specific.values())
            .chain([
                const {
                    &Thing {
                        extra_srcs: GlobSet::empty(),  // <---
                        flags: Vec::new(),
                        ...
                    }
                },
                ...
            ])
    }
}

Without being able to call GlobSet::empty() in const, I would need to do one of the following workarounds instead:

• Store Option<GlobSet> instead of just GlobSet, where None means the same thing as GlobSet::empty().

• Find a place to put my constant Thing at runtime, either inside ConfigFile (awkward) or some other storage that is passed in to things (awkward).

• Switch to Cow<'_, Thing> and sprinkle the implementation of things with Cow::Owned and map(Cow::Borrowed).

• Clone everything every time we iterate this data structure.

• Something with a static OnceLock.