pub fn to_platform_spec(&self) -> Result<PlatformSpec, TargetSpecError> {
        match self {
            PlatformSpecSummary::Always => Ok(PlatformSpec::Always),
            PlatformSpecSummary::Platform(platform) => {
                Ok(PlatformSpec::Platform(Arc::new(platform.to_platform()?)))
            }
            PlatformSpecSummary::Any => Ok(PlatformSpec::Any),
        }
    }
Converts `self` to a `PlatformSpec`.

Returns an `Error` if the platform was unknown.
guppy/src/platform/summaries.rs