Imply --all argument for virtual manifest.

[matklad]

If I run cargo build in a workspace, I often get:

error: manifest path `/home/user/projects/fall/Cargo.toml` is a virtual manifest, but this command requires running against an actual package in this workspace

The fix is to run cargo build --all instead. I think Cargo can figure this out automatically?

If the current package is virtual, and we are running one of --all enabled commands, --all should be the default.

The fix would involve changing this logic in all relevant commands (there's quite a bit of duplication:( ).

[alexcrichton]

👍

[rsertelon]

I've started to try to implement this. However, as I said in #4016, I'm a rust beginner and my code might not be idiomatic ;)

What I've tried for now is creating a method in cargo::core::Workspace to know if it's virtual or not, then simply used this method in the check for the --all flag. Like so:

pub fn is_virtual(&self) -> bool {
    match *self.packages.get(&self.current_manifest) {
        MaybePackage::Package(..) => false,
        MaybePackage::Virtual(..) => true
    }
}

and then instead of

let spec = if options.flag_all {
 // ...
}

I have this:

let spec = if options.flag_all || ws.is_virtual() {
 // ...
}

Does it seem sound to you? Or is there a better way for the is_virtual() method?

At first I thought of using the existing current_opt() method (and use the is_none() method of Option), but I thought the meaning of it wouldn't be explicit enough and might cause bugs somehow one day...