Adds the filter_target flag when gathering cargo metadata.

[amzn-aeline]

Unless building for all targets is specified, --filter_target will be added to the cargo command.
The target is chosen as follows:

1. If the user specified a target, that target is used
2. Otherwise, attempt to invoke rustc and determine the default target
3. If all else fails, build all targets.

Similar to: rust-lang/rust-analyzer#6912

[jonhoo]

For some more context on this kind of change, see rust-lang/rust-analyzer#6908.

Also, just for clarity, the Cargo flag here is --filter-platform, not --filter_target :)

[jonhoo]

I would probably just do:

Suggested change

	None
	} else {
	None
	}
	}
	None

[jonhoo]

Also, is it worth maybe giving a warning or something if this fails?

[pinkforest]

Thanks amzn-aeline for the contribution and thanks for the review jonhoo

And thanks for the context - so the --filter-platform was added in 1.38 ?

[jonhoo]

Yes, looks like it. Though since we're reading rustc -v already, we could perhaps return None on 1.37 and below as well?

[pinkforest]

Would be lovely if there is a crate to invoke / interrogate rustc metadata sans Command and duplicating the code here ?

In the long run would be happy to get rid of all Command's and use some another crate for it that handles all the version diff's

In 1.37 the behaviour is going to be different even if None ? Not too concerned about old rustc versions but just curiosity wise.

Cheers

[jonhoo]

I don't know of any unfortunately. Should be easy enough though. Maybe it could even be added as part of cargo_metadata?

I don't know what cargo metadata did by default before --filter-platform was added. I suspect it just always grabbed for all targets, which is the same thing None will do here, so I think that'd be okay.