@configuration syntax

What

Command

buck2 build //foo:bar@config//platform:linux-x86_64

should be equivalent to current syntax:

buck2 build //foo:bar --target-platforms=//platform:linux-x86_64

Why

Might be convenient if we define global (or per-target, as proposed in target configuration discovery RFC) alias. For example, if there's an alias

release=//config:linux-x86_64-release

The command above can be expressed as:

buck2 build //foo:bar@release

Additionally, if we have configuration expression RFC implemented, we can do something like:

buck2 build //foo:bar@release+gcc

Possible future extensions

For now, at-syntax only applies to command line arguments

• of build/targets/run/test commands
• probably cquery query

It would be reasonable to expect that this syntax should be allowed anywhere we need a target (e.g. in deps attribute), but this is out of scope of this proposal.