fix: bxl target patterns didn't accept multiple args

[lf-]

I was surprised to learn that my code which the target exprs were used as a list no longer worked after a buck2 upgrade. These are documented as being a list, yet they were not actually parsed as such! https://buck2.build/docs/api/bxl/cli_args/#target_expr

I actually wonder if buck2 was eating the subsequent list elements before (... but I remember it working before??), because this parser seems to never have contemplated multiple target args.

def _collect_nix_deps(ctx):
    print(ctx.cli_args)
    pass

main = bxl.main(
    impl = _collect_nix_deps,
    cli_args = {
        "target": bxl.cli_args.target_expr("Target(s) to build and import"),
    },
    doc = """
        Extracts the direct/buck2-transitive Nix dependency list of a buck2 target into a messy json file.
    """,
)

Before:

$ cargo r -p buck2 -- bxl test.bxl:main -- --target //:foo --target //:bar
   Compiling buck2_bxl v0.1.0 (/Users/jade/co/buck2/app/buck2_bxl)
   Compiling buck2 v0.1.0 (/Users/jade/co/buck2/app/buck2)
    Finished `dev` profile [optimized + debuginfo] target(s) in 5.84s
     Running `target/debug/buck2 bxl 'test.bxl:main' -- --target '//:foo' --target '//:bar'`
buck2 daemon constraint mismatch: Version mismatch; killing daemon... Starting new buck2 daemon...
Connected to new buck2 daemon.
error: the argument '--target <target>' cannot be used multiple times

After:

   Compiling buck2_bxl v0.1.0 (/Users/jade/co/buck2/app/buck2_bxl)
   Compiling buck2 v0.1.0 (/Users/jade/co/buck2/app/buck2)
    Finished `dev` profile [optimized + debuginfo] target(s) in 9.08s
     Running `target/debug/buck2 bxl 'test.bxl:main' -- --target '//:buck2' --target '//:buck2_bundle'`
buck2 daemon constraint mismatch: Version mismatch; killing daemon... Starting new buck2 daemon...
Connected to new buck2 daemon.
struct(target=[gh_facebook_buck2//:buck2, gh_facebook_buck2//:buck2_bundle])

I have no way to run the tests
(#1027), so you will have to
forgive my attempt to add a test case here; try getting Claude to fix
the bits I missed.

The OSS build is down due to April Fools, so while I've tested this
locally with some build fix hacks, I don't think the external CI will
say much useful.

[meta-codesync]

@facebook-github-bot has imported this pull request. If you are a Meta employee, you can view this in D99137385. (Because this pull request was imported automatically, there will not be any future comments.)

[cjhopman]

I wonder if this is a case where it would be better to just update the documentation.

The documentation says: "Takes an arg from the cli, and treats it as a target pattern, e.g. "cell//foo:bar", "cell//foo:", or "cell//foo/...". We will get a list of TargetLabel in bxl."

The "we will get a list" part is about the fact that a single expression like "cell//foo:" can resolve to multiple targets, and that bxl gets the resolved targets. I'd have expected this to give both the resolved list and the original expression, but :/

I think (but not sure) it's intentional that this doesn't accept multiple arguments, and where you'd want to accept multiples you'd use a cli_args.list(cli_args.target_expr()). The cli_args.list() documentation deserves some update... like how do i actually pass a list? does it mean i can pass the arg multiple times?

I wonder if we were previously swallowing this and fixed it to give an error.