How to use filegroup with target|file syntax in please build? - please.build

Question

I'm using the please build tool and I'm having trouble using the target|file syntax with a filegroup. Specifically, I want to use specific files from the filegroup output in a genrule, but the syntax doesn't seem to work. Here's an example of what I'm trying to do:
# subdir/BUILD.plz
this = package_name()
a = text_file(
name = 'a_file',
content = "aaaaaaaaa",
)
b = text_file(
name = 'b_file',
content = "bbbbbbbbb",
)
c = text_file(
name = 'c_file',
content = "ccccccccc",
)
filegroup(
name = this,
srcs = {
'A': [a],
'B': [b],
'C': [c],
},
visibility = ['PUBLIC']
)
# BUILD.plz
sub = '#//subdir'
a_and_c_concat = genrule(
name = 'a_and_c_concat',
srcs = {
'A': [f'{sub}|A'],
'C': [f'{sub}|C'],
},
outs = ["out"],
cmd = """
set -eux
cat "${SRCS_A}" "${SRCS_C}" > "${OUTS}"
""",
visibility = ['PUBLIC'],
)
It does the expected behavior if I change the filegroup to be a genrule and 're-export' srcs to outs like so:
# subdir/BUILD.plz
...
genrule(
name = this,
srcs = {
'A': [a],
'B': [b],
'C': [c],
},
outs = {
'A': ['a'],
'B': ['b'],
'C': ['c'],
},
cmd = """
set -eux
cp "${SRCS_A}" "${OUTS_A}"
cp "${SRCS_B}" "${OUTS_B}"
cp "${SRCS_C}" "${OUTS_C}"
""",
visibility = ['PUBLIC']
)
Is there a way to use the target|file syntax with filegroup in Please build, or is there another way to achieve the same effect another way?
Any help or advice would be greatly appreciated. Thank you!

Answer 1

This isn't implemented at the moment, but I think it's a reasonable thing for Please to do.
Filegroups are just a thin wrapper around build_rule(), so they accept all the same arguments, however they don't expose named sources as named outputs. I've created an issue here to track this as a feature request:
https://github.com/thought-machine/please/issues/2701