A way to get an execpath to a directory rather than a file

[illicitonion]

Description of the feature request:

Several times, I've wanted to be able to reference an execpath to a directory, for the purposes of adding a directory containing several files to an include search path. Something along the lines of:

cc_library(
    name = "lib",
    srcs = [...],
    copts = ["-I$(execpath @openssl//include:__pkg__)"],
)

If you're lucky, some tools support /.. so you can write something like:

cc_library(
    name = "lib",
    srcs = [...],
    copts = ["-I$(execpath @openssl//include:openssl/some_header.h)/../.."],
)

but many do not.

Note that we want to be able to be very precise about which directory is returned. In a world where you have:

@openssl//include:BUILD.bazel:

filegroup(
    name = "headers",
    srcs = [
        "openssl/header1.h",
        "openssl/header2.h",
    ],
)

it may be important to specifically reference the execpath of @openssl//include not openssl//include/headers because of how the headers are #included.

Which category does this issue belong to?

Rules API

Have you found anything relevant by searching the web?

See discussion in https://bazelbuild.slack.com/archives/C014RARENH0/p1722250875317499?thread_ts=1721325151.660949&cid=C014RARENH0

[fmeum]

A scheme that I think would be very easy to fit into the current model (exact syntax TBD):

$(execpath-1 @openssl//include:openssl/some_header.h) --> external/openssl/include/openssl
$(execpath-2 @openssl//include:openssl/some_header.h) --> external/openssl/include
$(execpath-0 @openssl//include:openssl/some_header.h) --> external/openssl/include/openssl/some_header.h
$(execpath-3 @openssl//include:openssl/some_header.h) --> external/openssl
$(execpath-4 @openssl//include:openssl/some_header.h) --> <fails>