Move default toolchain dependencies logic to .bzl file

[aszady]

Description

Purely refactoring change.
Encapsulates the logic of selecting dependencies into a function. Should come in handy in future changes.

Motivation

Originally #1290.
Partitioned from / inspired by #1552.

[simuons]

Please take a look at comments.

[simuons]

Was this "any" introduced to reduce duplication? I'm afraid this assumption that labels match between scala version might not always hold, leaving "any" obsolete.

I think some duplication here is fine. I would just list all major scala versions and deps lables for each dep_id.

imho that would look simpler. wdyt?

[aszady]

Indeed, any is here just to avoid duplication.

My goal was to keep the change minimal (here: not changing the logic of producing the relevant labels). This deduplication is not a new concept, as it expresses the same intention as the old version in the BUILD file, e.g.:

_SCALA_COMPILE_CLASSPATH_DEPS = [
    "@io_bazel_rules_scala_scala_compiler",
    "@io_bazel_rules_scala_scala_library",
] + (["@io_bazel_rules_scala_scala_reflect"] if SCALA_MAJOR_VERSION.startswith("2") else [
    "@io_bazel_rules_scala_scala_interfaces",
    "@io_bazel_rules_scala_scala_tasty_core",
    "@io_bazel_rules_scala_scala_asm",
    "@io_bazel_rules_scala_scala_library_2",
])

translated into:

    "scala_compile_classpath": {
        "any": [
            "@io_bazel_rules_scala_scala_compiler",
            "@io_bazel_rules_scala_scala_library",
        ],
        "2": [
            "@io_bazel_rules_scala_scala_reflect",
        ],
        "3": [
            "@io_bazel_rules_scala_scala_interfaces",
            "@io_bazel_rules_scala_scala_tasty_core",
            "@io_bazel_rules_scala_scala_asm",
            "@io_bazel_rules_scala_scala_library_2",
        ],
    },

[simuons]

I understand your intent. Probably it's just a matter of personal taste. Since it's private and encapsulated in a function I'm totally fine with that.

[simuons]

Is this new macro really needed? I mean scala_version attribute eventually should be passed to setup_scala_toolchain because it will be needed in target_setting. So maybe default classpaths should be added there if None was passed.

I think it is nice to have deps_id mappings extracted into separate bzl file with lookup exposed function. But for now I would avoid adding new macro and change setup_scala_toolchain to

setup_scala_toolchain(
    name = "default_toolchain",
    scala_compile_classpath = default_deps("scala_compile_classpath", SCALA_VERSION),
    scala_library_classpath = default_deps("scala_library_classpath", SCALA_VERSION),
    scala_macro_classpath = default_deps("scala_macro_classpath", SCALA_VERSION),
    use_argument_file_in_runner = True,
)

Similarly to what you did for declare_deps_provider

[aszady]

This macro is not necessary.

So since we have #1566 already merged (scala_version is now passed to setup_scala_toolchain), we can already proceed with default classpaths if None is provided.

I pushed the relevant revision.

[liucijus]

Thanks, @aszady!