Read default annotation values from package's Cargo.toml metadata

[dtolnay]

This PR allows Cargo package authors to specify metadata that replaces the need for crate_universe users to write annotations.

Crate_universe will pick up default values for various annotations fields from the metadata provided by the package author. It still remains possible for the crate_universe user to write their own annotations for the crate anyway, in which case these override the default values distributed by the package author.

For example the cxx crate contains some C++ source code that must get linked by the user. To accomplish this, all users of the crate from Bazel would previously have needed to fill in the following annotation:

crates_repository(
    name = "...",
    annotations = {
        "cxx": [crate.annotation(
            additive_build_file_content = """
            cc_library(
                name = "cxx_cc",
                srcs = ["src/cxx.cc"],
                hdrs = ["include/cxx.h"],
                include_prefix = "rust",
                includes = ["include"],
                linkstatic = True,
                strip_include_prefix = "include",
                visibility = ["//visibility:public"],
            )
            """,
            extra_aliased_targets = {"cxx_cc": "cxx_cc"},
            gen_build_script = False,
        )],
    },
    ...
)

Now cxx ships this annotation as part of the package metadata in its Cargo.toml, and Bazel users can pull in cxx without handwriting their own crate.annotation.

[package.metadata.bazel]
additive_build_file_content = """
cc_library(
    name = "cxx_cc",
    srcs = ["src/cxx.cc"],
    hdrs = ["include/cxx.h"],
    include_prefix = "rust",
    includes = ["include"],
    linkstatic = True,
    strip_include_prefix = "include",
    visibility = ["//visibility:public"],
)
"""
extra_aliased_targets = { cxx_cc = "cxx_cc" }
gen_build_script = false

[dtolnay]

I considered a more TOML-native form for additive_build_file_content, but just using multiline string literal containing Starlark seems all right.

Support for the structured form can be introduced separately if desired.

[package.metadata.bazel]
extra_aliased_targets = { cxx_cc = "cxx_cc" }
gen_build_script = false

[[package.metadata.bazel.additive_build_file_content]]
[package.metadata.bazel.additive_build_file_content.cc_library]
name = "cxx_cc"
srcs = ["src/cxx.cc"]
hdrs = ["include/cxx.h"]
include_prefix = "rust"
includes = ["include"]
linkstatic = true
strip_include_prefix = "include"
visibility = ["//visibility:public"]

[UebelAndre]

This is a pretty awesome change! Could you also add a unit test for this? I'm thinking a Cargo.toml file somewhere in crate_universe/test_data and have a test that reads the [package.metadata.bazel] section?

[dtolnay]

I made a couple attempts at a test, but I haven't been able to figure out how to get crate_universe to pay attention to local path dependencies. It seems to only want to generate targets for crates.io dependencies (version) and git.

Here is what I have so far: affc1b5.

[UebelAndre]

I was thinking that the testing would just be in this file and cover the changes around here. Not necessarily a brand new test target. I think that'd get sufficient coverage of the functionality, right?

[dtolnay]

Ah, I get it now. Good call. I have added that test.

[UebelAndre]

Fantastic! Thank you so much!