[Feature request] Add per_file_swiftcopt / host_ per_file_swiftcopt flags

[csmulhern]

Thes per_file_copt1 and host_per_file_copt2 flags are super useful for customizing C/C++ compiler flags based on source path. A common use case is to e.g. optimize external dependencies or suppress warnings for sources in external/. It would be useful to have an analogue for swiftc invocations provided by rules_swift.

[keith]

I think this is a reasonable ask. One part that's potentially tough is there's no regex starlark functionality, so we couldn't mirror its format exactly.

In the meantime there is a mechanism for doing this with swift_package_configuration, but you have to have control over your swift toolchain, and you have to use features:

swift_package_configuration(
    name = "third_party_config",
    packages = ["@some_dep//:__subpackages__"],
    configured_features = ["swift.suppress_warnings"],
)

swift_toolchain(
    name = "my_toolchain",
    # ... other attrs ...
    package_configurations = [":third_party_config"],
)

xcode_swift_toolchain(
    name = "my_xcode_toolchain",
    # ... other attrs ...
    package_configurations = [":third_party_config"],
)

[csmulhern]

Yeah not having regex functionality in starlark is a really bummer. One thing we could consider is starting with only prefix matching functionality, and then extending to full regex support in the future once regex is added to starlark. This is what I did when I added a similar flag to rules_rust. See e.g.: https://github.com/bazelbuild/rules_rust/blob/8fd5c0fc1daea902b7bc618d373338e9a8e97b85/rust/private/rustc.bzl#L2482.

In the meantime there is a mechanism for doing this with swift_package_configuration, but you have to have control over your swift toolchain, and you have to use features:

Didn't know this was possible, thanks. Defining my own toolchain in the meantime might be reasonably doable.