Allow multiple uses of extra_rustc_flags

[mypmc]

I would like to support the following use cases.

In .bazelrc.

build:dev --@rules_rust//:extra_rustc_flags="-C..."
build:test --config=dev
build:test --@rules_rust//:extra_rustc_flags="-C..." # add additional flags
build:fuzz --config=test
build:fuzz --@rules_rust//:extra_rustc_flags="-Cpasses=sancov,-Cllvm-args=-sanitizer-coverage-level=3,-Cllvm-args=-sanitizer-coverage-inline-8bit-counters,-Znew-llvm-pass-manager=no"

[mypmc]

out of luck :(

Bazel 4.0.0's baseline was cut on Nov 18. The allow_multiple parameter to config.string() was added on Dec 10. You can use a version of Bazel closer to head if you want to experiment with this feature.

[mypmc]

Whenever there is a new Bazel LTS release, all releases of rules_rust will maintain support for the older LTS version for at least 3 months unless Bazel doesn't allow this.

Bumped the min bazel version but I have no idea this met the rules_rust policy.

[hlopko]

Could you move the min supported bazel version bump to a separate PR please? It's a big-ish deal so it's good to not have it hidden in this PR.

[hlopko]

Otherwise looks good to me, but I'll kindly ask @krasimirgg to double check.

[krasimirgg]

I'm a bit concerned with the switch from a string list to a string and the "always-on" splitting-by-comma that this change introduces. IMO we need to have a good way to escape commas. An example is something like this where we want to pass arguments with commas in them though rustc all the way down to the linker, e.g., pass -Clink-arg=-Wl,-Bstatic as a single argument in an invocation like rustc ... -Clink-arg=-Wl,-Bstatic.

Is there a supported mechanism to allow multiple string_list flags in Bazel?

[mypmc]

-Clink-arg=-Wl,-Bstatic as a single argument

mm, I didn't take that case into consideration because:

• config.string_list is comma separated, IIUC no way to escape commas.
• config.string_list doesn't allow multiple uses.

But it is helpful to be able to pass arguments including commas.

[UebelAndre]

Perhaps this should be an upstream issue for Bazel to allow multiple string_list flags?

[mypmc]

related??

• Add "Representing repeatable Starlark flags as config.string_list" proposals#248
• Allow string_list flags to be set via repeated flag uses bazel#14911

---

I have an another idea. How about introducing new build setting e.g., extra_rustc_flag?

build --@rules_rust//:extra_rustc_flags="-C...,-C...,-Z..."

build:dev --@rules_rust//:extra_rustc_flag="-C..."
build:dev --@rules_rust//:extra_rustc_flag="-C..."

build:test --config=dev
build:test --@rules_rust//:extra_rustc_flag="-C..."

build:fuzz --config=test
build:fuzz --@rules_rust//:extra_rustc_flag="-C..."
build:fuzz --@rules_rust//:extra_rustc_flag="-C..."
build:fuzz --@rules_rust//:extra_rustc_flag="-Z..."

Looks verbose but straightforward to extend flags. What do you think?

[krasimirgg]

I have an another idea. How about introducing new build setting e.g., extra_rustc_flag?

I like this, to clarify a bit the semantics I read from your proposal: we keep the extra_rustc_flags as-is (string_list), and extra_rustc_flag is string + allow_multiple=true, and each extra_rustc_flag is singular (not split by comma), and the extra_rustc_flag-s are appended after the extra_rustc_flags.

[mypmc]

Created #1413, so I will close this PR. Thanks for your review.