Add extra_rustc_flag and extra_exec_rustc_flag

[mypmc]

Retake of #1407.
Each new flags are singular, and appended after the flags passed to plural ones.

Use case:

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..."

[krasimirgg]

How are we going to deal with the Min Bazel version failing CIs again?

[mypmc]

How are we going to deal with the Min Bazel version failing CIs again?

PR for it. #1414

[krasimirgg]

How are we going to deal with the Min Bazel version failing CIs again?

PR for it. #1414

Perfect! I believe we'll need to land that first, otherwise we won't be able to push this with ongoing failing buildbots.

[mypmc]

@krasimirgg ping; what should I do next??

[UebelAndre]

Can yo make sure the new flags are added to docs? You should only need to update https://github.com/bazelbuild/rules_rust/blob/0.6.0/docs/symbols.bzl and https://github.com/bazelbuild/rules_rust/blob/0.6.0/docs/BUILD.bazel#L46-L60. Then CI will tell you to run ./docs/update_docs.sh from the root of the repo.

[krasimirgg]

Latest version looks good.