Two changes:

• It was confusing (and not mentioned in the documentation) that rust_cargo_clippy_options required you to start the string with -- (such as -- -D warning). It now accepts both forms (so also -D warning).
• For some reason (AFAIK incorrectly) all cargo-specific flags were only supported for cargo check, while these are also valid for cargo build and cargo clippy. This now works as expected.

Aside from the unit tests, I tested both changes in my local Rust project, and they worked as expected.