The standard CONFLICTS mechanism excludes the port itself. The problem is Rust fails to build if a previous non-compatible version is installed.
I copied the original CONFLICTS mechanism in the pre-fetch target and remove the origin comparison. The goal is to keep the standard behaviour beside the origin comparison.
Another change in lang/rust only is "backported" from rust-nightly: the hash appended to libraries is computed in the Makefile and it becomes a variable in the PLIST. I plan to commit this change separately.