The leading commas thing is common in MS SQL. When it auto-generates SQL queries etc (in SSMS - SQL Server Management Studio etc), it, by default, uses the leading comma convention. So, there is at least some small amount of prior art. Also, as the OP mentioned, it is common in Haskell to use the leading comma convention, so, I’m not so sure the “No prior art” argument is as absolute as implied.
That being said, as I previously mentioned, I’m not particularly a fan of the leading comma convention and would be happier if it were not supported in Rust, but, I can think of no good reason (beyond personal preference) not to support it.