Use keep case for Rust union discriminant type. (#7321)
Use keep case for Rust union discriminant type. (#7321)

Fixes #7320.

I realize that Rust doesn't really follow the Namer convention, since
it uses Field case for methods... that's a future problem.

Co-authored-by: Casper Neo <9cb2c90f57de1edddf9d236217d1c2358404100c@google.com>
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