I found a LALR grammar for Rust on GitHub, compatible with Bison.

But it does not closely track the published Rust Grammar nor syntax rules in the Rust Reference. (See Rust-lang DOT org site.)

Although perhaps a Rust grammar could be extracted from the syntax rules in the Reference doc, it may be incomplete. For example, how do "as-yet-undocumented features" affect the current syntax rules in the Rust Reference?

So for now, it looks like C is easier to parse than Rust.