Another Rust operator precedence bug

My earlier patch to fix PR rust/29859 introduced a new operator
precedence bug in the Rust parser.  Assignment operators are
right-associative in Rust.  And, while this doesn't often matter, as
Rust assignments always have the value (), still as a matter of
principle we should get this correct.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29859