[rust] Cannot set the null value of enums, but can in c++, java, go

[espindola]

Given the included xml and reproducer script, one gets

java
out/java/repro/FooBarViaAttr.java:    NULL_VAL((short)254);
out/java/repro/FooBarViaType.java:    NULL_VAL((short)254);

cpp
out/cpp/repro/FooBarViaAttr.h:        NULL_VALUE = static_cast<std::uint8_t>(254)
out/cpp/repro/FooBarViaType.h:        NULL_VALUE = static_cast<std::uint8_t>(254)

go
out/golang/repro/FooBarViaAttr.go:var FooBarViaAttr = FooBarViaAttrValues{0, 1, 254}
out/golang/repro/FooBarViaType.go:var FooBarViaType = FooBarViaTypeValues{0, 1, 254}

rust
out/rust/repro/src/foo_bar_via_attr.rs:    NullVal = 0xff_u8, 
out/rust/repro/src/foo_bar_via_type.rs:    NullVal = 0xff_u8, 

So rust is the odd one out in not changing the encoding of the null value.

enum-null-value.xml

run.sh