Don't allow invalid chars in Symbol objects.

[brson]

What

Validate that large Symbol values do not contain chars that are not allowed in Symbol types.

Why

The conversion from str to Symbol falls back to the object path if a SymbolSmall cannot be created. The Host::symbol_new_from_slice function though does not do the required validation:

impl<E: Env> TryFromVal<E, &str> for Symbol {
    type Error = ConversionError;

    fn try_from_val(env: &E, v: &&str) -> Result<Self, Self::Error> {
        if let Ok(ss) = SymbolSmall::try_from_str(v) {
            Ok(Self(ss.0))
        } else if let Ok(so) = env.symbol_new_from_slice(v) {
            Ok(Self(so.0))
        } else {
            Err(ConversionError)
        }
    }
}

Symbol::new_from_slice creates an ScSymbol and puts it into the environment, but ScSymbol is (currently) allowed to contain invalid chars:

    fn symbol_new_from_slice(&self, s: &str) -> Result<SymbolObject, HostError> {
        self.add_host_object(ScSymbol(s.as_bytes().to_vec().try_into()?))
    }

This patch changes symbol_new_from_slice to prevent invalid ScSymbols from being injected into the environment.

This patch adds several test cases. Only the invalid_chars case was failing. The others I added because it wasn't obvious to me whether the cases were being checked or not; especially it was not obvious to me from the docs whether zero-length symbols are allowed (they are). The docs for Symbol should be updated to indicate zero-length symbols are ok, but that patch needs to live in the sdk.

Known limitations

na

[dmkozh]

Thanks for fixing this!

[leighmcculloch]

🎉