Stringy serialization fails with `Error { inner: UnsupportedType(None) }` about toml

[xmppftw]

Could it be because serializing a simple string without an associated key is not supported? I noticed that this also fails with the same error message:

    let foo = String::from("test");
    println!("{} - {:?}", &foo, toml::to_string(&foo));

I did not find something about string literals without keys in the TOML spec. If so, it's not a bug but rather a quality of life request to have an actually useful error message in this case saying that a key is required.

[epage]

Note: v0.5 and before, TOML values and documents were conflated together and this wouldn't have errrored but would have created an invalid TOML document. We've since made been more strict on the difference which also fixed some corner cases in what we can parse.

For toml, we've publicly exposed the error types. Technically, its exhaustive and we can add new variants but this is really a special case of UnsupportedType, so I'd rather not change away from that.

When we deserialize, we do so by converting it into a toml_edit::Value and then converting that into a toml_edit::Document. This allows us to reuse a lot of code but it prevents us from getting a more precise type to give to UnsupportedType.

We could hack in toml_edit::Value::type_name but I think that is misleading, telling the user a type in terms of TOML that is unsupported when the error is reporting things in terms of rust types.

I'm inclined to not act on this.