Support multiple attribute occurrences

[zohnannor]

Synopsis

It is impossible to feature-gate additional type conversions in From, Into derive macros. Code such as:

#[derive(From)]
#[from(types(u8, u16))]
#[cfg_attr(feature = "myfeature", from(types(u32)))]
struct MyIntExplicit(u64);

will fail to compile with Only a single attribute is allowed when myfeature is enabled.

Solution

Instead of err-ing on second occurrence of the attribute when building MetaInfo, just proceed and collect all additional attributes into it. Anyway user will see the error message if they'll try to provide types to attributes, expansions of which will result in conflicting implementations.

As I understand the inner working of the macro, that's all there is to it to support multiple attributes?

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)

[zohnannor]

Seems like this error will be fixed in #241, #248 PRs. I'll test if those resolve this issue.

[tyranron]

@zohnannor seems to be resolved in the latest beta releases. Feel free to reopen if something more is required.