Fix order sensitivity of Display attributes (#319)

[tyranron]

Resolves #319

Synopsis

See #319 (comment):

The following code used to work in 1.0.0-beta.3:

#[derive(Display)]
#[display("{name}@{tag}")]
#[display(bound(Tag: std::fmt::Display))]
struct Specifier<Tag> {
    name: String,
    tag: Tag,
}

However, in 1.0.0-beta.6, it emits the following error message:

error: multiple `#[display("...", ...)]` attributes aren't allowed

It took me a long time to figure out that the fix was to swap place between the 2 display attributes.

The regression lies here, where mem::replace doesn't provide the implied and expected "and" logic for merging two Options.

Solution

• Merge those two Options correctly.
• Recheck similar places over the code.

Checklist

• Documentation is updated (not required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (not required)