Allow only parentheses in #[reflect(...)]

[dloukadakis]

Objective

Fixes #8906

#8906 reports that the reflect attribute silently fails when using the #[reflect[...]] syntax. I wasn't able to reproduce that issue, so it was likely resolved in a prior change. This pull request addresses only the syntax issue.

Solution

Validate the Meta::List delimiter in reflect macro to allow only MacroDelimiter::Paren

Testing

• Did you test these changes? If so, how?
  I run cargo check --example reflection_types
• Are there any parts that need more testing?
  No
• How can other people (reviewers) test your changes? Is there anything specific they need to know?
  Run cargo check --example reflection_types

Then modify the reflection_types example by changing:

#[reflect(Hash, PartialEq, Clone)]
pub struct E {
    x: usize,
}

to

#[reflect[Hash, PartialEq, Clone]]
pub struct E {
    x: usize,
}

then run cargo check --example reflection_types again it should now fail with this error:

error: `#[reflect = "..."]` must use parenthesis `(` and `)`

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Please review the instructions for writing migration guides, then expand or revise the content in the migration guides directory to reflect your changes.

[alice-i-cecile]

Good fix, but I don't think that the migration guide is correct. It should instead focus on communicating that "this didn't work before either, but it led to silent bugs".

[dloukadakis]

#8906 reports that the reflect attribute silently fails when using the #[reflect[...]] syntax. I wasn't able to reproduce that issue, so it was likely resolved in a prior change. This pull request addresses only the syntax issue.

@alice-i-cecile I initially forgot to clarify why this pull request focuses only on the syntax, please see the updated pull request description.

[DaAlbrecht]

thanks! I think that makes it clear why it got removed