fix doc test in ref package

[Yu-zh]

No description provided.

[peter-jerry-ye-code-review]

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Looking at the git diff, here are the potential issues I've noticed:

1. Inconsistent Closure Syntax: The example code changes from fn!() to fn (). This could be problematic if fn!() was specifically required for macro expansion or special handling. The removal of the ! might affect the behavior of the closure.

2. Documentation Accuracy: The example in the documentation asserts that x.val equals 1, but it's not immediately clear if this is correct given the operations performed. The code sets the value to 3 within the protect function, so the assertion's correctness depends on whether protect is supposed to roll back changes, which should be explicitly documented.

3. Missing Return Type Documentation: The example code shows protect being used, but there's no clear documentation about what type R (the return type) should be or what the function returns in different scenarios. This could lead to confusion for users of the API.

These are potential issues that might need attention based on the context shown in the diff.

[coveralls]

Pull Request Test Coverage Report for Build 4811

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.126%

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Totals
Change from base Build 4797:	0.0%
Covered Lines:	4872
Relevant Lines:	5861

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💛 - Coveralls