Provide hints when struct/enum is never used

[fzyzcjy]

Changes

Close #1747

Checklist

• An issue to be fixed by this PR is listed above.
• New tests are added to ensure new features are working. Please refer to this page to see how to add a test.
• ./frb_internal precommit --mode slow (or fast) is run (it internal runs code generator, does auto formatting, etc).
• If this PR adds/changes features, documentations (in the ./website folder) are updated.
• CI is passing. Please refer to this page to see how to solve a failed CI.

Remark for PR creator

• ./frb_internal --help shows utilities for development.
• If fzyzcjy does not reply for a few days, maybe he just did not see it, so please ping him.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cbfb3eb) 99.25% compared to head (6c516d2) 99.25%.
Report is 30 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1763   +/-   ##
=======================================
  Coverage   99.25%   99.25%           
=======================================
  Files         357      357           
  Lines       14813    14827   +14     
=======================================
+ Hits        14703    14717   +14     
  Misses        110      110           

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