Honor library ignorefunction() settings when reporting unused functions.

[rgetz]

This change updates CheckUnusedFunctions to skip functions explicitly marked as ignored by the library configuration, both during per-file checks and whole-program analysis. This prevents false positives for known framework hooks, callbacks, or externally-used symbols that should be intentionally excluded from unused-function diagnostics.

I have found this very useful when looking at libraries, and creating a self.cfg file which looks like:

   <function name="iio_context_get_pdata">
     <ignorefunction>true</ignorefunction>
   </function>

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[chrchr-github]

Currently, the attribute is used in CheckClass to allow certain virtual functions to be called in constructors. I doubt that it is much used, but I'm not sure if we should repurpose/dual-purpose this feature. ignorefunction is also quite unspecific, and there is no documentation.
A test should be added.

[rgetz]

Thanks for pointing this out.

You’re right that ignorefunction is not documented in the reference PDF. However, it is a first-class element in the cfg schema itself, cppcheck/cfg/cppcheck-cfg.rng defines

<element name="ignorefunction">
    <ref name="DATA-BOOL"/>
</element>

which implies this attribute is intended to be a general library configuration signal rather than something check-specific or accidental. (happy to be corrected in my understanding if I'm wrong).

Given that:

• There is no documented restriction that ignorefunction is exclusive to CheckClass
• The name itself is generic (unlike e.g. constructor-specific attributes)
• The schema allows it for any function symbol

…using it to suppress unused-function diagnostics seems consistent with the schema-level intent: “this function should be ignored by certain analyses.” Today, unused-function reporting ignores that signal, which leads to false positives even when the user has explicitly annotated the function in the library config...

That said, I agree on two points:

• Documentation gap – the reference PDF should either document ignorefunction and describe its semantics, or explicitly scope it if it is meant to be limited.
• Tests – I will add a regression test covering unused-function suppression via ignorefunction (both per-file and whole-program modes). (based on feedback, if this is the right direction or not).

If folks prefer to keep ignorefunction narrowly scoped to class analysis, I’m open to introducing a more specific cfg attribute for unused-function suppression instead. But given the existing schema and naming, honoring it in CheckUnusedFunctions feels like the least surprising behavior for users today...

Just let me know.