Compiler warnings not shown on subsequent compilations

[rkost]

Describe the problem

Compiler warnings for code from non-.ino source files are not shown on subsequent sketch compilations.

To reproduce

When someone writes code like this (see below) he will correctly get two identical warnings:

warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]

When one compiles it again the warnings will all disappear so the user (me) thinks he is perfectly fine and writing clean code.

FunFact: This will not happen if all code is written to the main (.ino) file.

File DummyClass.h:

class DummyClass
{
public:
  char* getString();

  char* getSecondString();
};

File DummyClass.cpp:

char* DummyClass::getString()
{
  return "Some String";
}

char* DummyClass::getSecondString()
{
  return "Some other string";
}

File test.ino:

#include "DummyClass.h"
DummyClass dummy;

void setup()
{
  Serial.begin(115200);
}
void loop()
{
  Serial.println(dummy.getString());
  delay(1000);
}

Expected behavior

Compiler warnings are shown every time the sketch is compiled if the code would produce a warning.

Arduino CLI version

60a8aa9

Operating system

All

Operating system version

Any

Additional context

Additional reports

• https://forum.arduino.cc/t/arduino-ide-2-0-3-duplicate-tab-page/1084561/5
• https://forum.arduino.cc/t/warning-disappears-after-second-compilation/1139615
• https://forum.arduino.cc/t/bad-behaviour-using-keypad/1231737/9
• https://forum.arduino.cc/t/feature-request-clean-build-option/1291789/9

Related

• Do not recompile the sketch if there are no changes and libraries have not changed #2204

Issue checklist

• I searched for previous reports in the issue tracker
• I verified the problem still occurs when using the nightly build
• My report contains all necessary details

[matthijskooijman]

This is caused by caching: the second time the file is not recompiled if it was unchanged, so the warnings are not shown. The only way I can see to fix this (other than making warnings into errors), is to cache the stdout/stderr output as well, so it can be shown again even when not running the compiler. Not sure if this is the best way to approach this, though.

[facchinm]

@matthijskooijman is correct, caching elides warnings from previous compilations. I believe that ccache caches stdout/stderr and replays them even if the cached version is unchanged, so the approach could be ok, we only need to find a suitable "backend" for storing this information (taking care it also works in a cloud infrastructure)

[per1234]

There is some valuable related discussion here: arduino/arduino-builder#301

Matthijs was kind enough to add a link to here on that end, but it seems the backtrack reference generated by GitHub was lost when I transferred this issue from the arduino/Arduino repo.