[clang-tidy] Improved too-small-loop-variable with bit-field support

[PiotrZSL]

Summary

Implemented support for bit-field members as a loop variable
or upper limit. Supporting also non bit-field integer members.

Fixes issues: https://github.com/llvm/llvm-project/issues/58614

[PiotrZSL]

Ready for review.

[carlosgalvezp]

Thanks for the fix! I have some suggestions for improved readability.

clang-tools-extra/clang-tidy/bugprone/TooSmallLoopVariableCheck.cpp

109–110

Having an std::pair is a bit hard to read, as it's not clear what each element of the pair represents. Could you replace it with something like this, for improved readability and maintainability? Then you can also skip the utility header.

struct MagnitudeBits
{
  unsigned Width;
  bool IsBitField;
};

204

This is a bit confusing, could you keep it with %0 and %1 (which clearly represent "loop variable" and "upper bound"), and simply create a std::string with the appropriate content? Something like:

std::string type = LoopVarType.getAsString();
if (LoopVarMagnitudeBits.second) {
  type += ":" + std::to_string(LoopVarMagntiudeBits.second);
}
Msg << type;

215–220

Please create a helper function to remove duplication.

[Eugene.Zelenko]

@carlosgalvezp: Sorry, there are too much Clang specifics in this patch, so I could not be reviewer.

[carlosgalvezp]

In D142587#4152943, @Eugene.Zelenko wrote:

@carlosgalvezp: Sorry, there are too much Clang specifics in this patch, so I could not be reviewer.

No prob, thanks for letting us know :)

[PiotrZSL]

Review comments fix

[carlosgalvezp]

LGTM, thanks!

[Henric]

Hi
With this patch, the following code gives me a warning that I don't think is intentional.

typedef struct S {
  int x:4;
} S;

void foo(S s) {
  for (int i=10; i > s.x; --i) ;
}

loop_var.c:6:22: warning: loop variable has narrower type 'int:4' than iteration's upper bound 'int' [bugprone-too-small-loop-variable]
for (int i=10; i > s.x; --i) ;
                   ^

Looks like the logic i reversed here, so the loop variable gets the bit field type.

[PiotrZSL]

Thank you for information, I will look into this....
Indeed test for this scenario is missing.