Detect bugprone downcounting loops on unsigned loop variables

[LegalizeAdulthood]

Bugzilla Link	47975
Version	unspecified
OS	Windows NT

Extended Description

Suppose you have a loop like this:

for (int i = maxLevels; i >= 0; --i)
  doSomething(i);

This is perfectly fine as lone as i remains a signed quantity.

Now suppose someone changes the loop variable to unsigned, perhaps because doSomething takes an unsigned int argument and they think "static casts are ugly", so they change the loop to:

for (unsigned int i = maxLevels; i >= 0; --i)
  doSomething(i);

Now they have transformed a finite loop into an infinite loop. On the intended last iteration where i = 0, it will be "decremented", which could wraparound to MAX_UINT, resulting in the loop continuing to execute, possibly interminably.

Create a clang-tidy check that warns about downcounting loops on unsigned quantities where the loop termination test is >= 0.

[LegalizeAdulthood]

Does that check get reported by clang-tidy?

[llvmbot]

We already have the -Wtautological-unsigned-zero-compare flag (https://clang.llvm.org/docs/DiagnosticsReference.html#id763) that diagnoses cases where unsigned comparisons against zero always evaluate to a constant

[LegalizeAdulthood]

First pass on this check would be to just warn based on identifying the loop condition.

Second pass would be to not warn if the interior of the loop contains control flow that exits the loop when i == 0.