clang-format: AllowShortCaseLabelsOnASingleLine only for simple cases

[AdamBucior]

Hi,

I would like clang-format to put short case labels on single line only for simple single-case cases like:

switch (i) {
case 0: return 3;
case 1: return 7;
case 2: return 1;
case 3: return 5;
}

But not for more complicated multiple-case cases like:

switch (i) {
case 0:
case 1:
  return i;
case 2:
case 3:
  i *= 2;
  break;
}

Which clang-format -style="{AllowShortCaseLabelsOnASingleLine: true}" formats as:

switch (i) {
case 0:
case 1: return i;
case 2:
case 3: i *= 2; break;
}

which doesn't make it obvious that return i; is also done in case i is 0 (and similarly for case 2:).

The problem also exists for fallthrough cases like:

switch (i) {
case 0:
  g();
  [[fallthrough]];
case 1:
  f();
  break;
}

Which clang-format -style="{AllowShortCaseLabelsOnASingleLine: true}" formats as:

switch (i) {
case 0: g(); [[fallthrough]];
case 1: f(); break;
}

I propose to add a new option: AllowShortCaseLabelsOnASingleLine: SingleCases

The last example might be more challenging because clang-format would need a way to detect whether the previous case can fallthrough. I see 2 ways:

• Just check if there is a [[fallthrough]] attribute (this option would cause wrong formatting if someone forgets [[fallthrough]])
• Check for return or break (this option might be harder to implement correctly)

I'm fine with both ways.

[llvmbot]

@llvm/issue-subscribers-clang-format

[mydeveloperday]

https://reviews.llvm.org/D133571