[clang] Diagnostic message are incorrectly wrapped if colors are enabled

[tbaederr]

Because OS.tell() doesn't know about colors so the control characters count into the length as well, which means in

llvm-project/clang/lib/Frontend/TextDiagnostic.cpp

Lines 665 to 680 in 57ccb46

	uint64_t StartOfLocationInfo = OS.tell();
	// Emit the location of this particular diagnostic.
	if (Loc.isValid())
	emitDiagnosticLoc(Loc, PLoc, Level, Ranges);
	if (DiagOpts.ShowColors)
	OS.resetColor();
	if (DiagOpts.ShowLevel)
	printDiagnosticLevel(OS, Level, DiagOpts.ShowColors);
	printDiagnosticMessage(OS,
	/*IsSupplemental*/ Level == DiagnosticsEngine::Note,
	Message, OS.tell() - StartOfLocationInfo,
	DiagOpts.MessageLength, DiagOpts.ShowColors);
	}

we calculate the value for CurrentColumn incorrectly. On my terminal $COLUMNS is currently 120 (and llvm figures that out correctly), but the diagnostic message is wrapped even though it would fit in to the previous line just fine (as proven by passing -fmessage-length=130 or -fno-color-diagnostics):

[tbaederr]

CC @Sirraide? I think you've been working in this space lately. Not sure what to do about this issue. Colors are handled in llvm's stream stuff.

[Sirraide]

Yeah, I’ve run into that issue many times before in other projects; my solution is typically to parse the string we’re outputting and compute its column width manually by skipping ANSI escape sequences.

formatted_raw_ostream::UpdatePosition() already does this; I’d imagine that (at least on most platforms), we should be able to extend that function to recognise and skip ANSI excape sequences if colours are enabled.

[Sirraide]

skip ANSI excape sequences

ANSI escape sequences that denote colours are of the form "\033" "[" <characters> "m"; recognising that format and ignoring any characters that are part of them from the length calculation should be enough for everything Clang is doing (not all escape codes have that form, but I don’t think Clang is doing things like ‘move the cursor to row 3, column 7’).