indentWithTabs ought to preserve C-style multiline comments

[nedtwigg]

As noted here, our indentWithTabs space has some surprising behavior when used with C-style multiline comments, for example:

/**
 * A simple greeting task.
 */
abstract class GreetingTask extends DefaultTask {}

Gets errored out because Spotless dumps the leading spaces

/**
-·*·A·simple·greeting·task.
-·*/
+*·A·simple·greeting·task.
+*/

A hard limitation is that the indentWithTabs step can't know what language it is indenting. But it is pretty easy to detect that the leading whitespace of a line was terminated by a *, and then note that the indentation had one more space than expected, and just preserve that space.

You would detect that the first non-whitespace character is a * here to set boolean mightBeMultilineComment = true/false.

spotless/lib/src/main/java/com/diffplug/spotless/generic/IndentStep.java

Lines 88 to 102 in b3a6e36

	char c;
	while (contentStart < raw.length() && isSpaceOrTab(c = raw.charAt(contentStart))) {
	switch (c) {
	case ' ':
	++numSpaces;
	break;
	case '\t':
	numSpaces += state.numSpacesPerTab;
	break;
	default:
	throw new IllegalArgumentException("Unexpected char " + c);
	}
	++contentStart;
	}

And then do some remainder math here to add an extra space if necessary.

spotless/lib/src/main/java/com/diffplug/spotless/generic/IndentStep.java

Lines 111 to 115 in b3a6e36

	case TAB:
	for (int i = 0; i < numSpaces / state.numSpacesPerTab; ++i) {
	builder.append('\t');
	}
	break;

I don't plan to implement this, but I'm happy to merge a PR for this (current test is here).

[nedtwigg]

@sormuras was nice enough to contribute something similar to this back in #82, but I rejected it at that time as a breaking change. The key change in scope that I believe makes this a ### Fixed rather than a **BREAKING CHANGE** is:

• detects that first non-whitespace characters is *
• then and only then it will permit a single extra space character to persist

[vpavic]

I opened #1072 to address this.

The proposed solution does this:

• detects that first non-whitespace characters is *
• then and only then it will permit a single extra space character to persist