Rewrite the missing-space rule without a negative lookahead

`//foo` gets a space after its slashes unless the comment is an IDE
marker, `//noinspection` or `//$NON-NLS-n$`, which the tools recognise
only as written. The exemption was a negative lookahead inside the
matching pattern; it is now a second pattern that names the markers,
checked after the first one matches. The two forms accept the same
lines: the first pattern needs a non-space, non-slash character right
after the slashes, so the lookahead was always evaluated after the whole
run of slashes, which is where the second pattern looks as well.

Taken from google/google-java-format#1460, whose motivation stands here
too: regex engines without backtracking do not support lookahead, and
the markers are easier to read, and to share with doc/Comment as #23
asks, as a pattern of their own. The rest of that change does not apply:
`Strings.repeat` is already `String.repeat` here, and the markdown
branch belongs to google-java-format's own `///` handling.

FormatterTest pins the behaviour before and after: both markers, a
marker followed by text, `//foo`, `///foo`, and comments that already
have their space.

Author: eamonnmcmanus

open-java-format/src/main/java/com/palantir/javaformat/java/JavaCommentsHelper.java

@@ -167,10 +167,12 @@ private static boolean isJBangDirective(String text) {
         return matcher.lookingAt() && (matcher.group(1) != null || JBANG_DIRECTIVE_NAMES.contains(matcher.group(2)));
     }
 
+    private static final Pattern LINE_COMMENT_MISSING_SPACE_PREFIX = Pattern.compile("^(//+)[^\\s/]");
+
     // Preserve special `//noinspection` and `//$NON-NLS-x$` comments used by IDEs, which cannot
     // contain leading spaces.
-    private static final Pattern LINE_COMMENT_MISSING_SPACE_PREFIX =
-            Pattern.compile("^(//+)(?!noinspection|\\$NON-NLS-\\d+\\$)[^\\s/]");
+    private static final Pattern LINE_COMMENT_NO_SPACE_PREFIX =
+            Pattern.compile("^//+(noinspection|\\$NON-NLS-\\d+\\$)");
 
     private static String lineCommentPrefix(String line) {
         int prefixLength = 0;
@@ -185,7 +187,7 @@ private List<String> wrapLineComments(List<String> lines, int column0) {
         for (String line : lines) {
             // Add missing leading spaces to line comments: `//foo` -> `// foo`.
             Matcher matcher = LINE_COMMENT_MISSING_SPACE_PREFIX.matcher(line);
-            if (matcher.find()) {
+            if (matcher.find() && !LINE_COMMENT_NO_SPACE_PREFIX.matcher(line).find()) {
                 int length = matcher.group(1).length();
                 line = "/".repeat(length) + " " + line.substring(length);
             }

open-java-format/src/test/java/com/palantir/javaformat/java/FormatterTest.java

@@ -367,6 +367,33 @@ void onlyWrapLineCommentOnWhitespace_noLeadingWhitespace() throws Exception {
                         + "}\n");
     }
 
+    @Test
+    void lineCommentGetsItsMissingSpaceExceptIdeMarkers() throws Exception {
+        // `//foo` gets a space after any number of slashes, while the IDE markers `//noinspection` and `//$NON-NLS-n$`
+        // are recognised by their tools only without one and stay as written.
+        String input = "class T {\n"
+                + "  //noinspection unchecked\n"
+                + "  //$NON-NLS-1$ //$NON-NLS-2$\n"
+                + "  //$NON-NLS-12$ and more\n"
+                + "  //foo\n"
+                + "  ///foo\n"
+                + "  // bar\n"
+                + "  //  two spaces\n"
+                + "  void m() {}\n"
+                + "}\n";
+        String expected = "class T {\n"
+                + "  //noinspection unchecked\n"
+                + "  //$NON-NLS-1$ //$NON-NLS-2$\n"
+                + "  //$NON-NLS-12$ and more\n"
+                + "  // foo\n"
+                + "  /// foo\n"
+                + "  // bar\n"
+                + "  //  two spaces\n"
+                + "  void m() {}\n"
+                + "}\n";
+        assertThat(Formatter.create().formatSource(input)).isEqualTo(expected);
+    }
+
     @Test
     void throwsFormatterException() throws Exception {
         assertThatThrownBy(() -> Formatter.create().formatSourceAndFixImports("package foo; public class {"))