[clang-format] Treat empty for/while loops as short loops (#70768)

A for/while loop with only a semicolon as its body should be treated as
an empty loop.

Fixes #61708.

Author: owenca

clang/lib/Format/UnwrappedLineParser.cpp

@@ -3117,9 +3117,16 @@ void UnwrappedLineParser::parseForOrWhileLoop(bool HasParens) {
       FormatTok->setFinalizedType(TT_ConditionLParen);
     parseParens();
   }
-  // Event control.
-  if (Style.isVerilog())
+
+  if (Style.isVerilog()) {
+    // Event control.
     parseVerilogSensitivityList();
+  } else if (Style.AllowShortLoopsOnASingleLine && FormatTok->is(tok::semi) &&
+             Tokens->getPreviousToken()->is(tok::r_paren)) {
+    nextToken();
+    addUnwrappedLine();
+    return;
+  }
 
   handleAttributes();
   parseLoopBody(KeepBraces, /*WrapRightBrace=*/true);

clang/unittests/Format/FormatTest.cpp

@@ -1355,18 +1355,20 @@ TEST_F(FormatTest, FormatIfWithoutCompoundStatementButElseWith) {
 }
 
 TEST_F(FormatTest, FormatLoopsWithoutCompoundStatement) {
+  verifyFormat("while (true)\n"
+               "  ;");
+  verifyFormat("for (;;)\n"
+               "  ;");
+
   FormatStyle AllowsMergedLoops = getLLVMStyle();
   AllowsMergedLoops.AllowShortLoopsOnASingleLine = true;
+
   verifyFormat("while (true) continue;", AllowsMergedLoops);
   verifyFormat("for (;;) continue;", AllowsMergedLoops);
   verifyFormat("for (int &v : vec) v *= 2;", AllowsMergedLoops);
   verifyFormat("BOOST_FOREACH (int &v, vec) v *= 2;", AllowsMergedLoops);
-  verifyFormat("while (true)\n"
-               "  ;",
-               AllowsMergedLoops);
-  verifyFormat("for (;;)\n"
-               "  ;",
-               AllowsMergedLoops);
+  verifyFormat("while (true);", AllowsMergedLoops);
+  verifyFormat("for (;;);", AllowsMergedLoops);
   verifyFormat("for (;;)\n"
                "  for (;;) continue;",
                AllowsMergedLoops);
@@ -1404,6 +1406,7 @@ TEST_F(FormatTest, FormatLoopsWithoutCompoundStatement) {
                "  a++;\n"
                "while (true);",
                AllowsMergedLoops);
+
   // Without braces labels are interpreted differently.
   verifyFormat("{\n"
                "  do\n"
@@ -1412,6 +1415,17 @@ TEST_F(FormatTest, FormatLoopsWithoutCompoundStatement) {
                "  while (true);\n"
                "}",
                AllowsMergedLoops);
+
+  // Don't merge if there are comments before the null statement.
+  verifyFormat("while (1) //\n"
+               "  ;",
+               AllowsMergedLoops);
+  verifyFormat("for (;;) /**/\n"
+               "  ;",
+               AllowsMergedLoops);
+  verifyFormat("while (true) /**/\n"
+               "  ;",
+               "while (true) /**/;", AllowsMergedLoops);
 }
 
 TEST_F(FormatTest, FormatShortBracedStatements) {