transform: LoopToForLoop - fix bad emission

For loops only support assignments or function calls for the continuing statement.

Fixed: tint:1064
Change-Id: I07065b2119e7b9f97ca7e46b1464fd72333ca429
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/60212
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>

src/transform/loop_to_for_loop.cc

@@ -89,7 +89,8 @@ void LoopToForLoop::Run(CloneContext& ctx, const DataMap&, DataMap&) {
       return nullptr;
     }
 
-    // The continuing block must be empty or contain a single statement
+    // The continuing block must be empty or contain a single, assignment or
+    // function call statement.
     ast::Statement* continuing = nullptr;
     if (auto* loop_cont = loop->continuing()) {
       if (loop_cont->statements().size() != 1) {
@@ -97,6 +98,10 @@ void LoopToForLoop::Run(CloneContext& ctx, const DataMap&, DataMap&) {
       }
 
       continuing = loop_cont->statements()[0];
+      if (!continuing
+               ->IsAnyOf<ast::AssignmentStatement, ast::CallStatement>()) {
+        return nullptr;
+      }
 
       // And the continuing statement must not use any of the variables declared
       // in the loop body.

src/transform/loop_to_for_loop_test.cc

@@ -208,6 +208,32 @@ fn f() {
   EXPECT_EQ(expect, str(got));
 }
 
+TEST_F(LoopToForLoopTest, NoTransform_ContinuingIsCompound) {
+  auto* src = R"(
+fn f() {
+  var i : i32;
+  i = 0;
+  loop {
+    if ((i < 15)) {
+      break;
+    }
+    ignore(123);
+
+    continuing {
+      if (false) {
+      }
+    }
+  }
+}
+)";
+
+  auto* expect = src;
+
+  auto got = Run<LoopToForLoop>(src);
+
+  EXPECT_EQ(expect, str(got));
+}
+
 TEST_F(LoopToForLoopTest, NoTransform_ContinuingMultipleStmts) {
   auto* src = R"(
 fn f() {