Don't split return type if there is a comment after function metadata. (#1579)

This situation rarely occurs in practice, since users typically put the
comment before the metadata too (which is why we never noticed this bug
until now).

But if it does occur, it's definitely wrong to force the return type to
split unnecessarily.

Author: munificent

lib/src/front_end/piece_factory.dart

@@ -616,6 +616,19 @@ mixin PieceFactory {
       return;
     }
 
+    // Hoist any comments before the function so they don't force a split
+    // between the return type and function. In most cases, this doesn't matter
+    // because the [SequenceBuilder] for the surrounding code will separate out
+    // the leading comment. But if there is a metadata annotation followed by
+    // a comment, then the function, then the comment doesn't get captured by
+    // the [SequenceBuilder], as in:
+    //
+    //     @meta
+    //     // Weird place for comment.
+    //     int f() {}
+    var leadingComments =
+        pieces.takeCommentsBefore(returnType.firstNonCommentToken);
+
     var returnTypePiece = pieces.build(() {
       for (var keyword in modifiers) {
         pieces.modifier(keyword);
@@ -628,7 +641,8 @@ mixin PieceFactory {
       writeFunction();
     });
 
-    pieces.add(VariablePiece(returnTypePiece, [signature], hasType: true));
+    pieces.add(prependLeadingComments(leadingComments,
+        VariablePiece(returnTypePiece, [signature], hasType: true)));
   }
 
   /// If [parameter] has a [defaultValue] then writes a piece for the parameter

test/tall/function/comment.unit

@@ -21,4 +21,12 @@ main() {
       // nested
     }
   }
+}
+>>> Don't split return type if comment before.
+// Comment.
+int f() {;}
+<<<
+// Comment.
+int f() {
+  ;
 }

test/tall/function/metadata.unit

@@ -95,4 +95,14 @@ f([
   @metadata
   @another(argument, argument)
   callback() = constantFunction,
-]) {}
+]) {}
+>>> Don't split return type if comment after metadata.
+@meta
+// Comment.
+int f() {;}
+<<<
+@meta
+// Comment.
+int f() {
+  ;
+}

test/tall/regression/other/dart.unit

@@ -39,4 +39,19 @@ main() {
           .transform(utf8.decoder)
           .transform(const LineSplitter())
           .asBroadcastStream();
+}
+>>> Don't split return type if comment after metadata annotation.
+class Benchmark {
+  @override
+  // A rate of one run per 2s, with a millisecond of noise.  Some variation is
+  // needed for Golem's noise-based filtering and regression detection.
+  double
+  measure() => (2000 + Random().nextDouble() - 0.5) * 1000;
+}
+<<<
+class Benchmark {
+  @override
+  // A rate of one run per 2s, with a millisecond of noise.  Some variation is
+  // needed for Golem's noise-based filtering and regression detection.
+  double measure() => (2000 + Random().nextDouble() - 0.5) * 1000;
 }