Add NodeStart/NodeEnd debug events to WriterEvents

Author: nojaf

src/Fantomas.Core.Tests/CodeFormatterTests.fs

@@ -127,3 +127,28 @@ let b = 0
     match oak.ModulesOrNamespaces.[0].Declarations.[0] with
     | ModuleDecl.TopLevelBinding node -> Assert.That(node.HasContentBefore, Is.True)
     | _ -> Assert.Fail()
+
+[<Test>]
+let ``GetWriterEventsAsync emits NodeStart and NodeEnd events`` () =
+    let events =
+        CodeFormatter.GetWriterEventsAsync(false, "let x = 1") |> Async.RunSynchronously
+
+    let nodeStarts =
+        events
+        |> Array.choose (function
+            | WriterEvent.NodeStart(nodeType, _) -> Some nodeType
+            | _ -> None)
+
+    let nodeEnds =
+        events
+        |> Array.choose (function
+            | WriterEvent.NodeEnd(nodeType, _) -> Some nodeType
+            | _ -> None)
+
+    Assert.That(nodeStarts, Is.Not.Empty)
+    Assert.That(nodeEnds, Is.Not.Empty)
+    // Every NodeStart should have a matching NodeEnd
+    Assert.That(nodeStarts.Length, Is.EqualTo nodeEnds.Length)
+    // Oak is the root node
+    Assert.That(nodeStarts.[0], Is.EqualTo "Oak")
+    Assert.That(nodeEnds |> Array.last, Is.EqualTo "Oak")

src/Fantomas.Core/CodeFormatter.fs

@@ -110,7 +110,11 @@ type CodeFormatter =
             let ast, _ = asts.[0]
             let oak = ASTTransformer.mkOak (Some sourceText) ast
             let oak = Trivia.enrichTree config sourceText ast oak
-            let context = Context.Context.Create config
+
+            let context =
+                { Context.Context.Create config with
+                    DebugMode = true }
+
             let ctx = context |> CodePrinter.genFile oak
             return Context.dumpEvents ctx
         }

src/Fantomas.Core/CodePrinter.fs

@@ -137,7 +137,11 @@ let leaveNode<'n when 'n :> Node> (n: 'n) =
     col sepNone n.ContentAfter (genTrivia n)
 
 let genNode<'n when 'n :> Node> (n: 'n) (f: Context -> Context) =
-    enterNode n +> recordCursorNode f n +> leaveNode n
+    onlyIfCtx (fun ctx -> ctx.DebugMode) (writerEvent (NodeStart(n.GetType().Name, sprintf "%O" n.Range)))
+    +> enterNode n
+    +> recordCursorNode f n
+    +> leaveNode n
+    +> onlyIfCtx (fun ctx -> ctx.DebugMode) (writerEvent (NodeEnd(n.GetType().Name, sprintf "%O" n.Range)))
 
 let genSingleTextNode (node: SingleTextNode) = !-node.Text |> genNode node
 

src/Fantomas.Core/Context.fs

@@ -116,6 +116,8 @@ module WriterModel =
             | RestoreAtColumn c -> { m with AtColumn = c }
             | SetIndent c -> { m with Indent = c }
             | RestoreIndent c -> { m with Indent = c }
+            | NodeStart _
+            | NodeEnd _ -> m
 
         match m.Mode with
         | Dummy
@@ -171,17 +173,23 @@ module WriterEvents =
 
 [<System.Diagnostics.DebuggerDisplay("\"{Dump()}\""); NoComparison>]
 type Context =
-    { Config: FormatConfig
-      WriterModel: WriterModel
-      WriterEvents: Queue<WriterEvent>
-      FormattedCursor: pos option }
+    {
+        Config: FormatConfig
+        WriterModel: WriterModel
+        WriterEvents: Queue<WriterEvent>
+        FormattedCursor: pos option
+        /// When enabled, genNode emits NodeStart/NodeEnd WriterEvents around each Oak node.
+        /// Only used by CodeFormatter.GetWriterEventsAsync for diagnostic output.
+        DebugMode: bool
+    }
 
     /// Initialize with a string writer and use space as delimiter
     static member Default =
         { Config = FormatConfig.Default
           WriterModel = WriterModel.init
           WriterEvents = Queue.empty
-          FormattedCursor = None }
+          FormattedCursor = None
+          DebugMode = false }
 
     static member Create config : Context =
         { Context.Default with Config = config }

src/Fantomas.Core/Context.fsi

@@ -39,10 +39,15 @@ type WriterModel =
 
 [<System.Diagnostics.DebuggerDisplay("\"{Dump()}\""); NoComparison>]
 type Context =
-    { Config: FormatConfig
-      WriterModel: WriterModel
-      WriterEvents: Queue<WriterEvent>
-      FormattedCursor: pos option }
+    {
+        Config: FormatConfig
+        WriterModel: WriterModel
+        WriterEvents: Queue<WriterEvent>
+        FormattedCursor: pos option
+        /// When enabled, genNode emits NodeStart/NodeEnd WriterEvents around each Oak node.
+        /// Only used by CodeFormatter.GetWriterEventsAsync for diagnostic output.
+        DebugMode: bool
+    }
 
     /// Initialize with a string writer and use space as delimiter
     static member Default: Context

src/Fantomas.Core/WriterEvent.fs

@@ -16,3 +16,5 @@ type WriterEvent =
     | RestoreIndent of restoreIndent: int
     | SetAtColumn of setAtColumn: int
     | RestoreAtColumn of restoreAtColumn: int
+    | NodeStart of nodeType: string * range: string
+    | NodeEnd of endNodeType: string * endRange: string