Merge pull request fsprojects#3234 from nojaf/fix-2944

Fix indentation loss for comments in no-break infix expressions

Author: nojaf

CHANGELOG.md

@@ -14,6 +14,7 @@
 - Dynamic operator on result of qualified function call causes formatting error. [#3135](https://github.com/fsprojects/fantomas/issues/3135)
 - Long delegate type with generic args no longer breaks around arrow. [#2468](https://github.com/fsprojects/fantomas/issues/2468)
 - Idempotency problem when formatting NUnit Assert.That with lambda argument. [#1740](https://github.com/fsprojects/fantomas/issues/1740)
+- Comment between lines of no-break infix expression no longer loses indentation. [#2944](https://github.com/fsprojects/fantomas/issues/2944)
 
 ## [8.0.0-alpha-002] - 2025-12-15
 

src/Fantomas.Core.Tests/OperatorTests.fs

@@ -1422,3 +1422,21 @@ let allDecls =
     @+ iimplsLs
     @+ ctorLs
 """
+
+[<Test>]
+let ``comment between lines breaks indentation, 2944`` () =
+    formatSourceString
+        """
+5 =
+  // a comment
+  5
+"""
+        { config with IndentSize = 2 }
+    |> prepend newline
+    |> should
+        equal
+        """
+5 =
+  // a comment
+  5
+"""

src/Fantomas.Core.Tests/QuotationTests.fs

@@ -122,10 +122,10 @@ test
             .Replace("8.12", "8.13") // CRAP score rounding
             .Replace("4.12", "4.13") // CRAP score rounding
             .Trim([| '\u00FF' |]) = expected
-            .Replace('\r', '\u00FF')
-            .Replace('\n', '\u00FF')
-            .Replace("\u00FF\u00FF", "\u00FF")
-            .Trim([| '\u00FF' |])
+                .Replace('\r', '\u00FF')
+                .Replace('\n', '\u00FF')
+                .Replace("\u00FF\u00FF", "\u00FF")
+                .Trim([| '\u00FF' |])
     @>
 """
 

src/Fantomas.Core/CodePrinter.fs

@@ -812,36 +812,55 @@ let genExpr (e: Expr) =
                 ctx
 
     | Expr.InfixApp node ->
-        let genOnelinerInfixExpr (node: ExprInfixAppNode) =
-            let genExpr e =
-                match e with
-                | Expr.Record _
-                | Expr.AnonStructRecord _ -> atCurrentColumnIndent (genExpr e)
-                | _ -> genExpr e
+        let genRhsExpr e =
+            match e with
+            | Expr.Record _
+            | Expr.AnonStructRecord _ -> atCurrentColumnIndent (genExpr e)
+            | _ -> genExpr e
 
+        let genShortInfixExpr =
             genExpr node.LeftHandSide
             +> sepSpace
             +> genSingleTextNode node.Operator
             +> sepNlnWhenWriteBeforeNewlineNotEmpty
             +> sepSpace
-            +> genExpr node.RightHandSide
+            +> genRhsExpr node.RightHandSide
+
+        let isNewLineInfixOp = newLineInfixOps.Contains node.Operator.Text
+        let isNoBreakInfixOp = noBreakInfixOps.Contains node.Operator.Text
 
-        if
-            isLambdaOrIfThenElse node.LeftHandSide
-            && newLineInfixOps.Contains node.Operator.Text
-        then
+        // Lambdas or if/then/else on the LHS of pipe-like operators (|>, ||>, >>)
+        // always use the multiline layout to avoid confusing indentation.
+        if isLambdaOrIfThenElse node.LeftHandSide && isNewLineInfixOp then
             genNode node (genMultilineInfixExpr node)
+        // No-break operators (=, >, <, %) keep the operator on the same line as the LHS.
+        // When the expression doesn't fit on one line, indent the RHS to preserve
+        // correct indentation when trivia (comments) precedes it. See #2944.
+        elif isNoBreakInfixOp then
+            let genLongNoBreakInfixExpr =
+                genExpr node.LeftHandSide
+                +> sepSpace
+                +> genSingleTextNode node.Operator
+                +> indent
+                +> sepNlnWhenWriteBeforeNewlineNotEmpty
+                +> sepSpace
+                +> genRhsExpr node.RightHandSide
+                +> unindent
+
+            fun ctx ->
+                genNode
+                    node
+                    (isShortExpression ctx.Config.MaxInfixOperatorExpression genShortInfixExpr genLongNoBreakInfixExpr)
+                    ctx
+        // All other infix operators place the operator at the start of the next line.
         else
             fun ctx ->
                 genNode
                     node
                     (isShortExpression
                         ctx.Config.MaxInfixOperatorExpression
-                        (genOnelinerInfixExpr node)
-                        (ifElse
-                            (noBreakInfixOps.Contains(node.Operator.Text))
-                            (genOnelinerInfixExpr node)
-                            (genMultilineInfixExpr node)))
+                        genShortInfixExpr
+                        (genMultilineInfixExpr node))
                     ctx
 
     | Expr.IndexWithoutDot node ->

src/Fantomas.Core/SyntaxOak.fs

@@ -97,7 +97,14 @@ type NodeBase(range: range) =
             if hasChildren then
                 for n in x.Children do
                     match n with
-                    | :? SingleTextNode as stn -> sb.Append(contentIndent).Append(stn.ToString()).AppendLine() |> ignore
+                    | :? SingleTextNode as stn ->
+                        for tn in stn.ContentBefore do
+                            sb.Append(contentIndent).Append(tn.ToString()).AppendLine() |> ignore
+
+                        sb.Append(contentIndent).Append(stn.ToString()).AppendLine() |> ignore
+
+                        for tn in stn.ContentAfter do
+                            sb.Append(contentIndent).Append(tn.ToString()).AppendLine() |> ignore
                     | :? NodeBase as nb ->
                         nb.AppendToStringWithIndent(sb, depth + 1)
                         sb.AppendLine() |> ignore