Add space before arrow when trivia before is present.

CHANGELOG.md

@@ -5,6 +5,9 @@
 ### Changed
 * Update FCS to 'Prototype signature files for lex files', commit 5b74995780a0e9d24a5db025e68b31e6d910f71f
 
+### Fixed
+* Missing space before arrow. [#2888](https://github.com/fsprojects/fantomas/issues/2888)
+
 ## [6.0.3] - 2023-05-14
 
 ### Fixed

src/Fantomas.Core.Tests/PatternMatchingTests.fs

@@ -2321,3 +2321,26 @@ let f () =
       tail -> 1
     | _ -> None
 """
+
+[<Test>]
+let ``multiline trivia for arrow, 2888`` () =
+    formatSourceString
+        false
+        """
+match subcategory with
+| BuildPhaseSubcategory.Internal
+// Getting here means the compiler has ICE-d. Let's not pile on by showing the unknownSubcategory assert below.
+// Just treat as an unknown-to-LanguageService error.
+ -> false
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+match subcategory with
+| BuildPhaseSubcategory.Internal
+// Getting here means the compiler has ICE-d. Let's not pile on by showing the unknownSubcategory assert below.
+// Just treat as an unknown-to-LanguageService error.
+ -> false
+"""

src/Fantomas.Core/CodePrinter.fs

@@ -2013,7 +2013,16 @@ let genClause (isLastItem: bool) (node: MatchClauseNode) =
 
                         expressionFitsOnRestOfLine (sepSpace +> genExpr node.BodyExpr) long
 
-                    (genSingleTextNodeWithSpaceSuffix sepSpace node.Arrow
+                    let sepArrow =
+                        if not node.Arrow.HasContentBefore then
+                            genSingleTextNodeWithSpaceSuffix sepSpace node.Arrow
+                        else
+                            // In the weird case where the arrow has content before it and there is no multiline whenExpr,
+                            // we need to ensure a space was written before the arrow to avoid offset errors.
+                            // See https://github.com/fsprojects/fantomas/issues/2888
+                            !- $" {node.Arrow.Text} " |> genNode node.Arrow
+
+                    (sepArrow
                      +> ifElse
                          (ctx.Config.ExperimentalKeepIndentInBranch && isLastItem)
                          genKeepIndentInBranch