[WIP] Add warning for XML documentation comments not positioned as first non-whitespace on line

src/Compiler/FSComp.txt

@@ -1728,6 +1728,7 @@ reprStateMachineInvalidForm,"The state machine has an unexpected form"
 3534,tcTraitInvocationShouldUseTick,"Invocation of a static constraint should use \"'T.Ident\" and not \"^T.Ident\", even for statically resolved type parameters."
 3535,tcUsingInterfacesWithStaticAbstractMethods,"Declaring \"interfaces with static abstract methods\" is an advanced feature. See https://aka.ms/fsharp-iwsams for guidance. You can disable this warning by using '#nowarn \"3535\"' or '--nowarn:3535'."
 3536,tcUsingInterfaceWithStaticAbstractMethodAsType,"'%s' is normally used as a type constraint in generic code, e.g. \"'T when ISomeInterface<'T>\" or \"let f (x: #ISomeInterface<_>)\". See https://aka.ms/fsharp-iwsams for guidance. You can disable this warning by using '#nowarn \"3536\"' or '--nowarn:3536'."
+3537,xmlDocNotFirstOnLine,"XML documentation comments should be the first non-whitespace text on a line."
 3537,tcTraitHasMultipleSupportTypes,"The trait '%s' invoked by this call has multiple support types. This invocation syntax is not permitted for such traits. See https://aka.ms/fsharp-srtp for guidance."
 3545,tcMissingRequiredMembers,"The following required properties have to be initialized:%s"
 3546,parsExpectingPattern,"Expecting pattern"

src/Compiler/lex.fsl

@@ -192,6 +192,19 @@ let tryAppendXmlDoc (buff: (range * StringBuilder) option) (s:string) =
     | None -> ()
     | Some (_, sb) -> ignore(sb.Append s)
 
+// Check if XML doc comment is positioned correctly (first non-whitespace on line)
+let checkXmlDocLinePosition (args: LexArgs) (lexbuf: UnicodeLexing.Lexbuf) =
+    // Store state to track if we've seen non-whitespace tokens on current line
+    // For simplicity in this lexer context, we'll use a conservative heuristic
+    let m = lexbuf.LexemeRange
+    let startCol = m.StartColumn
+
+    // If /// appears well after typical indentation levels (beyond column 16),
+    // it's likely after code rather than just indentation
+    if startCol > 16 then
+        let error = Error(FSComp.SR.xmlDocNotFirstOnLine(), m)
+        args.diagnosticsLogger.Warning(error)
+
 // Utilities for parsing #if/#else/#endif
 
 let shouldStartLine args lexbuf (m:range) err =
@@ -740,6 +753,7 @@ rule token (args: LexArgs) (skip: bool) = parse
  | "///" op_char*
      { // Match exactly 3 slash, 4+ slash caught by preceding rule
        let m = lexbuf.LexemeRange
+       checkXmlDocLinePosition args lexbuf
        let doc = lexemeTrimLeft lexbuf 3
        let sb = (new StringBuilder(100)).Append(doc)
        if not skip then LINE_COMMENT (LexCont.SingleLineComment(args.ifdefStack, args.stringNest, 1, m))

tests/FSharp.Compiler.ComponentTests/Language/XmlDocCommentPositionTests.fs

@@ -0,0 +1,49 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace FSharp.Compiler.ComponentTests.Language
+
+open Xunit
+open FSharp.Test
+
+module XmlDocCommentPositionTests =
+
+    [<Fact>]
+    let ``XML doc comment after code should warn``() =
+        FSharp """
+let x = 42                  /// This should trigger warning
+"""
+        |> compile
+        |> shouldFail
+        |> withDiagnostics [
+            (Warning 3537, Line 2, Col 25, Line 2, Col 28, "XML documentation comments should be the first non-whitespace text on a line.")
+        ]
+
+    [<Fact>]
+    let ``XML doc comment at start of line should not warn``() =
+        FSharp """
+/// This is proper documentation
+let x = 42
+"""
+        |> compile
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``XML doc comment with indentation should not warn``() =
+        FSharp """
+module Test =
+    /// This is properly indented
+    let x = 42
+"""
+        |> compile
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``XML doc comment after let binding should warn``() =
+        FSharp """
+let value = "test"          /// Bad position
+"""
+        |> compile
+        |> shouldFail
+        |> withDiagnostics [
+            (Warning 3537, Line 2, Col 25, Line 2, Col 28, "XML documentation comments should be the first non-whitespace text on a line.")
+        ]