Fix 1954: val mutable in signature loses 'mutable'

src/Fantomas.Tests/SignatureTests.fs

@@ -171,6 +171,24 @@ module Tainted
 val GetHashCodeTainted: (Tainted<'T> -> int) when 'T: equality
 """
 
+[<Test>]
+let ``should keep mutable in type signature, 1954`` () =
+    formatSourceString
+        true
+        """
+module Tainted
+val mutable showParserStackOnParseError: bool
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+module Tainted
+
+val mutable showParserStackOnParseError: bool
+"""
+
 [<Test>]
 let ``should keep access modifiers in signatures seperated`` () =
     formatSourceString

src/Fantomas/CodePrinter.fs

@@ -867,7 +867,7 @@ and genMemberFlagsForMemberBinding astContext (mf: MemberFlags) (rangeOfBindingA
             |> Option.defaultValue (!- "override ")
         <| ctx
 
-and genVal astContext (Val (ats, px, ao, s, identRange, t, vi, isInline, _, eo) as node) =
+and genVal astContext (Val (ats, px, ao, s, identRange, t, vi, isInline, isMutable, _, eo) as node) =
     let range, synValTyparDecls =
         match node with
         | ValSpfn (_, _, synValTyparDecls, _, _, _, _, _, _, _, range) -> range, synValTyparDecls
@@ -883,6 +883,7 @@ and genVal astContext (Val (ats, px, ao, s, identRange, t, vi, isInline, _, eo)
     +> genAttributes astContext ats
     +> (!- "val "
         +> onlyIf isInline (!- "inline ")
+        +> onlyIf isMutable (!- "mutable ")
         +> opt sepSpace ao genAccess
         -- s
         +> genericParams
@@ -4045,7 +4046,7 @@ and genMemberSig astContext node =
         | SynMemberSig.NestedType (_, r) -> r, SynMemberSig_NestedType
 
     match node with
-    | MSMember (Val (ats, px, ao, s, _, t, vi, isInline, ValTyparDecls (tds, _, tcs), eo), mf) ->
+    | MSMember (Val (ats, px, ao, s, _, t, vi, isInline, _, ValTyparDecls (tds, _, tcs), eo), mf) ->
         let (FunType namedArgs) = (t, vi)
 
         let isFunctionProperty =

src/Fantomas/SourceParser.fs

@@ -1510,9 +1510,19 @@ let (|MSMember|MSInterface|MSInherit|MSValField|MSNestedType|) =
     | SynMemberSig.NestedType (tds, _) -> MSNestedType tds
 
 let (|Val|)
-    (ValSpfn (ats, (IdentOrKeyword (OpNameFullInPattern (s, _)) as ident), tds, t, vi, isInline, _, px, ao, eo, _))
+    (ValSpfn (ats,
+              (IdentOrKeyword (OpNameFullInPattern (s, _)) as ident),
+              tds,
+              t,
+              vi,
+              isInline,
+              isMutable,
+              px,
+              ao,
+              eo,
+              _))
     =
-    (ats, px, ao, s, ident.idRange, t, vi, isInline, tds, eo)
+    (ats, px, ao, s, ident.idRange, t, vi, isInline, isMutable, tds, eo)
 
 // Misc