Use autoIndentNlnByFuture for let bang. Fixes fsprojects#615

src/Fantomas.Tests/ComputationExpressionTests.fs

@@ -180,3 +180,50 @@ observable {
     and! b = bar
     return a + b }
 """
+
+[<Test>]
+let ``let bang should be formatted as regular let, 615`` () =
+    formatSourceString false """
+let f =
+  async {
+    // Without binding newline after assignment sign preserved, which is expected behavior
+    let r =
+      match 0 with
+      | _ -> ()
+
+    return r
+  }
+
+let f2 =
+  async {
+    // When binding, newline force-removed, which makes the whole expression
+    // on the right side to be indented.
+    let! r = match 0 with
+             | _ -> () |> async.Return
+
+    return r
+  }
+"""  config
+    |> prepend newline
+    |> should equal """
+let f =
+    async {
+        // Without binding newline after assignment sign preserved, which is expected behavior
+        let r =
+            match 0 with
+            | _ -> ()
+
+        return r
+    }
+
+let f2 =
+    async {
+        // When binding, newline force-removed, which makes the whole expression
+        // on the right side to be indented.
+        let! r =
+            match 0 with
+            | _ -> () |> async.Return
+
+        return r
+    }
+"""

src/Fantomas/CodePrinter.fs

@@ -1439,17 +1439,16 @@ and genExpr astContext synExpr =
         addParenIfAutoNln e1 (genExpr astContext) -- sprintf " <- " +> genExpr astContext e2
 
     | LetOrUseBang(isUse, p, e1, ands, e2) ->
-
         let genAndList astContext (ands: list<SequencePointInfoForBinding * bool * bool * SynPat * SynExpr * range>) =
             colPost sepNln sepNln
                 ands
                 (fun (_,_,_,pat,expr,_) -> !- "and! " +> genPat astContext pat -- " = " +> genExpr astContext expr)
 
-        atCurrentColumn (ifElse isUse (!- "use! ") (!- "let! ")
-            +> genPat astContext p -- " = " +> genExpr astContext e1 +> sepNln
-            +> genAndList astContext ands
-            +> genExpr astContext e2
-        )
+        ifElse isUse (!- "use! ") (!- "let! ") +> genPat astContext p -- " = "
+        +> autoIndentNlnByFuture (genExpr astContext e1)
+        +> sepNln
+        +> genAndList astContext ands
+        +> genExpr astContext e2
 
     | ParsingError r ->
         raise <| FormatException (sprintf "Parsing error(s) between line %i column %i and line %i column %i"