Crash when using --keepNewlineAfter

[knocte]

Description

I've tried to run fantomas 3.0 in a project inside my solution, and it crashed with:

$ fantomas . --recurse --keepNewlineAfter
./Presentation.fs has been written.
The following exception occurs while formatting stdin: Fantomas.FormatConfig+FormatException: Parsing failed with errors: [|/tmp.fsx (46,90)-(46,92) parse error Unexpected keyword 'if' in binding. Expected incomplete structured construct at or before this point or other token.;
  /tmp.fsx (61,1)-(61,4) parse error Incomplete structured construct at or before this point in binding. Expected incomplete structured construct at or before this point or other token.;
  /tmp.fsx (38,1)-(38,4) parse error Incomplete value or function definition. If this is in an expression, the body of the expression must be indented to the same column as the 'let' keyword.|]
And options: { SourceFiles = [|"/tmp.fsx"|]
  ConditionalCompilationDefines = []
  ErrorSeverityOptions = { WarnLevel = 3
                           GlobalWarnAsError = false
                           WarnOff = []
                           WarnOn = []
                           WarnAsError = []
                           WarnAsWarn = [] }
  IsInteractive = false
  LightSyntax = None
  CompilingFsLib = false
  IsExe = false }
   at Fantomas.CodeFormatterImpl.parse@61-2.Invoke(FSharpParseFileResults _arg2) in C:\Users\nojaf\Projects\fantomas\src\Fantomas\CodeFormatterImpl.fs:line 66
   at Microsoft.FSharp.Control.AsyncPrimitives.CallThenInvokeNoHijackCheck[a,b](AsyncActivation`1 ctxt, FSharpFunc`2 userCode, b result1) in E:\A\_work\130\s\src\fsharp\FSharp.Core\async.fs:line 417
   at <StartupCode$FSharp-Compiler-Service>.$Service.ParseFile@424-3.Invoke(AsyncActivation`1 ctxt)
   at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in E:\A\_work\130\s\src\fsharp\FSharp.Core\async.fs:line 109
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.FSharp.Control.AsyncResult`1.Commit() in E:\A\_work\130\s\src\fsharp\FSharp.Core\async.fs:line 350
   at Microsoft.FSharp.Control.AsyncPrimitives.RunSynchronouslyInCurrentThread[a](CancellationToken cancellationToken, FSharpAsync`1 computation) in E:\A\_work\130\s\src\fsharp\FSharp.Core\async.fs:line 882
   at Microsoft.FSharp.Control.AsyncPrimitives.RunSynchronously[T](CancellationToken cancellationToken, FSharpAsync`1 computation, FSharpOption`1 timeout) in E:\A\_work\130\s\src\fsharp\FSharp.Core\async.fs:line 890
   at Microsoft.FSharp.Control.FSharpAsync.RunSynchronously[T](FSharpAsync`1 computation, FSharpOption`1 timeout, FSharpOption`1 cancellationToken) in E:\A\_work\130\s\src\fsharp\FSharp.Core\async.fs:line 1154
   at Fantomas.Cmd.Program.processSourceString(Boolean isFsiFile, String s, FSharpChoice`2 tw, FormatConfig config) in C:\Users\nojaf\Projects\fantomas\src\Fantomas.CoreGlobalTool\Program.fs:line 85
   at Fantomas.Cmd.Program.stringToFile@196(FSharpRef`1 force, FSharpRef`1 profile, String s, String outFile, FormatConfig config) in C:\Users\nojaf\Projects\fantomas\src\Fantomas.CoreGlobalTool\Program.fs:line 204
Force writing original contents to ./Program.fs.
./UserInteraction.fs has been written.
./Properties/AssemblyInfo.fs has been written.

Repro code

1. Clone https://github.com/knocte/geewallet
2. Switch to the 'stable' branch.
3. cd src\GWallet.Frontend.Console
4. Run fantomas . --recurse --keepNewlineAfter

Current results: crash.
Expected results: shouldn't crash.

[VelocityRa]

The issue is not a crash, but an error when formatting.

The first time you run fantomas, it indents these lines incorrectly (appends 84 spaces to each one): https://github.com/knocte/geewallet/blob/stable/src/GWallet.Frontend.Console/Program.fs#L43-L52

And then after this if you run it again, you get the crash because the code is invalid.

After a bit of testing, the formatting issue happens when any comment (even an empty one) appears right after this line like so:

    let signedTransaction = Account.LoadSignedTransactionFromFile fileToReadFrom.FullName
    //

(has to be right after, no newlines)

[VelocityRa]

Reduced down to

let foo() =
    let bar = 6
    //
    baz
    baz2

which gets formatted to

let foo() =
    let bar = 6
               //
               baz
               baz2

[smoothdeveloper]

It doesn't seem related to (edit: only) single line comments, just an empty line causes it to glitch too

let a () =
    let q = 1
    
    q
    b

turns into

let a() =
    let q = 1

             q
             b

you need a sequence of expressions for it to happen, when arriving there:

fantomas/src/Fantomas/CodePrinter.fs

Line 1068 in c2338f3

| SequentialSimple es -> atCurrentColumn (colAutoNlnSkip0 sepSemiNln es (genExpr astContext))

the ctx.Writer.Column is 9 (in above sample) and should probably be reset before handling printing of trivia and AST nodes. I'm not familiar with the codebase to know where and how to handle this transformation of the Context.

edit: There is easy fix (#526) for this case without comment but it still fails the same with the comment.

[smoothdeveloper]

@knocte you may give a shot at version in master, it looks like it still compiles after running that version.