Cons pattern in `let` bindings is converted to invalid code

[clkbug]

Issue created from fantomas-online

Code

let x::y = [0]

Result

let (::) x, y = [ 0 ]

Problem description

After formatting, fantomas output the following error:

The following exception occurs while formatting stdin: Fantomas.FormatConfig+FormatException: Formatted content is not valid F# code
   at Fantomas.Extras.FakeHelpers.formatContentInternalAsync@95-5.Invoke(Boolean _arg2) in C:\Users\fverdonck\Projects\fantomas\src\Fantomas.Extras\FakeHelpers.fs:line 96
   at Microsoft.FSharp.Control.AsyncPrimitives.CallThenInvokeNoHijackCheck[a,b](AsyncActivation`1 ctxt, FSharpFunc`2 userCode, b result1) in D:\workspace\_work\1\s\src\fsharp\FSharp.Core\async.fs:line 404
   at Fantomas.CodeFormatterImpl.isValidFSharpCode@377-7.Invoke(AsyncActivation`1 ctxt) in C:\Users\fverdonck\Projects\fantomas\src\Fantomas\CodeFormatterImpl.fs:line 377
   at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in D:\workspace\_work\1\s\src\fsharp\FSharp.Core\async.fs:line 104
--- End of stack trace from previous location ---
   at Microsoft.FSharp.Control.AsyncResult`1.Commit() in D:\workspace\_work\1\s\src\fsharp\FSharp.Core\async.fs:line 337
   at Microsoft.FSharp.Control.AsyncPrimitives.RunSynchronouslyInCurrentThread[a](CancellationToken cancellationToken, FSharpAsync`1 computation) in D:\workspace\_work\1\s\src\fsharp\FSharp.Core\async.fs:line 870   
   at Microsoft.FSharp.Control.AsyncPrimitives.RunSynchronously[T](CancellationToken cancellationToken, FSharpAsync`1 computation, FSharpOption`1 timeout) in D:\workspace\_work\1\s\src\fsharp\FSharp.Core\async.fs:line 878
   at Microsoft.FSharp.Control.FSharpAsync.RunSynchronously[T](FSharpAsync`1 computation, FSharpOption`1 timeout, FSharpOption`1 cancellationToken) in D:\workspace\_work\1\s\src\fsharp\FSharp.Core\async.fs:line 1142
   at Program.processSourceString(Boolean isFsiFile, String s, FSharpChoice`2 tw, FormatConfig config) in C:\Users\fverdonck\Projects\fantomas\src\Fantomas.CoreGlobalTool\Program.fs:line 114
   at Program.stringToFile@346(Boolean force, Boolean profile, String s, String outFile, FormatConfig config) in C:\Users\fverdonck\Projects\fantomas\src\Fantomas.CoreGlobalTool\Program.fs:line 358
Formatted content is not valid F# code

Extra information

• The formatted result breaks by code.
• The formatted result gives compiler warnings.
• I or my company would be willing to help fix this.

Options

Fantomas 4.6 branch at 12/04/2021 18:09:39 - 9e35634

Default Fantomas configuration

_{Did you know that you can ignore files when formatting from fantomas-tool or the FAKE targets by using a .fantomasignore file?}

[nojaf]

Hello @clkbug, thank you for reporting this issue.
This is an interesting case from an AST point of view.

@dsyme I believe this is a case where the PreIdent would be useful.
If you have:

let x::y = [0]
let op_ColonColon x y = [0]

leads to

LongIdent (LongIdentWithDots ([op_ColonColon], []), None, None, ... )
LongIdent (LongIdentWithDots ([op_ColonColon], []), None, None, ... )

So, inside the pattern node, having the original syntax would be beneficial.
And when we detect the :: we should treat it as a special case in Fantomas.

[auduchinok]

It'd likely be easier to work with :: if it was a separate pattern case in FCS SyntaxTree, but it would probably require a lot of changes across the compiler.

[clkbug]

This problem no longer seems to occur, doesn't it?

[nojaf]

No, the AST got updated and SynPat.ListCons got introduced to represent this.

To close this issue properly, it would be great to include a regression test. That way, we can make sure that the changes made to the AST and the new SynPat.ListCons don't cause any unexpected issues or bugs.

If you're up for it, would you be interested in sending a PR (pull request)? It would be awesome to have your contribution to this fix!