bugfix/issue-2485-sequentials-stack-overflow

[njlr]

• Switches Sequentials to an imperative impl for efficiency and to not blow up stack

[nojaf]

Hello, thank you for taking the initiative to solve this problem.

I'm going to sound very purist here but I'm not a fan of the solution.
We have solved these problems in the past via tail recursion, I typically use this blog post as a reference.

This PR also misses a test case to illustrate the problem is indeed solved.
Could you add a new test file ASTTransformerTests.fs (module Fantomas.Core.Tests.ASTTransformerTests)

with some unit test like:

#if RELEASE
open FSharp.Compiler.Text
open FSharp.Compiler.Xml
open FSharp.Compiler.Syntax
open FSharp.Compiler.SyntaxTrivia

[<Test>]
let ``avoid stack-overflow in long array/list, 2485`` () =
    let testIdent = Ident("Test", Range.Zero)

    let mkStringExpr () =
        SynExpr.Const(
            SynConst.String((System.Guid.NewGuid().ToString("N"), SynStringKind.Regular, Range.Zero)),
            Range.Zero
        )

    let longArrayExpr: SynExpr =
        let rec mkArray count childExpr =
            if count = 20_000 then
                childExpr
            else
                mkArray
                    (count + 1)
                    (SynExpr.Sequential(
                        DebugPointAtSequential.SuppressNeither,
                        true,
                        mkStringExpr (),
                        childExpr,
                        Range.Zero
                    ))

        SynExpr.ArrayOrListComputed(true, mkArray 0 (mkStringExpr ()), Range.Zero)

    let rootNode =
        Fantomas.Core.AstTransformer.astToNode
            Range.Zero
            []
            [ SynModuleOrNamespace(
                  [ testIdent ],
                  false,
                  SynModuleOrNamespaceKind.AnonModule,
                  [ SynModuleDecl.Expr(longArrayExpr, Range.Zero) ],
                  PreXmlDoc.Empty,
                  [],
                  None,
                  Range.Zero,
                  { ModuleKeyword = None
                    NamespaceKeyword = None }
              ) ]

    Assert.Pass()
#endif

Notice that #if RELEASE will be required for that test to succeed.

Also please take a look at the PR checklist

Many thanks

[njlr]

Thanks @nojaf I have followed the steps in the checklist 👍

[nojaf]

Thanks!