Recursive sequence expression containing recursive definition doesn't run in finite space

[kbattocchi]

This small example causes a stack overflow, but shouldn't:

let rec loop r = seq {
    if r > 0 then
        let rec unused() = unused()
        yield r
        yield! loop r
}

printfn "%i" (Seq.nth 10000000 (loop 1))

Removing the conditional or replacing the unused recursive definition with an unused non-recursive definition both appear to result in the expected behavior of constant stack space usage.

[dsyme]

This appears to be due to the fact that "let rec" causes the sequence expression to be compiled into combinatory form, but the last two portions are still compiled into state-machine form, and the recurrent thunk in-between the two forms causes the stack usage to grow.

One fix is to add Expr.LetRec into the forms compiled by LowerSeqExpr in visualfsharp\src\fsharp\LowerCallsAndSeqs.fs. I'm not sure how hard that is though.

[dsyme]

@kbattocchi I finally fixed this one in #3536, please take a look

[dsyme]

Reopening as the fix was wrong, see #3749