Record with lambda in non-last field collapses to single line producing invalid code

[randrag]

Fantomas collapses a multi-line record expression to a single line when it fits within the line length limit. When a non-last field has a fun lambda value, this produces invalid F# code because the fun keyword's body extends to the end of the expression, swallowing subsequent fields.

Issue created from fantomas-online

Code

type Rec = {
    A : int
    B : int -> int
    C : int
}

let test () : Rec =
    {
        A = 1
        B = fun x -> x + 1
        C = 3
    }

Result

type Rec = { A: int; B: int -> int; C: int }

let test () : Rec =
    { A = 1
      B =
        fun x ->
            x + 1
            C = 3 }

Problem description

F# parses fun x -> as extending to the end of the enclosing expression. The semicolons become sequential expressions inside the lambda body:

{ A = 1; B = fun x -> (x + 1; C = 3) }

Suggested fix (I have a PR ready)

Keep the single-line format but wrap the lambda in parentheses to delimit the body:

let test () : Rec = { A = 1; B = (fun x -> x + 1); C = 3 }

Extra information

• The formatted result breaks my code.
• The formatted result gives compiler warnings.
• I or my company would be willing to help fix this. I have a (Claude code) PR ready, have reviewed it, will submit as draft.
• I would like a release if this problem is solved.

Reproduced on version 7.0.3 and 8.0.0-alpha-003.

Options

Fantomas main branch at 1/1/1990

Default Fantomas configuration