Warn when a function is an interpolated string argument / when a type that isn't IFormattable gets format arguments

I propose we put a warning here -

(from https://github.com/dotnet/fsharp/issues/18749 @esbenbjerre)

open System

type DateTimeRange =
  {
    Start: DateTime
    End: DateTime
  }

  override _.ToString() = $"{_.Start:O} / {_.End:O}" // Warn on _.Start and _.End

let range =
  {
    Start = DateTime.UtcNow
    End = DateTime.UtcNow.AddSeconds 2.0
  }

$"{range}"

This code looks like it's using _ as a self-identifier when in fact the self-identifier is discarded and _.Start expands to fun x -> x.Start. Similar pitfalls exist when a partially-applied function is used as a string interpolation argument.

The existing way of approaching this problem in F# is having this confuse people.

Pros and Cons

The advantage of making this adjustment to F# is robustness (less bugs)

The disadvantages of making this adjustment to F# is needing to #nowarn when you really want to get readable information about lambdas. I haven't encountered a need for this though.

Extra information

Estimated cost (XS, S, M, L, XL, XXL): XS

Related suggestion:

• https://github.com/fsharp/fslang-suggestions/issues/1327

Affidavit (please submit!)

Please tick these items by placing a cross in the box:

• This is not a question (e.g. like one you might ask on StackOverflow) and I have searched StackOverflow for discussions of this issue
• This is a language change and not purely a tooling change (e.g. compiler bug, editor support, warning/error messages, new warning, non-breaking optimisation) belonging to the compiler and tooling repository
• This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it
• I have searched both open and closed suggestions on this site and believe this is not a duplicate

Please tick all that apply:

• This is not a breaking change to the F# language design
• I or my company would be willing to help implement and/or test this

For Readers

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Start with the interpolated-string examples in the issue and review the related suggestion at fsharp/fslang-suggestions#1327. Determine the language-design scope for warnings involving discarded self-identifiers, partially applied functions, and non-IFormattable format arguments; done means the warning behavior and cases are specified clearly enough to implement and test.