[WIP] [RFC FS-1060] Nullness checking

[dsyme]

Continuation of #5790 and #6804

This is a prototype implementation of RFC FS-1060 nullable reference types

See tests\adhoc\nullness for testing and samples including baselines of outputs from

• existing compiler
• updated compiler
• updated compiler with /langversion:preview
• updated compiler with /langversion:preview /checknulls

TODO:

• revise RFC and resolve all unresolved issues (@dsyme)
• implement syntax string | null and : not null (@T-Gro )
• implement import of .NET metadata (@T-Gro )
• implement emit of .NET metadata (@T-Gro )
• Import+Export TODOs (to test and adjust if needed)
• Nullness used in inheritance (e.g. inherit System.Collections.Generic.List<string | null>)
• Soundness of import of CLI EventHandlers (possibly overly defensive)
• Nullness and import of TypeProvider-generated types + In general the overall TP scenario whet it comes to nullness support - do in separate PR
• Object overrides can change nullness (e.g. boolean.ToString() does or StringBuilder.ToString()), make it work to avoid false alarms
• Type.GetType(..) |> Option.ofObj produces a false alarm, fix that
• printfn "%s" someNullableString produces a false alarm, check the printing pattern annotations
• (+) operator for strings gives false alarms on nullable strings, should allow nullables since String.Concat also does
• Pattern matching flow analysis on match x with | null -> Support this as a scenario
• Allow incoming arrays to be null (this causes false alarms as of now, it's a bug)
• resolve all // TODO NULLNESS (@dsyme)
• use it in the codebase and get it green
• there's a case in tests\adhoc\nullness we're getting The types 'System.String (...)' and 'System.String (...)' do not have compatible nullability. which is wrong - either the nullability aren't shown for some reason or the types should be considered compatible

Testing:

• add proper testing, moving tests across from tests\adhoc\nullness
• test and check signature compatibility. While integrating master I noticed a case in the compiler where a signature file had a non-nullable string type for ther return of a function and an implementation had a nullable string type (and a later soundness problem arose)

To be moved to RFC and resolved, then tested here:

• should nullness be supported on ref tuples and anon tuples
• work out what to do about compat of Option.ofObj and others
• the "type equiv" relation is currently directional only giving a nullness warning if "actual" has a null and "expected" is non-null. This includes a dubious change of direction in SolveFunType thing - for the contravariance of inputs. This has caused a few outputs to change in tests. This is both unsound in the general case (nested cases of equivalence should demand true equivalence) and is dubious for functions because the "MatchingOnly" case of type equivalence is also directional.
• assess nullness checking in conditionals, e..g if x then null else "" and if x then "" else null

[dsyme]

@vzarytovskii @T-Gro I squahed this and also fixed a nasty bug with nullness inference that I spotted while walking through with Vlad on Thursday

[dsyme]

@vzarytovskii @T-Gro This is basically green now :) And much, much trimmed down, with some bugs fixed. I've also revised the TODO list to what would be needed for bringing into preview

(It's possible we could even ship preview using the string __withnull syntax then incrementally improve after)

[vzarytovskii]

@vzarytovskii @T-Gro This is basically green now :) And much, much trimmed down, with some bugs fixed. I've also revised the TODO list to what would be needed for bringing into preview

I'm going to take care of metadata part

[ken-okabe]

I came across an article about F# Nullness support . As someone who's eagerly awaiting Nullable types including reference types in F#, I want to thank you for this contributions.

https://github.com/ken-okabe/vanfs?tab=readme-ov-file#nullable
https://github.com/ken-okabe/vanfs/blob/main/README-whatisNull.md

[vzarytovskii]

I came across an article about F# Nullness support . As someone who's eagerly awaiting Nullable types including reference types in F#, I want to thank you for this contributions.

https://github.com/ken-okabe/vanfs?tab=readme-ov-file#nullable

https://github.com/ken-okabe/vanfs/blob/main/README-whatisNull.md

Thank you for kind words. We hoped to release it earlier, but feature turned out to be much more complex and big than we anticipated :(