[RFC FS-1060] Nullness checking by dsyme · Pull Request #6804 · dotnet/fsharp ·...

Continuation of #5790

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

See tests\fsharp\core\nullness\test.fsx for testing and samples.

TODO:

• implement import of .NET metadata
• implement emit of .NET metadata
• resolve all unresolved issues in RFC
• implement match x with null -> ... | x -> ... implied use of NonNull pattern
• make null and NonNull be considered disjunctive discriminators in pattern matching
• resolve all // TODO NULLNESS
• ambivalent/oblivious annotations are not being shown at all in visual output. Consider what to do about these. They should likely be dropped but give a supplementary sentence "Some types shown are ambivalent to null checking for compatibility reasons".
• add testing
• get it green
• performance testing
• should nullness be supported on ref tuples and anon tuples
• 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)
• Work out what to do about compat of Option.ofObj and others
• Fix and re-enable test Test ProjectForWitnesses4 GetWitnessPassingInfo

DONE:

• allow Foo<string?>, currently Foo< string? > with a space between ? and > is needed, need to token smash ?> token in tyargs

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