Improve Error Reporting: Make Nullness Warning 3261 less intimidating

[isaacabraham]

What

The standard FS3261 warning for e.g.

let x : string | null = null
x.Length // unsafe access - FS3261

is as follows:

warning FS3261: Nullness warning: The types 'string' and 'string | null' do not have compatible nullability.

Why

This error is not especially friendly or easy to understand, nor does it give a solution to fix the issue.

How

Consider replacing with:

warning FS3261: The symbol `x` may be null, and accessing the `Length` member could lead to a runtime exception. Consider pattern matching and / or converting to an Option using 'Option.ofObj' to safely access the value.

[smoothdeveloper]

warning FS3261: Nullness warning: The types 'string' and 'string | null' do not have compatible nullability.

->

warning FS3261: The symbol x may be null, and accessing the Length member could lead to a runtime exception. Consider pattern matching and / or converting to an Option using 'Option.ofObj' to safely access the value.

->

warning FS3261: Nullness warning: Accessing Length on x is not null-safe and can fail at runtime. More about null-safe member access: URL

url: a doc page explaining null checks, non-null, more details about use cases, performance implication, etc.

I don't feel we should recommend Option.ofObj or a specific way to make the code null-safe, but rather link to a dedicated and comprehensive page.

[isaacabraham]

@smoothdeveloper That's a good point - I had a similar thought. However, I'm not aware of this being done anywhere else in error messages for F# (although we have started doing this in SAFE Stack packages). You're right that Option.ofObj is opinionated, but realistically, what other ways of safely handling nullness in F# is there - pattern match on null or convert to Option and then use pattern matching / map etc.?