null flow state should be shown at declaration site as well as use site

[YairHalberstadt]

Version Used: VS 16.6 preview 1

Steps to Reproduce:

In VS copy the following:

#nullable enable
public class C
{
    string M()
    {
        var str = "hello";
        return str;
    }
}

Hover over str in return str.

You get the following popup:

Hover over str in var str = "hello";

You get the following popup:

Note it doesn't mention the null flow state, only the null-type of the variable.

I found this extremely confusing since str is very clearly not null. I wasted a few minutes trying to find out if the method I was calling returned null, before I realized it was because var is always inferred as nullable.

I think this would be less confusing if the null flow state was also shown in declaration so it would say:

(local variable) string? str
str is not null here"

[CyrusNajmabadi]

@jcouv i agree. This is def a bit confusing. Hopefully this is a low-cost thing to implement.

[sharwell]

We need to think about the way this feature interacts with pre- and post-conditions.

• Some cases have the same pre- and post-conditions (e.g. most reads)

• Some cases only have post-conditions (e.g. variable initializers)

• Some cases have both pre- and post-conditions, e.g. the flow state of value here:

  Debug.Assert(value is object);

  For this expression, the post-condition is always "not null", but the precondition could be any of "not null", "maybe null", or "unknown (oblivious)".

[sharwell]

Design review conclusion: (all examples here show the not-null case)

Overview

By default, show the following:

'str' is not null here.

If the post-condition of the use changes the flow analysis state, show the following:

'str' might be null here.
'str' is not null after this use.

If the use is the target of an assignment (where the value is written but not read):

'str' is not null after this assignment.

Special cases

• Only show the pre- and post-conditions if the use is not just an assignment and the flow analysis state changes as a result of the use.