Replace null-forgiving operator uses with debug assertions

[atifaziz]

This PR introduces Debug.Assert that's defined similarly to System.Diagnostics.Debug.Assert to help eliminate the bulk uses of the null-forgiving operator (!). The assertion more explicitly states the illegal case, and if triggered, will also relay the offending condition in the message. It also eliminates needing to suppress CS8602 and CS8603 in targets earlier than .NET 6.

This PR also adds Assume.NotNull, that's a Debug.Assert in disguise, but which specifically asserts the not-null-reference case. The idea is to make these cases easily searchable in code.

[codecov]

Codecov Report

Merging #890 (2163141) into master (d587416) will decrease coverage by 0.14%.
The diff coverage is 72.97%.

@@            Coverage Diff             @@
##           master     #890      +/-   ##
==========================================
- Coverage   92.53%   92.38%   -0.15%     
==========================================
  Files         108      110       +2     
  Lines        3428     3441      +13     
  Branches     1020     1020              
==========================================
+ Hits         3172     3179       +7     
- Misses        194      200       +6     
  Partials       62       62              

Impacted Files	Coverage Δ
MoreLinq/FallbackIfEmpty.cs	88.67% <ø> (ø)
MoreLinq/Pad.cs	100.00% <ø> (ø)
MoreLinq/Partition.cs	98.24% <ø> (ø)
MoreLinq/Lookup.cs	61.87% <33.33%> (-2.26%)	⬇️
MoreLinq/Assume.cs	100.00% <100.00%> (ø)
MoreLinq/Debug.cs	100.00% <100.00%> (ø)
MoreLinq/Fold.cs	98.07% <100.00%> (ø)
MoreLinq/PartialSort.cs	100.00% <100.00%> (ø)

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[viceroypenguin]

While I am not opposed to Debug.Assert(bool condition), I would suggest adding a static T AssertNotNull<T>([NotNull, AllowNull] T? object) where T : class, which receives an object, throws if null, and returns the object unchanged otherwise. The nullability analyzer will treat the returned object as not-null, which means it can be used immediately.

Example use case:

var g = Debug.AssertNotNull(_lastGrouping);

[atifaziz]

While I am not opposed to Debug.Assert(bool condition), I would suggest adding a static T AssertNotNull<T>([NotNull, AllowNull] T? object) where T : class, which receives an object, throws if null, and returns the object unchanged otherwise. The nullability analyzer will treat the returned object as not-null, which means it can be used immediately.

Example use case:

var g = Debug.AssertNotNull(_lastGrouping);

You can't do that because such assertion methods are conditional methods that get compiled out of release builds and therefore they must return void. See also the remarks section of the ConditionalAttribute docs:

Applying ConditionalAttribute to a method indicates to compilers that a call to the method should not be compiled into Microsoft intermediate language (MSIL) unless the conditional compilation symbol that is associated with ConditionalAttribute is defined. _You will get a compilation error in Visual Studio if you apply this attribute to a method that does not return void.

[viceroypenguin]

Ah, I missed the [Conditional]. I'll ponder this further.

[viceroypenguin]

Approved for now. I have specific recommendations for different improvements for several of these, but those can be done later.

[atifaziz]

which receives an object, throws if null, and returns the object unchanged otherwise

I've added this idea with 7e6fa33 and updated the initial description with:

This PR also adds Assume.NotNull, that's a Debug.Assert in disguise, but which specifically asserts the not-null-reference case. The idea is to make these cases easily searchable in code.

[viceroypenguin]

Finished product LGTM