Allow changing context for subsequent assertions

[cmeeren]

Background and motivation

Based on #2223.

It would be nice to have an API to be able to change the context for custom assertions. For example, I have the following extension to ReferenceTypeAssertions that allows me to transform the value before asserting further:

type ReferenceTypeAssertions<'TSubject, 'TAssertions
    when 'TAssertions :> ReferenceTypeAssertions<'TSubject, 'TAssertions>> with

    member this.WhenTransformed(f: 'TSubject -> 'TTransformed) = f this.Subject

This allows me to do this:

result
    .Should()
    .MatchRegex(@"^\d{6}-\d{4}$", "the value should be formatted correctly")
    .And.WhenTransformed(String.replace "-" "")
    .Should()
    .Be(input, "the value should be equal to the input ignoring spaces")

However, if the last one fails, it still uses the variable name result as the context. This is confusing, because the value is no longer result.

I know I can override the context using AssertionScope, but I can't use that in WhenTransformed because I don't actually assert anything there.

Another example is a custom assertions type OptionAssertions I have made to assert (using a method BeSome) an F# option value to be Some _, and then use Which to continue asserting on the inner value. I can't use AssertionScope because it's disposed once the AndWhichConstraint is returned from BeSome.

API Proposal

I really don't know how this can best be solved. But, at the very least, it seems to me I have to be able to call a new API in custom *Assertions extension members, and that this API can not be based on IDisposable.

API Usage

See above.

Alternative Designs

No response

Risks

No response

[cmeeren]

For what it's worth, I am no longer interested in this. As detailed in https://github.com/fluentassertions/fluentassertions/issues/2225#issuecomment-1636796736, I am creating a brand new F# assertions library inspired by FluentAssertions. That library will automatically support this feature with no end user hassle. Feel free to close.