assert: Add a self-describing Matcher

[pdhammond42]

Summary

Add a Matcher interface and associated assert.Matches function

Changes

Add interface, assertion functoin and an exmaple matcher implementation.

Motivation

This allows users to extend the assertions in possibly highly domain specific ways, without having to modify the assert package, and provide a suitably descriptive failure message to help pinpoint the problem.

Related issues

[dolmen]

Please run the code generator. Check the Actions result.

[dolmen]

• This extends the API surface
• This can be built on top of Testify, in a separate package

At this point I don't want to extend the API surface.
So, closing.

[pdhammond42]

Hi @dolmen , I don't see how this can be built on top of Testify. The point is to allow new tests to be built on top of Testify, using Testify's description of the failure. Of course I could do something similar using

stringStartingFoo:=matches.StringStarting("foo")
assert.True(t, stringStartingFoo.Test(x), stringStartingFoo.Describe(x))

but then I could also do

if c := matches.StringStarting("foo"); !c.Test(x) {
  t.Error(c.Describe(x)
}

But it's your code base, your decision. Thanks for looking

[dolmen]

"build on top of Testify" means "create your own module that uses the testify/assert API".