Add code for calculating stable identifiers

.rubocop.yml

@@ -12,3 +12,5 @@ Metrics/AbcSize:
   Enabled: false
 Lint/RescueException:
   Enabled: false
+Style/MixinUsage:
+  Enabled: false

AWFUL-HACKS.md

@@ -93,3 +93,22 @@ so I've monkey-patched their build system in our Rakefile in order
 to make it error properly in this case.
 
 Can be removed when: The linked issue is fixed.
+
+
+### Stable identifiers from RSpec
+
+Another "I did terrible things to RSpec" entry, sorry. RSpec's
+design here is totally reasonable and sensible and honestly
+probably *is* how you should pass state around, but seems
+to make it impossible to get access to the Example object from
+inside an it block without actually being the definer of the
+block.
+
+See `hypothesis_stable_identifier` for details, but basically I
+couldn't figure out how to get the name of a currently executing
+spec in RSPec from a helper function without some fairly brutal
+hacks where we extract information badly from self.inspect, because
+it's there stored as a string that gets passed in for inspection.
+
+Can be removed when: Someone shows me a better way, or a
+feature is added to RSpec to make this easier.

Rakefile

@@ -5,8 +5,9 @@ require 'bundler/setup'
 require 'rspec/core/rake_task'
 require 'rubocop/rake_task'
 require 'helix_runtime/build_task'
+require 'rake/testtask'
 
-RSpec::Core::RakeTask.new(:test)
+RSpec::Core::RakeTask.new(:spec)
 
 RuboCop::RakeTask.new
 
@@ -24,7 +25,12 @@ end
 
 HelixRuntime::BuildTask.new
 
-task test: :build
+Rake::TestTask.new(minitests: :build) do |t|
+  t.test_files = FileList['minitests/**/test_*.rb']
+  t.verbose = true
+end
+
+task test: %i[build spec minitests]
 
 task :format do
   sh 'bundle exec rubocop -a'

lib/hypothesis.rb

@@ -7,6 +7,62 @@
 require 'hypothesis/debug'
 
 module Hypothesis
+  HYPOTHESIS_LOCATION = File.dirname(__FILE__)
+
+  def hypothesis_stable_identifier
+    # Attempt to get a "stable identifier" for any given
+    # call into hypothesis. We use these to create
+    # database keys (or will when we have a database) that
+    # are stable across runs, so that when a test that
+    # previously failed is rerun, we can fetch and reuse
+    # the previous examples.
+
+    # Note that essentially any answer to this method is
+    # "fine" in that the failure mode is that sometiems we
+    # just won't run the same test, but it's nice to keep
+    # this as stable as possible if the code isn't changing.
+
+    # Minitest makes it nice and easy to create a stable
+    # test identifier, because it follows the classic xunit
+    # pattern where a test is just a method invocation on a
+    # fresh test class instance and it's easy to find out
+    # which invocation that was.
+    return "#{self.class.name}::#{@NAME}" if defined? @NAME
+
+    # If we are running in an rspec example then, sadly,
+    # rspec take the entirely unreasonable stance that
+    # the correct way to pass data to a test is by passing
+    # it as a function argument. Honestly, what is this,
+    # Haskell? Ahem. Perfectly reasonable design decisions
+    # on rspec's part, this creates some annoying difficulties
+    # for us. We solve this through brute force and ignorance
+    # by relying on the information we want being in the
+    # inspect for the Example object, even if it's just there
+    # as a string.
+    begin
+      is_rspec = is_a? RSpec::Core::ExampleGroup
+    rescue NameError
+      is_rspec = false
+    end
+
+    if is_rspec
+      return [
+        self.class.description,
+        inspect.match(/"([^"]+)"/)[1]
+      ].join(' ')
+    end
+
+    # Fallback time! We just walk the stack until we find the
+    # entry point into code we control. This will typically be
+    # where "hypothesis" was called.
+    Thread.current.backtrace.each do |line|
+      return line unless line.include?(Hypothesis::HYPOTHESIS_LOCATION)
+    end
+    # This should never happen unless something very strange is
+    # going on.
+    raise 'BUG: Somehow we have no caller!'
+  end
+
   def hypothesis(options = {}, &block)
     unless World.current_engine.nil?
       raise UsageError, 'Cannot nest hypothesis calls'

minitests/test_stable_identifier.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+require 'minitest/autorun'
+require 'hypothesis'
+
+class TestIdentifiers < Minitest::Test
+  include Hypothesis
+
+  def test_abc
+    assert_equal hypothesis_stable_identifier, 'TestIdentifiers::test_abc'
+  end
+end

spec/stable_identifier_spec.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class SomeClass
+  include Hypothesis
+  def stuff
+    hypothesis_stable_identifier
+  end
+end
+
+RSpec.describe 'stable identifiers' do
+  it 'are the full rspec string' do
+    expect(hypothesis_stable_identifier).to eq(
+      'stable identifiers are the full rspec string'
+    )
+  end
+
+  it 'fall back to a traceback' do
+    ident = SomeClass.new.stuff
+    expect(ident).to include(__FILE__)
+    expect(ident).to include('6')
+  end
+end