Expecting a hash to include another when using composable matchers gives a misleading diff for the subset of the hash that did match:

  1) includes should include {:a => /2/, :b => "bar"}
     Failure/Error:
       expect(
         {
           a: '123',
           b: 'foo',
         }
       ).to include(a: /2/, b: 'bar')
     
       expected {:a => "123", :b => "foo"} to include {:b => "bar"}
       Diff:
       @@ -1,3 +1,3 @@
       -:a => /2/,
       -:b => "bar",
       +:a => "123",
       +:b => "foo",

Notice that

-:a => /2/,
+:a => "123",

is in the diff, despite "123" being matched by /2/ - this can make it harder to determine what the problem is, with a larger hash.

N.b. that the expected {:a => "123", :b => "foo"} to include {:b => "bar"} part of the failure message is good, and does indicate the problem - this issue is only about the "Diff:" part.

Your environment

Resolving dependencies...
Using bundler 1.17.3
Using diff-lcs 1.3
Using rspec-support 3.7.1
Using rspec-core 3.7.1
Using rspec-expectations 3.7.0
Using rspec-mocks 3.7.0
Using rspec 3.7.0
Ruby version is: 2.5.3

Steps to reproduce

# frozen_string_literal: true

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"

  gem "rspec", "3.7.0" # Activate the gem and version you are reporting the issue against.
end

puts "Ruby version is: #{RUBY_VERSION}"
require 'rspec/autorun'

RSpec.describe 'includes' do
  it do
    expect(
      {
        a: '123',
        b: 'foo',
      }
    ).to include(a: /2/, b: 'bar')
  end
end

Expected behavior

The diff does not highlight matched entries as different:

@@ -1,3 +1,3 @@
 :a => "123",
-:b => "bar",
+:b => "foo",

Actual behavior

The diff does highlight matched entries as different:

@@ -1,3 +1,3 @@
-:a => /2/,
-:b => "bar",
+:a => "123",
+:b => "foo",