Improve code coverage and fix some test mutations

Src/FluentAssertions/Xml/XDocumentAssertions.cs

@@ -132,7 +132,7 @@ public AndConstraint<XDocumentAssertions> NotBeEquivalentTo(XDocument unexpected
             params object[] becauseArgs)
         {
             Guard.ThrowIfArgumentIsNull(expected, nameof(expected),
-                "Cannot assert the document has a root element if the element name is <null>*");
+                "Cannot assert the document has a root element if the expected name is <null>*");
 
             return HaveRoot(XNamespace.None + expected, because, becauseArgs);
         }
@@ -157,7 +157,7 @@ public AndConstraint<XDocumentAssertions> NotBeEquivalentTo(XDocument unexpected
             }
 
             Guard.ThrowIfArgumentIsNull(expected, nameof(expected),
-                "Cannot assert the document has a root element if the element name is <null>*");
+                "Cannot assert the document has a root element if the expected name is <null>*");
 
             XElement root = Subject.Root;
 
@@ -189,7 +189,7 @@ public AndConstraint<XDocumentAssertions> NotBeEquivalentTo(XDocument unexpected
             params object[] becauseArgs)
         {
             Guard.ThrowIfArgumentIsNull(expected, nameof(expected),
-                "Cannot assert the document has an element if the element name is <null>*");
+                "Cannot assert the document has an element if the expected name is <null>*");
 
             return HaveElement(XNamespace.None + expected, because, becauseArgs);
         }
@@ -217,7 +217,7 @@ public AndConstraint<XDocumentAssertions> NotBeEquivalentTo(XDocument unexpected
             }
 
             Guard.ThrowIfArgumentIsNull(expected, nameof(expected),
-                    "Cannot assert the document has an element if the element name is <null>*");
+                    "Cannot assert the document has an element if the expected name is <null>*");
 
             Execute.Assertion
                 .ForCondition(Subject.Root is not null)

Src/FluentAssertions/Xml/XmlNodeFormatter.cs

@@ -1,5 +1,4 @@
-using System.Collections.Generic;
-using System.Xml;
+using System.Xml;
 using FluentAssertions.Common;
 using FluentAssertions.Formatting;
 

Tests/FluentAssertions.Specs/Collections/CollectionAssertionSpecs.ContainSingle.cs

@@ -13,8 +13,6 @@ namespace FluentAssertions.Specs.Collections
     /// </content>
     public partial class CollectionAssertionSpecs
     {
-        #region Contain Single
-
         [Fact]
         public void When_injecting_a_null_predicate_into_ContainSingle_it_should_throw()
         {
@@ -272,6 +270,22 @@ public void When_collection_is_IEnumerable_it_should_be_evaluated_only_once()
             act.Should().NotThrow();
         }
 
-        #endregion
+        [Fact]
+        public void When_an_assertion_fails_on_ContainSingle_succeeding_message_should_be_included()
+        {
+            // Act
+            Action act = () =>
+            {
+                using var _ = new AssertionScope();
+                var values = new List<int>();
+                values.Should().ContainSingle();
+                values.Should().ContainSingle();
+            };
+
+            // Assert
+            act.Should().Throw<XunitException>()
+                .WithMessage("Expected*to contain a single item, but the collection is empty*" +
+                "Expected*to contain a single item, but the collection is empty*");
+        }
     }
 }

Tests/FluentAssertions.Specs/Collections/GenericDictionaryAssertionSpecs.cs

@@ -3,6 +3,7 @@
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Linq;
+using FluentAssertions.Execution;
 using Xunit;
 using Xunit.Sdk;
 
@@ -1570,6 +1571,24 @@ public void When_a_dictionary_checks_a_list_of_keys_not_to_be_present_it_will_ho
             // Assert
             act.Should().Throw<XunitException>();
         }
+
+        [Fact]
+        public void When_an_assertion_fails_on_ContainKey_succeeding_message_should_be_included()
+        {
+            // Act
+            Action act = () =>
+            {
+                using var _ = new AssertionScope();
+                var values = new Dictionary<int, int>();
+                values.Should().ContainKey(0);
+                values.Should().ContainKey(1);
+            };
+
+            // Assert
+            act.Should().Throw<XunitException>()
+                .WithMessage("Expected*to contain key 0*Expected*to contain key 1*");
+        }
+
         #endregion
 
         #region ContainValue