Add System.Diagnostics.CodeAnalysis to common targets

BSIPA.sln

@@ -24,6 +24,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution
 		.github\workflows\docs.yml = .github\workflows\docs.yml
 		README.md = README.md
 		.github\release_draft.yml = .github\release_draft.yml
+		System.Diagnostics.CodeAnalysis.cs = System.Diagnostics.CodeAnalysis.cs
 		.github\workflows\tag_docs.yml = .github\workflows\tag_docs.yml
 	EndProjectSection
 EndProject

Common.targets

@@ -1,4 +1,8 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project>
 
+  <ItemGroup>
+    <Compile Include="$(MSBuildThisFileDirectory)\System.Diagnostics.CodeAnalysis.cs"/>
+  </ItemGroup>
+  
 </Project>

System.Diagnostics.CodeAnalysis.cs

@@ -0,0 +1,76 @@
+#if !NETSTANDARD2_1
+namespace System.Diagnostics.CodeAnalysis
+{
+    // Effectively the Microsoft implementation for when it doesn't exist for my convenience
+
+    /// <summary>
+    /// Specifies that when a method returns <see cref="ReturnValue"/>,
+    /// the parameter may be <see langword="null"/> even if the corresponding type disallows it.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class MaybeNullWhenAttribute : Attribute
+    {
+        /// <summary>
+        /// Initializes the attribute with the specified return value condition.
+        /// </summary>
+        /// <param name="returnValue">The return value condition. If the method returns this
+        /// value, the associated parameter may be null.</param>
+        public MaybeNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        /// <summary>
+        /// Gets the return value condition.
+        /// </summary>
+        /// <value>The return value condition. If the method returns this value, the
+        /// associated parameter may be null.</value>
+        public bool ReturnValue { get; }
+    }
+    /// <summary>
+    /// Specifies that when a method returns <see cref="ReturnValue"/>,
+    /// the parameter is not <see langword="null"/> even if the corresponding type allows it.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class NotNullWhenAttribute : Attribute
+    {
+        /// <summary>
+        /// Initializes the attribute with the specified return value condition.
+        /// </summary>
+        /// <param name="returnValue">The return value condition. If the method returns this
+        /// value, the associated parameter is not null.</param>
+        public NotNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        /// <summary>
+        /// Gets the return value condition.
+        /// </summary>
+        /// <value>The return value condition. If the method returns this value, the
+        /// associated parameter is not null.</value>
+        public bool ReturnValue { get; }
+    }
+    /// <summary>
+    /// Specifies that an output may be <see langword="null"/> even if the corresponding type disallows it.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+    internal sealed class MaybeNullAttribute : Attribute
+    {
+    }
+    /// <summary>
+    /// Specifies that the method will not return if the associated Boolean parameter is passed the specified value.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class DoesNotReturnIfAttribute : Attribute
+    {
+        /// <summary>
+        /// Initializes the attribute with the specified parameter value.
+        /// </summary>
+        /// <param name="parameterValue">
+        /// The condition parameter value. Code after the method will be considered unreachable by diagnostics if the argument to
+        /// the associated parameter matches this value.
+        /// </param>
+        public DoesNotReturnIfAttribute(bool parameterValue) => ParameterValue = parameterValue;
+
+        /// <summary>
+        /// Gets the condition parameter value.
+        /// </summary>
+        public bool ParameterValue { get; }
+    }
+}
+#endif