[Version 10.0] Feature support for CallerArgumentExpression attribute

standard/attributes.md

@@ -494,7 +494,7 @@
 - `System.Diagnostics.ConditionalAttribute` ([§23.5.3](attributes.md#2353-the-conditional-attribute)), is a multi-use attribute class which is used to define conditional methods and conditional attribute classes. This attribute indicates a condition by testing a conditional compilation symbol.
 - `System.ObsoleteAttribute` ([§23.5.4](attributes.md#2354-the-obsolete-attribute)), which is used to mark a member as obsolete.
 - `System.Runtime.CompilerServices.AsyncMethodBuilderAttribute` ([§23.5.5](attributes.md#2355-the-asyncmethodbuilder-attribute)), which is used to establish a task builder for an async method.
-- `System.Runtime.CompilerServices.CallerLineNumberAttribute` ([§23.5.6.2](attributes.md#23562-the-callerlinenumber-attribute)), `System.Runtime.CompilerServices.CallerFilePathAttribute` ([§23.5.6.3](attributes.md#23563-the-callerfilepath-attribute)), and `System.Runtime.CompilerServices.CallerMemberNameAttribute` ([§23.5.6.4](attributes.md#23564-the-callermembername-attribute)), which are used to supply information about the calling context to optional parameters.
+- `System.Runtime.CompilerServices.CallerLineNumberAttribute` ([§23.5.6.2](attributes.md#23562-the-callerlinenumber-attribute)), `System.Runtime.CompilerServices.CallerFilePathAttribute` ([§23.5.6.3](attributes.md#23563-the-callerfilepath-attribute)), `System.Runtime.CompilerServices.CallerMemberNameAttribute` ([§23.5.6.4](attributes.md#23564-the-callermembername-attribute)), and `System.Runtime.CompilerServices.CallerArgumentExpressionAttribute` (§callargexpattr), which are used to supply information about the calling context to optional parameters.
 - `System.Runtime.CompilerServices.EnumeratorCancellationAttribute` ([§23.5.8](attributes.md#2358-the-enumeratorcancellation-attribute)), which is used to specify parameter for the cancellation token in an asynchronous iterator.
 - `System.Runtime.CompilerServices.ModuleInitializer` ([§23.5.9](attributes.md#2359-the-moduleinitializer-attribute)), which is used to mark a method as a module initializer.
 
@@ -845,6 +845,79 @@
 
 For an invocation that occurs within a local function, the name of the method that calls that local function is used. Consider the following: if method `M` calls local function `F1`, which in turn calls local function `F2`, and `F2` has a parameter marked with this attribute, the method name passed to `F2` is `M`, because a local function is *not* a function member!
 
+#### §callargexpattr The CallerArgumentExpression attribute
+
+The attribute `System.Runtime.CompilerServices.CallerArgumentExpression` is applied to a *target parameter*, and can result in the capture of the source-code text of a sibling parameter’s argument as a string, referred to here as the *captured string*.
+
+Except when it is the first parameter in an extension method, the target parameter shall have a *default_argument*.
+
+Consider the following method declaration:
+
+<!-- Example: {template:"standalone-lib-without-using", name:"CallerArgumentAttr1"} -->
+```csharp
+using System;
+using System.Runtime.CompilerServices;
+#nullable enable
+class Test
+{
+    public static void M(int val = 0, [CallerArgumentExpression("val")] string? text = null)
+    {
+        Console.WriteLine($"val = {val}, text = <{text}>");
+    }
+}
+```
+
+in which the target parameter is `text` and the sibling parameter is `val`, whose corresponding argument’s source-code text can be captured in `text` when `M` is called.
+
+The attribute constructor takes an argument of type `string`. That string
+
+- Shall contain the name of a sibling parameter; otherwise, the attribute is ignored.
+- Shall omit the leading `@` from a parameter name having that prefix.
+
+A *parameter_list* may contain multiple target parameters.
+
+The type of the target parameter shall have a standard conversion from `string`.
+
+> *Note:* This means no user-defined conversions from `string` are allowed, and in practice means the type of such a parameter must be `string`, `object`, or an interface implemented by `string`. *end note*
+
+If an explicit argument is passed for the target parameter, no string is captured, and that parameter takes on that argument’s value. Otherwise, the text for the argument corresponding to the sibling parameter is converted to a captured string, according to the following rules:
+
+- Leading and trailing white space is removed both before and after any outermost grouping parentheses are removed.
+- All outermost grouping parentheses are removed both before and after any leading and trailing white space is removed.
+- All other *input_element*s are retained verbatim (including white space, comments, *Unicode_Escape_Sequence*s, and `@` prefixes on identifiers).
+
+The captured string is then passed as the argument corresponding to the target parameter. However, if the argument for the sibling parameter is omitted, the target parameter takes on its *default_argument* value.
+
+> *Example*: Given the declaration of `M` above, consider the following calls to `M`:
+>
+> <!-- Example: {template:"standalone-console", name:"CallerArgumentAttr2", inferOutput:true, additionalFiles:["CallerArgumentAttrM.cs"]} -->
+> ```csharp
+> Test.M();
+> Test.M(123);
+> Test.M(123, null);
+> Test.M(123, "xyz");
+> Test.M(  1  +      2 );
+> Test.M(( ( (123) + 0) ) );
+> int local = 10;
+> Test.M(l\u006fcal /*...*/ + // xxx
+>   5);
+> ```
+>
+> the output produced is
+>
+> ```console
+> val = 0, text = <>
+> val = 123, text = <123>
+> val = 123, text = <>
+> val = 123, text = <xyz>
+> val = 3, text = <1  +      2>
+> val = 123, text = <(123) + 0>
+> val = 15, text = <l\u006fcal /*...*/ + // xxx
+>   5>
+> ```
+>
+> *end example*
+
 ### 23.5.7 Code analysis attributes
 
 #### 23.5.7.1 General
@@ -967,7 +1040,7 @@
 > ```csharp
 > #nullable enable
 > public class X
 > {
 >     private void ThrowIfNull([DoesNotReturnIf(true)] bool isNull, string argumentName)
 >     {
 >         if (!isNull)
@@ -1192,5 +1265,3 @@
 > Now, the indexer’s name is `TheItem`.
 >
 > *end example*
-
-

standard/standard-library.md

@@ -190,7 +190,7 @@
     public class NullReferenceException : Exception
     {
         public NullReferenceException();
         public NullReferenceException(string? message);
         public NullReferenceException(string? message, Exception? innerException);
     }
 
@@ -238,7 +238,7 @@
     public sealed class StackOverflowException : Exception
     {
         public StackOverflowException();
         public StackOverflowException(string? message);
         public StackOverflowException(string? message, Exception? innerException);
     }
 
@@ -247,7 +247,7 @@
         public int Length { get; }
         public char this [int index] { get; }
         public static string Format(string format, params object?[] args);
         System.Collections.IEnumerator IEnumerable.GetEnumerator();
         System.Collections.Generic.IEnumerator<char> IEnumerable<char>.GetEnumerator();
     }
 
@@ -407,7 +407,7 @@
     public class OperationCanceledException : Exception
     {
         public OperationCanceledException();
         public OperationCanceledException(string? message);
         public OperationCanceledException(string? message, Exception? innerException);
     }
 
@@ -680,7 +680,7 @@
     }
 
     public interface IAsyncEnumerable<out T>
     {
         IAsyncEnumerator<T> GetAsyncEnumerator(CancellationToken token = default);
     }
 
@@ -747,7 +747,7 @@
       System.AttributeTargets.Property, AllowMultiple=true, Inherited=false)]
     public sealed class MemberNotNullWhenAttribute : Attribute
     {
         public MemberNotNullWhenAttribute(bool returnValue, string member) {}
         public MemberNotNullWhenAttribute(bool returnValue, params string[] members) {}
     }
 
@@ -793,7 +793,14 @@
 
         public Type BuilderType { get; }
     }
 
+    [System.AttributeUsage(System.AttributeTargets.Parameter, AllowMultiple=false,
+        Inherited=false)]
+    public sealed class CallerArgumentExpressionAttribute : Attribute
+    {
+        public CallerArgumentExpressionAttribute (string parameterName);
+    }
+
     [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
     public sealed class CallerFilePathAttribute : Attribute
     {
@@ -1394,6 +1401,7 @@
 - `global::System.Linq.Expressions.Expression<TDelegate>`
 - `global::System.Reflection.MemberInfo`
 - `global::System.Runtime.CompilerServices.AsyncMethodBuilderAttribute`
+- `global::System.Runtime.CompilerServices.CallerArgumentExpressionAttribute`
 - `global::System.Runtime.CompilerServices.CallerFileAttribute`
 - `global::System.Runtime.CompilerServices.CallerLineNumberAttribute`
 - `global::System.Runtime.CompilerServices.CallerMemberNameAttribute`

tools/example-templates/additional-files/CallerArgumentAttrM.cs

@@ -0,0 +1,10 @@
+using System;
+using System.Runtime.CompilerServices;
+#nullable enable
+class Test
+{
+    public static void M(int val = 0, [CallerArgumentExpression("val")] string? text = null)
+    {
+        Console.WriteLine($"val = {val}, text = <{text}>");
+    }
+}