[Debug] Add user friendly diagnostic when @DebugDescription is atached to a generic (#75302)

  - **Explanation**:
Emit a user friendly diagnostic when `@DebugDescription` is attached to a generic type 
definition.
  - **Scope**:
This will not break existing code. Use of `@DebugDescription` previously produced an 
unhelpful error diagnostic, this change makes the error diagnostic clear.
  - **Original PRs**:
#75307
  - **Risk**:
No risk.
  - **Testing**:
Swift tests, CI
  - **Reviewers**:
@hborla, @stephentyrone, @DougGregor

Author: kastiglione

lib/Macros/Sources/SwiftMacros/DebugDescriptionMacro.swift

@@ -17,9 +17,35 @@ import SwiftDiagnostics
 public enum DebugDescriptionMacro {}
 public enum _DebugDescriptionPropertyMacro {}
 
+/// The member role is used only to perform diagnostics. The member role ensures any diagnostics are emitted once per
+/// type. The macro's core behavior begins with the `MemberAttributeMacro` conformance.
+extension DebugDescriptionMacro: MemberMacro {
+  public static func expansion(
+    of node: AttributeSyntax,
+    providingMembersOf declaration: some DeclGroupSyntax,
+    in context: some MacroExpansionContext
+  )
+  throws -> [DeclSyntax]
+  {
+    guard !declaration.is(ProtocolDeclSyntax.self) else {
+      let message: ErrorMessage = "cannot be attached to a protocol"
+      context.diagnose(node: node, error: message)
+      return []
+    }
+
+    guard declaration.asProtocol(WithGenericParametersSyntax.self)?.genericParameterClause == nil else {
+      let message: ErrorMessage = "cannot be attached to a generic definition"
+      context.diagnose(node: node, error: message)
+      return []
+    }
+
+    return []
+  }
+}
+
 /// A macro which orchestrates conversion of a description property to an LLDB type summary.
 ///
-/// The job of conversion is split across two macros. This macro performs some analysis on the attached
+/// The process of conversion is split across multiple macros/roles. This role performs some analysis on the attached
 /// type, and then delegates to `@_DebugDescriptionProperty` to perform the conversion step.
 extension DebugDescriptionMacro: MemberAttributeMacro {
   public static func expansion(
@@ -31,8 +57,12 @@ extension DebugDescriptionMacro: MemberAttributeMacro {
   throws -> [AttributeSyntax]
   {
     guard !declaration.is(ProtocolDeclSyntax.self) else {
-      let message: ErrorMessage = "cannot be attached to a protocol"
-      context.diagnose(node: node, error: message)
+      // Diagnostics for this case are emitted by the `MemberMacro` conformance.
+      return []
+    }
+
+    guard declaration.asProtocol(WithGenericParametersSyntax.self)?.genericParameterClause == nil else {
+      // Diagnostics for this case are emitted by the `MemberMacro` conformance.
       return []
     }
 

stdlib/public/core/DebuggerSupport.swift

@@ -64,6 +64,7 @@ import SwiftShims
 ///   and other arbitrary computation are not supported. Of note, conditional
 ///   logic and computed properties are not supported.
 /// * Overloaded string interpolation cannot be used.
+@attached(member)
 @attached(memberAttribute)
 public macro DebugDescription() =
   #externalMacro(module: "SwiftMacros", type: "DebugDescriptionMacro")

test/Macros/DebugDescription/error_generic_definition.swift

@@ -0,0 +1,12 @@
+// REQUIRES: swift_swift_parser
+
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -swift-version 5 -module-name main -disable-availability-checking -typecheck -verify -plugin-path %swift-plugin-dir
+
+// expected-error @+1 {{cannot be attached to a generic definition}}
+@DebugDescription
+struct MyGeneric<T> {
+  var debugDescription: String {
+    "zero zero zero"
+  }
+}