[Debug] Print only description of value types that conform to CustomStringConvertible

stdlib/public/core/DebuggerSupport.swift

@@ -214,18 +214,22 @@ public enum _DebuggerSupport {
 
     print(String(repeating: " ", count: indent), terminator: "", to: &target)
 
-    // do not expand classes with no custom Mirror
-    // yes, a type can lie and say it's a class when it's not since we only
-    // check the displayStyle - but then the type would have a custom Mirror
-    // anyway, so there's that...
+    // 1. Do not expand classes, unless they conform to CustomReflectable.
+    // 2. Do not expand value types that conform to CustomStringConvertible
+    //    or CustomDebugStringConvertible, unless the type also conform to
+    //    CustomReflectable.
     let isNonClass = mirror.displayStyle != .`class`
+    let isStringConvertible: Bool
     let isCustomReflectable: Bool
     if let value = value {
       isCustomReflectable = value is CustomReflectable
+      isStringConvertible =
+        value is CustomStringConvertible || value is CustomDebugStringConvertible
     } else {
       isCustomReflectable = true
+      isStringConvertible = false
     }
-    let willExpand = isNonClass || isCustomReflectable
+    let willExpand = (isNonClass && !isStringConvertible) || isCustomReflectable
 
     let count = mirror.children.count
     let bullet = isRoot && (count == 0 || !willExpand) ? ""

test/stdlib/DebuggerSupport.swift

@@ -10,6 +10,11 @@ struct StructWithMembers {
   var b = "Hello World"
 }
 
+struct StructWithMembersAndDescription: CustomStringConvertible {
+  var a = 1
+  var description: String { "Hello World" }
+}
+
 class ClassWithMembers {
   var a = 1
   var b = "Hello World"
@@ -61,6 +66,11 @@ StringForPrintObjectTests.test("StructWithMembers") {
   expectEqual(printed, "▿ StructWithMembers\n  - a : 1\n  - b : \"Hello World\"\n")
 }
 
+StringForPrintObjectTests.test("StructWithMembersAndDescription") {
+  let printed = _stringForPrintObject(StructWithMembersAndDescription())
+  expectEqual(printed, "Hello World\n")
+}
+
 #if _runtime(_ObjC)
 StringForPrintObjectTests.test("ClassWithMembers") {
   let printed = _stringForPrintObject(ClassWithMembers())