[test] check deprecation warnings for undesirable overloads

Author: glessard

stdlib/public/core/CString.swift

@@ -381,6 +381,7 @@ extension String {
   @_specialize(where Encoding == Unicode.UTF16)
   @inlinable // Fold away specializations
   @_alwaysEmitIntoClient
+  @available(*, deprecated)
   public init<Encoding: Unicode.Encoding>(
     decodingCString nullTerminatedCodeUnits: inout Encoding.CodeUnit,
     as sourceEncoding: Encoding.Type

test/stdlib/StringAPICStringDiagnostics.swift

@@ -0,0 +1,33 @@
+// RUN: %target-typecheck-verify-swift -verify-ignore-unknown
+
+import StdlibUnittest
+
+func checkStringOverloadCompilationDiagnostics() {
+
+  _ = String(cString: "string") // expected-warning {{'init(cString:)' is deprecated: Operate directly on the String}}
+
+  _ = String(validatingUTF8: "string") // expected-warning {{init(validatingUTF8:)' is deprecated: Operate directly on the String}}
+
+  _ = String.decodeCString("string", as: Unicode.UTF8.self) // expected-warning {{'decodeCString(_:as:repairingInvalidCodeUnits:)' is deprecated: Operate directly on the String}}
+
+  _ = String(decodingCString: "string", as: Unicode.UTF8.self) // expected-warning {{'init(decodingCString:as:)' is deprecated: Operate directly on the String}}
+}
+
+func checkInoutConversionOverloadCompilationDiagnostics() {
+
+  var i = UInt8.zero
+
+  _ = String(cString: &i) // expected-warning {{'init(cString:)' is deprecated}}
+
+  var c = CChar.zero
+
+  _ = String(cString: &c) // expected-warning {{'init(cString:)' is deprecated}}
+
+  _ = String(validatingUTF8: &c) // expected-warning {{init(validatingUTF8:)' is deprecated}}
+
+  var u = Unicode.UTF8.CodeUnit.zero
+
+  _ = String.decodeCString(&u, as: Unicode.UTF8.self) // expected-warning {{'decodeCString(_:as:repairingInvalidCodeUnits:)' is deprecated}}
+
+  _ = String(decodingCString: &u, as: Unicode.UTF8.self) // expected-warning {{'init(decodingCString:as:)' is deprecated}}
+}