Frontend: Introduce a -warn-soft-deprecated option.

This option should cause references to declarations that are deprecated in
future OS versions to be diagnosed.

Author: tshortli

include/swift/Basic/LangOptions.h

@@ -462,6 +462,9 @@ namespace swift {
     /// Diagnose implicit 'override'.
     bool WarnImplicitOverrides = false;
 
+    /// Diagnose use of declarations that are soft-deprecated.
+    bool WarnSoftDeprecated = false;
+
     /// Diagnose uses of NSCoding with classes that have unstable mangled names.
     bool EnableNSKeyedArchiverDiagnostics = true;
 

include/swift/Option/Options.td

@@ -838,6 +838,11 @@ def warn_implicit_overrides :
   Flags<[FrontendOption, DoesNotAffectIncrementalBuild]>,
   HelpText<"Warn about implicit overrides of protocol members">;
 
+def warn_soft_deprecated :
+  Flag<["-"], "warn-soft-deprecated">,
+  Flags<[FrontendOption, DoesNotAffectIncrementalBuild, HelpHidden]>,
+  HelpText<"Warn when soft-deprecated declarations are referenced">;
+
 def typo_correction_limit : Separate<["-"], "typo-correction-limit">,
   Flags<[FrontendOption, HelpHidden]>,
   MetaVarName<"<n>">,

lib/Frontend/CompilerInvocation.cpp

@@ -1168,6 +1168,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.WarnImplicitOverrides =
     Args.hasArg(OPT_warn_implicit_overrides);
 
+  Opts.WarnSoftDeprecated = Args.hasArg(OPT_warn_soft_deprecated);
+
   Opts.EnableNSKeyedArchiverDiagnostics =
       Args.hasFlag(OPT_enable_nskeyedarchiver_diagnostics,
                    OPT_disable_nskeyedarchiver_diagnostics,