Frontend: Introduce the -unavailable-decl-optimization flag.

Part of rdar://106674022

Author: tshortli

include/swift/Basic/LangOptions.h

@@ -105,6 +105,22 @@ namespace swift {
     TaskToThread,
   };
 
+  /// Describes the code size optimization behavior for code associated with
+  /// declarations that are marked unavailable.
+  enum class UnavailableDeclOptimization : uint8_t {
+    /// No optimization. Unavailable declarations will contribute to the
+    /// resulting binary by default in this mode.
+    None,
+
+    /// Avoid generating any code for unavailable declarations.
+    ///
+    /// NOTE: This optimization can be ABI breaking for a library evolution
+    /// enabled module because existing client binaries built with a
+    /// pre-Swift 5.9 compiler may depend on linkable symbols associated with
+    /// unavailable declarations.
+    Complete,
+  };
+
   /// A collection of options that affect the language dialect and
   /// provide compiler debugging facilities.
   class LangOptions final {
@@ -171,6 +187,10 @@ namespace swift {
     /// Disable API availability checking.
     bool DisableAvailabilityChecking = false;
 
+    /// Optimization mode for unavailable declarations.
+    UnavailableDeclOptimization UnavailableDeclOptimizationMode =
+        UnavailableDeclOptimization::None;
+
     /// Causes the compiler to use weak linkage for symbols belonging to
     /// declarations introduced at the deployment target.
     bool WeakLinkAtTarget = false;

include/swift/Option/Options.td

@@ -490,6 +490,13 @@ def check_api_availability_only : Flag<["-"], "check-api-availability-only">,
   Flags<[HelpHidden, FrontendOption, NoInteractiveOption]>,
   HelpText<"Deprecated, has no effect">;
 
+def unavailable_decl_optimization_EQ : Joined<["-"], "unavailable-decl-optimization=">,
+  MetaVarName<"<complete,none>">,
+  Flags<[FrontendOption, NoInteractiveOption]>,
+  HelpText<"Specify the optimization mode for unavailable declarations. The "
+           "value may be 'none' (no optimization) or 'complete' (code is not "
+           "generated at all unavailable declarations)">;
+
 def library_level : Separate<["-"], "library-level">,
   MetaVarName<"<level>">,
   Flags<[HelpHidden, FrontendOption, ModuleInterfaceOption]>,

lib/Frontend/CompilerInvocation.cpp

@@ -552,6 +552,20 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Diags.diagnose(SourceLoc(), diag::warn_flag_deprecated,
                    "-check-api-availability-only");
 
+  if (const Arg *A = Args.getLastArg(OPT_unavailable_decl_optimization_EQ)) {
+    auto value =
+        llvm::StringSwitch<Optional<UnavailableDeclOptimization>>(A->getValue())
+            .Case("none", UnavailableDeclOptimization::None)
+            .Case("complete", UnavailableDeclOptimization::Complete)
+            .Default(None);
+
+    if (value)
+      Opts.UnavailableDeclOptimizationMode = *value;
+    else
+      Diags.diagnose(SourceLoc(), diag::error_invalid_arg_value,
+                     A->getAsString(Args), A->getValue());
+  }
+
   Opts.WeakLinkAtTarget |= Args.hasArg(OPT_weak_link_at_target);
 
   if (auto A = Args.getLastArg(OPT_enable_conformance_availability_errors,