Add -enable-experimental-lifetime-dependence-inference option

Until stdlib can be built with noncopyable generics, stdlib types
can appear as ~Escapable and ~Copyable, leading to invalid inference.

Use a flag to test implicit lifetime dependence

Author: meg-gupta

include/swift/Basic/LangOptions.h

@@ -556,6 +556,9 @@ namespace swift {
     /// Enable experimental associated type inference improvements.
     bool EnableExperimentalAssociatedTypeInference = false;
 
+    /// Enable implicit lifetime dependence for ~Escapable return types.
+    bool EnableExperimentalLifetimeDependenceInference = false;
+
     /// Enables dumping type witness systems from associated type inference.
     bool DumpTypeWitnessSystems = false;
 

include/swift/Option/FrontendOptions.td

@@ -660,6 +660,14 @@ def disable_experimental_associated_type_inference :
   Flag<["-"], "disable-experimental-associated-type-inference">,
   HelpText<"Disable experimental associated type inference improvements">;
 
+def enable_experimental_lifetime_dependence_inference :
+  Flag<["-"], "enable-experimental-lifetime-dependence-inference">,
+  HelpText<"Enable experimental lifetime dependence inference">;
+
+def disable_experimental_lifetime_dependence_inference :
+  Flag<["-"], "disable-experimental-lifetime-dependence-inference">,
+  HelpText<"Disable experimental lifetime dependence inference">;
+
 def disable_availability_checking : Flag<["-"],
   "disable-availability-checking">,
   HelpText<"Disable checking for potentially unavailable APIs">;

lib/Frontend/CompilerInvocation.cpp

@@ -1346,6 +1346,11 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   if (Args.hasArg(OPT_disable_experimental_associated_type_inference))
     Opts.EnableExperimentalAssociatedTypeInference = false;
 
+  if (Args.hasArg(OPT_enable_experimental_lifetime_dependence_inference))
+    Opts.EnableExperimentalLifetimeDependenceInference = true;
+  if (Args.hasArg(OPT_disable_experimental_lifetime_dependence_inference))
+    Opts.EnableExperimentalLifetimeDependenceInference = false;
+
   Opts.DumpTypeWitnessSystems = Args.hasArg(OPT_dump_type_witness_systems);
 
   for (auto &block: FrontendOpts.BlocklistConfigFilePaths)