Frontend: Lift restrictions on lazy typechecking options.

The `-experimental-skip-all-function-bodies` flag is specified when producing
modules for indexing. These modules are not used for compilation, so it should
be safe to allow `-experimental-lazy-typecheck` and
`-experimental-skip-non-exportable-decls` as well without
`-enable-library-evolution`.

Resolves rdar://128706306

Author: tshortli

lib/Frontend/CompilerInvocation.cpp

@@ -1154,14 +1154,20 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   Opts.EnableSkipExplicitInterfaceModuleBuildRemarks = Args.hasArg(OPT_remark_skip_explicit_interface_build);
 
-  if (Args.hasArg(OPT_enable_library_evolution)) {
-    Opts.SkipNonExportableDecls |=
-        Args.hasArg(OPT_experimental_skip_non_exportable_decls);
-  } else {
-    if (Args.hasArg(OPT_experimental_skip_non_exportable_decls))
+  if (Args.hasArg(OPT_experimental_skip_non_exportable_decls)) {
+    // Only allow -experimental-skip-non-exportable-decls if either library
+    // evolution is enabled (in which case the module's ABI is independent of
+    // internal declarations) or when -experimental-skip-all-function-bodies is
+    // present. The latter implies the module will not be used for code
+    // generation, so omitting details needed for ABI should be safe.
+    if (Args.hasArg(OPT_enable_library_evolution) ||
+        Args.hasArg(OPT_experimental_skip_all_function_bodies)) {
+      Opts.SkipNonExportableDecls |= true;
+    } else {
       Diags.diagnose(SourceLoc(), diag::ignoring_option_requires_option,
                      "-experimental-skip-non-exportable-decls",
                      "-enable-library-evolution");
+    }
   }
 
   Opts.AllowNonResilientAccess =
@@ -1707,13 +1713,18 @@ static bool ParseTypeCheckerArgs(TypeCheckerOptions &Opts, ArgList &Args,
   Opts.DebugGenericSignatures |= Args.hasArg(OPT_debug_generic_signatures);
   Opts.DebugInverseRequirements |= Args.hasArg(OPT_debug_inverse_requirements);
 
-  if (Args.hasArg(OPT_enable_library_evolution)) {
-    Opts.EnableLazyTypecheck |= Args.hasArg(OPT_experimental_lazy_typecheck);
-  } else {
-    if (Args.hasArg(OPT_experimental_lazy_typecheck))
+  if (Args.hasArg(OPT_experimental_lazy_typecheck)) {
+    // Same restrictions as -experimental-skip-non-exportable-decls. These
+    // could be relaxed in the future, since lazy typechecking is probably not
+    // inherently unsafe without these options.
+    if (Args.hasArg(OPT_enable_library_evolution) ||
+        Args.hasArg(OPT_experimental_skip_all_function_bodies)) {
+      Opts.EnableLazyTypecheck |= Args.hasArg(OPT_experimental_lazy_typecheck);
+    } else {
       Diags.diagnose(SourceLoc(), diag::ignoring_option_requires_option,
                      "-experimental-lazy-typecheck",
                      "-enable-library-evolution");
+    }
   }
 
   if (LangOpts.AllowNonResilientAccess &&

test/Serialization/lazy-typecheck.swift

@@ -11,6 +11,11 @@
 // RUN: %target-swift-frontend -swift-version 5 %S/../Inputs/lazy_typecheck.swift -module-name lazy_typecheck -enable-library-evolution -parse-as-library -package-name Package -DFLAG -emit-module -emit-module-path %t/lazy-skip-all/lazy_typecheck.swiftmodule -debug-forbid-typecheck-prefix NoTypecheck -experimental-lazy-typecheck -experimental-skip-all-function-bodies -experimental-skip-non-exportable-decls
 // RUN: %target-swift-frontend -package-name Package -typecheck -verify %S/../Inputs/lazy_typecheck_client.swift -DFLAG -I %t/lazy-skip-all
 
+// (2a) Verify that -experimental-lazy-typecheck and -experimental-skip-non-exportable-decls do not require
+// -enable-library-evolution if -experimental-skip-all-function-bodies is specified.
+// RUN: %target-swift-frontend -swift-version 5 %S/../Inputs/lazy_typecheck.swift -module-name lazy_typecheck -parse-as-library -package-name Package -DFLAG -emit-module -emit-module-path %t/lazy-skip-all-non-resilient/lazy_typecheck.swiftmodule -debug-forbid-typecheck-prefix NoTypecheck -experimental-lazy-typecheck -experimental-skip-all-function-bodies -experimental-skip-non-exportable-decls
+// RUN: %target-swift-frontend -package-name Package -typecheck -verify %S/../Inputs/lazy_typecheck_client.swift -DFLAG -I %t/lazy-skip-all-non-resilient
+
 // (3) Verify that a module built with -experimental-lazy-typecheck, -experimental-skip-non-inlinable-function-bodies,
 //     and -experimental-skip-non-exportable-decls can be used by the same client as in (1).
 // RUN: %target-swift-frontend -swift-version 5 %S/../Inputs/lazy_typecheck.swift -module-name lazy_typecheck -enable-library-evolution -parse-as-library -package-name Package -DFLAG -emit-module -emit-module-path %t/lazy-skip-non-inlinable/lazy_typecheck.swiftmodule -debug-forbid-typecheck-prefix NoTypecheck -experimental-lazy-typecheck -experimental-skip-non-inlinable-function-bodies -experimental-skip-non-exportable-decls