diff --git a/docs/reference.md b/docs/reference.md
index afd928c..92d1f57 100644
--- a/docs/reference.md
+++ b/docs/reference.md
@@ -151,6 +151,7 @@
     *   [testonly: [boolean] Declares a target must only be used for testing.](#var_testonly)
     *   [visibility: [label list] A list of labels that can depend on a target.](#var_visibility)
     *   [walk_keys: [string list] Key(s) for managing the metadata collection walk.](#var_walk_keys)
+    *   [weak_frameworks: [name list] Name of frameworks that must be weak linked.](#var_weak_frameworks)
     *   [write_runtime_deps: Writes the target's runtime_deps to the given path.](#var_write_runtime_deps)
     *   [xcode_extra_attributes: [scope] Extra attributes for Xcode projects.](#var_xcode_extra_attributes)
     *   [xcode_test_application_name: [string] Name for Xcode test target.](#var_xcode_test_application_name)
@@ -355,10 +356,6 @@
 #### **Command-specific switches**
 
 ```
-  --force
-      Ignores specifications of "check_includes = false" and checks all
-      target's files that match the target label.
-
   --check-generated
       Generated files are normally not checked since they do not exist
       until after a build. With this flag, those generated files that
@@ -367,6 +364,18 @@
   --check-system
      Check system style includes (using <angle brackets>) in addition to
      "double quote" includes.
+
+  --default-toolchain
+      Normally wildcard targets are matched in all toolchains. This
+      switch makes wildcard labels with no explicit toolchain reference
+      only match targets in the default toolchain.
+
+      Non-wildcard inputs with no explicit toolchain specification will
+      always match only a target in the default toolchain if one exists.
+
+  --force
+      Ignores specifications of "check_includes = false" and checks all
+      target's files that match the target label.
 ```
 
 #### **What gets checked**
@@ -465,7 +474,7 @@
   gn check out/Default "//foo/*
       Check only the files in targets in the //foo directory tree.
 ```
-### <a name="cmd_clean"></a>**gn clean &lt;out_dir&gt;**
+### <a name="cmd_clean"></a>**gn clean &lt;out_dir&gt;...**
 
 ```
   Deletes the contents of the output directory except for args.gn and
@@ -521,6 +530,7 @@
   testonly
   visibility
   walk_keys
+  weak_frameworks
 
   runtime_deps
       Compute all runtime deps for the given target. This is a computed list
@@ -533,15 +543,15 @@
 ```
 
 #### **Shared flags**
-```
-  --all-toolchains
-      Normally only inputs in the default toolchain will be included.
-      This switch will turn on matching all toolchains.
 
-      For example, a file is in a target might be compiled twice:
-      once in the default toolchain and once in a secondary one. Without
-      this flag, only the default toolchain one will be matched by
-      wildcards. With this flag, both will be matched.
+```
+  --default-toolchain
+      Normally wildcard targets are matched in all toolchains. This
+      switch makes wildcard labels with no explicit toolchain reference
+      only match targets in the default toolchain.
+
+      Non-wildcard inputs with no explicit toolchain specification will
+      always match only a target in the default toolchain if one exists.
 
   --format=json
       Format the output as JSON instead of text.
@@ -553,8 +563,9 @@
   --blame
       Used with any value specified on a config, this will name the config that
       causes that target to get the flag. This doesn't currently work for libs,
-      lib_dirs, frameworks and framework_dirs because those are inherited and
-      are more complicated to figure out the blame (patches welcome).
+      lib_dirs, frameworks, weak_frameworks and framework_dirs because those are
+      inherited and are more complicated to figure out the blame (patches
+      welcome).
 ```
 
 #### **Configs**
@@ -584,6 +595,7 @@
   --all
       Collects all recursive dependencies and prints a sorted flat list. Also
       usable with --tree (see below).
+
   --as=(buildfile|label|output)
       How to print targets.
 
@@ -610,6 +622,7 @@
 
       Tree output can not be used with the filtering or output flags: --as,
       --type, --testonly.
+
   --type=(action|copy|executable|group|loadable_module|shared_library|
           source_set|static_library)
       Restrict outputs to targets matching the given type. If
@@ -810,6 +823,12 @@
 #### **Compilation Database**
 
 ```
+  --export-rust-project
+      Produces a rust-project.json file in the root of the build directory
+      This is used for various tools in the Rust ecosystem allowing for the
+      replay of individual compilations independent of the build system.
+      This is an unstable format and likely to change without warning.
+
   --export-compile-commands[=<target_name1,target_name2...>]
       Produces a compile_commands.json file in the root of the build directory
       containing an array of “command objects”, where each command object
@@ -842,7 +861,7 @@
   gn help --markdown all
       Dump all help to stdout in markdown format.
 ```
-### <a name="cmd_ls"></a>**gn ls &lt;out_dir&gt; [&lt;label_pattern&gt;] [\--all-toolchains] [\--as=...]**
+### <a name="cmd_ls"></a>**gn ls &lt;out_dir&gt; [&lt;label_pattern&gt;] [\--default-toolchain] [\--as=...]**
 ```
       [--type=...] [--testonly=...]
 
@@ -870,14 +889,13 @@
           Prints the first output file for the target relative to the
           root build directory.
 
-  --all-toolchains
-      Normally only inputs in the default toolchain will be included.
-      This switch will turn on matching all toolchains.
+  --default-toolchain
+      Normally wildcard targets are matched in all toolchains. This
+      switch makes wildcard labels with no explicit toolchain reference
+      only match targets in the default toolchain.
 
-      For example, a file is in a target might be compiled twice:
-      once in the default toolchain and once in a secondary one. Without
-      this flag, only the default toolchain one will be matched by
-      wildcards. With this flag, both will be matched.
+      Non-wildcard inputs with no explicit toolchain specification will
+      always match only a target in the default toolchain if one exists.
 
   --testonly=(true|false)
       Restrict outputs to targets with the testonly flag set
@@ -910,10 +928,6 @@
 
   gn ls out/Debug "//base/*" --as=output | xargs ninja -C out/Debug
       Builds all targets in //base and all subdirectories.
-
-  gn ls out/Debug //base --all-toolchains
-      Lists all variants of the target //base:base (it may be referenced
-      in multiple toolchains).
 ```
 ### <a name="cmd_meta"></a>**gn meta**
 
@@ -1075,7 +1089,7 @@
 
 ```
   gn refs <out_dir> (<label_pattern>|<label>|<file>|@<response_file>)*
-          [--all] [--all-toolchains] [--as=...] [--testonly=...] [--type=...]
+          [--all] [--default-toolchain] [--as=...] [--testonly=...] [--type=...]
 
   Finds reverse dependencies (which targets reference something). The input is
   a list containing:
@@ -1111,14 +1125,6 @@
       directly or indirectly on that file.
 
       When used with --tree, turns off eliding to show a complete tree.
-  --all-toolchains
-      Normally only inputs in the default toolchain will be included.
-      This switch will turn on matching all toolchains.
-
-      For example, a file is in a target might be compiled twice:
-      once in the default toolchain and once in a secondary one. Without
-      this flag, only the default toolchain one will be matched by
-      wildcards. With this flag, both will be matched.
 
   --as=(buildfile|label|output)
       How to print targets.
@@ -1132,10 +1138,19 @@
           Prints the first output file for the target relative to the
           root build directory.
 
+  --default-toolchain
+      Normally wildcard targets are matched in all toolchains. This
+      switch makes wildcard labels with no explicit toolchain reference
+      only match targets in the default toolchain.
+
+      Non-wildcard inputs with no explicit toolchain specification will
+      always match only a target in the default toolchain if one exists.
+
   -q
      Quiet. If nothing matches, don't print any output. Without this option, if
      there are no matches there will be an informational message printed which
      might interfere with scripts processing the output.
+
   --testonly=(true|false)
       Restrict outputs to targets with the testonly flag set
       accordingly. When unspecified, the target's testonly flags are
@@ -1147,6 +1162,7 @@
 
       Tree output can not be used with the filtering or output flags: --as,
       --type, --testonly.
+
   --type=(action|copy|executable|group|loadable_module|shared_library|
           source_set|static_library)
       Restrict outputs to targets matching the given type. If
@@ -1956,8 +1972,7 @@
 ### <a name="func_source_set"></a>**source_set**: Declare a source set target.
 
 ```
-  The language of a source_set target is determined by the extensions present
-  in its sources.
+  Only C-language source sets are supported at the moment.
 ```
 
 #### **C-language source_sets**
@@ -1985,15 +2000,6 @@
   when linking multiple static libraries into a shared library.
 ```
 
-#### **Rust-language source_sets**
-
-```
-  A Rust source set is a collection of sources that get passed along to the
-  final target that depends on it. No compilation is performed, and the source
-  files are simply added as dependencies on the eventual rustc invocation that
-  would produce a binary.
-```
-
 #### **Variables**
 
 ```
@@ -2890,14 +2896,14 @@
   with a double slash like "//foo/bar"), you should use the get_path_info()
   function. This function won't work because it will always make relative
   paths, and it needs to support making paths relative to the source root, so
-  can't also generate source-absolute paths without more special-cases.
+  it can't also generate source-absolute paths without more special-cases.
 ```
 
 #### **Arguments**
 
 ```
   input
-      A string or list of strings representing file or directory names These
+      A string or list of strings representing file or directory names. These
       can be relative paths ("foo/bar.txt"), system absolute paths
       ("/foo/bar.txt"), or source absolute paths ("//foo/bar.txt").
 
@@ -3462,6 +3468,7 @@
           "-lfreetype -lexpat".
 
     framework_switch [string, optional, link tools only]
+    weak_framework_switch [string, optional, link tools only]
     framework_dir_switch [string, optional, link tools only]
         Valid for: Linker tools
 
@@ -3471,11 +3478,14 @@
 
         If you specified:
           framework_switch = "-framework "
+          weak_framework_switch = "-weak_framework "
           framework_dir_switch = "-F"
-        then the "{{libs}}" expansion for
-          [ "UIKit.framework", "$root_out_dir/Foo.framework" ]
-        would be
-          "-framework UIKit -F. -framework Foo"
+        and:
+          framework_dirs = [ "$root_out_dir" ]
+          frameworks = [ "UIKit.framework", "Foo.framework" ]
+          weak_frameworks = [ "MediaPlayer.framework" ]
+        would be:
+          "-F. -framework UIKit -framework Foo -weak_framework MediaPlayer"
 
     outputs  [list of strings with substitutions]
         Valid for: Linker and compiler tools (required)
@@ -3757,7 +3767,8 @@
         Shared libraries packaged as framework bundle. This is principally
         used on Apple's platforms (macOS and iOS). All name must be ending
         with ".framework" suffix; the suffix will be stripped when expanding
-        {{frameworks}} and each item will be preceded by "-framework".
+        {{frameworks}} and each item will be preceded by "-framework" or
+        "-weak_framework".
 
   The static library ("alink") tool allows {{arflags}} plus the common tool
   substitutions.
@@ -6382,6 +6393,42 @@
 
   See "gn help generated_file".
 ```
+### <a name="var_weak_frameworks"></a>**weak_frameworks**: [name list] Name of frameworks that must be weak linked.
+
+```
+  A list of framework names.
+
+  The frameworks named in that list will be weak linked with any dynamic link
+  type target. Weak linking instructs the dynamic loader to attempt to load
+  the framework, but if it is not able to do so, it leaves any imported symbols
+  unresolved. This is typically used when a framework is present in a new
+  version of an SDK but not on older versions of the OS that the software runs
+  on.
+```
+
+#### **Ordering of flags and values**
+
+```
+  1. Those set on the current target (not in a config).
+  2. Those set on the "configs" on the target in order that the
+     configs appear in the list.
+  3. Those set on the "all_dependent_configs" on the target in order
+     that the configs appear in the list.
+  4. Those set on the "public_configs" on the target in order that
+     those configs appear in the list.
+  5. all_dependent_configs pulled from dependencies, in the order of
+     the "deps" list. This is done recursively. If a config appears
+     more than once, only the first occurence will be used.
+  6. public_configs pulled from dependencies, in the order of the
+     "deps" list. If a dependency is public, they will be applied
+     recursively.
+```
+
+#### **Example**
+
+```
+  weak_frameworks = [ "OnlyOnNewerOSes.framework" ]
+```
 ### <a name="var_write_runtime_deps"></a>**write_runtime_deps**: Writes the target's runtime_deps to the given path.
 
 ```
diff --git a/src/gn/c_tool.cc b/src/gn/c_tool.cc
index 4046e33..7c06364 100644
--- a/src/gn/c_tool.cc
+++ b/src/gn/c_tool.cc
@@ -21,6 +21,7 @@
     : Tool(n), depsformat_(DEPS_GCC), precompiled_header_type_(PCH_NONE) {
   CHECK(ValidateName(n));
   set_framework_switch("-framework ");
+  set_weak_framework_switch("-weak_framework ");
   set_framework_dir_switch("-F");
   set_lib_dir_switch("-L");
   set_lib_switch("-l");
@@ -184,6 +185,8 @@
 
   if (!ReadDepsFormat(scope, err) || !ReadPrecompiledHeaderType(scope, err) ||
       !ReadString(scope, "framework_switch", &framework_switch_, err) ||
+      !ReadString(scope, "weak_framework_switch", &weak_framework_switch_,
+                  err) ||
       !ReadString(scope, "framework_dir_switch", &framework_dir_switch_, err) ||
       !ReadString(scope, "lib_switch", &lib_switch_, err) ||
       !ReadString(scope, "lib_dir_switch", &lib_dir_switch_, err) ||
diff --git a/src/gn/command_desc.cc b/src/gn/command_desc.cc
index 081ae83..278d45d 100644
--- a/src/gn/command_desc.cc
+++ b/src/gn/command_desc.cc
@@ -299,6 +299,7 @@
           {variables::kDataKeys, DefaultHandler},
           {variables::kRebase, DefaultHandler},
           {variables::kWalkKeys, DefaultHandler},
+          {variables::kWeakFrameworks, DefaultHandler},
           {variables::kWriteOutputConversion, DefaultHandler},
           {"runtime_deps", DefaultHandler}};
 }
@@ -386,6 +387,7 @@
   HandleProperty(variables::kDataKeys, handler_map, v, dict);
   HandleProperty(variables::kRebase, handler_map, v, dict);
   HandleProperty(variables::kWalkKeys, handler_map, v, dict);
+  HandleProperty(variables::kWeakFrameworks, handler_map, v, dict);
   HandleProperty(variables::kWriteOutputConversion, handler_map, v, dict);
 
 #undef HandleProperty
@@ -447,6 +449,7 @@
   HandleProperty(variables::kLibDirs, handler_map, v, dict);
   HandleProperty(variables::kPrecompiledHeader, handler_map, v, dict);
   HandleProperty(variables::kPrecompiledSource, handler_map, v, dict);
+  HandleProperty(variables::kWeakFrameworks, handler_map, v, dict);
 
 #undef HandleProperty
 
@@ -508,6 +511,7 @@
   testonly
   visibility
   walk_keys
+  weak_frameworks
 
   runtime_deps
       Compute all runtime deps for the given target. This is a computed list
@@ -533,8 +537,9 @@
   --blame
       Used with any value specified on a config, this will name the config that
       causes that target to get the flag. This doesn't currently work for libs,
-      lib_dirs, frameworks and framework_dirs because those are inherited and
-      are more complicated to figure out the blame (patches welcome).
+      lib_dirs, frameworks, weak_frameworks and framework_dirs because those are
+      inherited and are more complicated to figure out the blame (patches
+      welcome).
 
 Configs
 
diff --git a/src/gn/compile_commands_writer.cc b/src/gn/compile_commands_writer.cc
index df5e73f..8d6c7fb 100644
--- a/src/gn/compile_commands_writer.cc
+++ b/src/gn/compile_commands_writer.cc
@@ -85,6 +85,9 @@
   flags.frameworks = FlagsGetter<std::string>(
       target, &ConfigValues::frameworks,
       FrameworksWriter(ESCAPE_SPACE, true, "-framework"));
+  flags.frameworks += FlagsGetter<std::string>(
+      target, &ConfigValues::weak_frameworks,
+      FrameworksWriter(ESCAPE_SPACE, true, "-weak_framework"));
 
   flags.includes = FlagsGetter<SourceDir>(target, &ConfigValues::include_dirs,
                                           IncludeWriter(path_output));
diff --git a/src/gn/config_values.cc b/src/gn/config_values.cc
index d4f597e..ba5ddf7 100644
--- a/src/gn/config_values.cc
+++ b/src/gn/config_values.cc
@@ -30,6 +30,7 @@
   VectorAppend(&cflags_objcc_, append.cflags_objcc_);
   VectorAppend(&defines_, append.defines_);
   VectorAppend(&frameworks_, append.frameworks_);
+  VectorAppend(&weak_frameworks_, append.weak_frameworks_);
   VectorAppend(&framework_dirs_, append.framework_dirs_);
   VectorAppend(&include_dirs_, append.include_dirs_);
   VectorAppend(&inputs_, append.inputs_);
diff --git a/src/gn/config_values.h b/src/gn/config_values.h
index efd70eb..ef57b26 100644
--- a/src/gn/config_values.h
+++ b/src/gn/config_values.h
@@ -44,6 +44,7 @@
   STRING_VALUES_ACCESSOR(defines)
   DIR_VALUES_ACCESSOR(framework_dirs)
   STRING_VALUES_ACCESSOR(frameworks)
+  STRING_VALUES_ACCESSOR(weak_frameworks)
   DIR_VALUES_ACCESSOR(include_dirs)
   STRING_VALUES_ACCESSOR(ldflags)
   DIR_VALUES_ACCESSOR(lib_dirs)
@@ -88,6 +89,7 @@
   std::vector<SourceDir> include_dirs_;
   std::vector<SourceDir> framework_dirs_;
   std::vector<std::string> frameworks_;
+  std::vector<std::string> weak_frameworks_;
   std::vector<SourceFile> inputs_;
   std::vector<std::string> ldflags_;
   std::vector<SourceDir> lib_dirs_;
diff --git a/src/gn/config_values_generator.cc b/src/gn/config_values_generator.cc
index 52ef61f..c8070d2 100644
--- a/src/gn/config_values_generator.cc
+++ b/src/gn/config_values_generator.cc
@@ -44,6 +44,34 @@
   (config_values->*accessor)().swap(result);
 }
 
+void GetFrameworksList(Scope* scope,
+                       const char* var_name,
+                       ConfigValues* config_values,
+                       std::vector<std::string>& (ConfigValues::*accessor)(),
+                       Err* err) {
+  const Value* value = scope->GetValue(var_name, true);
+  if (!value)
+    return;
+
+  std::vector<std::string> frameworks;
+  if (!ExtractListOfStringValues(*value, &frameworks, err))
+    return;
+
+  // All strings must end with ".frameworks".
+  for (const std::string& framework : frameworks) {
+    std::string_view framework_name = GetFrameworkName(framework);
+    if (framework_name.empty()) {
+      *err = Err(*value,
+                 "This frameworks value is wrong."
+                 "All listed frameworks names must not include any\n"
+                 "path component and have \".framework\" extension.");
+      return;
+    }
+  }
+
+  (config_values->*accessor)().swap(frameworks);
+}
+
 }  // namespace
 
 ConfigValuesGenerator::ConfigValuesGenerator(ConfigValues* dest_values,
@@ -105,27 +133,10 @@
   }
 
   // Frameworks
-  const Value* frameworks_value =
-      scope_->GetValue(variables::kFrameworks, true);
-  if (frameworks_value) {
-    std::vector<std::string> frameworks;
-    if (!ExtractListOfStringValues(*frameworks_value, &frameworks, err_))
-      return;
-
-    // All strings must end with ".frameworks".
-    for (const std::string& framework : frameworks) {
-      std::string_view framework_name = GetFrameworkName(framework);
-      if (framework_name.empty()) {
-        *err_ = Err(*frameworks_value,
-                    "This frameworks value is wrong."
-                    "All listed frameworks names must not include any\n"
-                    "path component and have \".framework\" extension.");
-        return;
-      }
-    }
-
-    config_values_->frameworks().swap(frameworks);
-  }
+  GetFrameworksList(scope_, variables::kFrameworks, config_values_,
+                    &ConfigValues::frameworks, err_);
+  GetFrameworksList(scope_, variables::kWeakFrameworks, config_values_,
+                    &ConfigValues::weak_frameworks, err_);
 
   // Precompiled headers.
   const Value* precompiled_header_value =
diff --git a/src/gn/desc_builder.cc b/src/gn/desc_builder.cc
index 8ef77a6..9590975 100644
--- a/src/gn/desc_builder.cc
+++ b/src/gn/desc_builder.cc
@@ -569,6 +569,16 @@
                                      std::move(frameworks));
       }
     }
+    if (what(variables::kWeakFrameworks)) {
+      const auto& weak_frameworks = target_->all_weak_frameworks();
+      if (!weak_frameworks.empty()) {
+        auto frameworks = std::make_unique<base::ListValue>();
+        for (size_t i = 0; i < weak_frameworks.size(); i++)
+          frameworks->AppendString(weak_frameworks[i]);
+        res->SetWithoutPathExpansion(variables::kWeakFrameworks,
+                                     std::move(frameworks));
+      }
+    }
 
     if (what(variables::kFrameworkDirs)) {
       const auto& all_framework_dirs = target_->all_framework_dirs();
diff --git a/src/gn/function_toolchain.cc b/src/gn/function_toolchain.cc
index 9efff8e..92721dc 100644
--- a/src/gn/function_toolchain.cc
+++ b/src/gn/function_toolchain.cc
@@ -408,6 +408,7 @@
           "-lfreetype -lexpat".
 
     framework_switch [string, optional, link tools only]
+    weak_framework_switch [string, optional, link tools only]
     framework_dir_switch [string, optional, link tools only]
         Valid for: Linker tools
 
@@ -417,11 +418,14 @@
 
         If you specified:
           framework_switch = "-framework "
+          weak_framework_switch = "-weak_framework "
           framework_dir_switch = "-F"
-        then the "{{libs}}" expansion for
-          [ "UIKit.framework", "$root_out_dir/Foo.framework" ]
-        would be
-          "-framework UIKit -F. -framework Foo"
+        and:
+          framework_dirs = [ "$root_out_dir" ]
+          frameworks = [ "UIKit.framework", "Foo.framework" ]
+          weak_frameworks = [ "MediaPlayer.framework" ]
+        would be:
+          "-F. -framework UIKit -framework Foo -weak_framework MediaPlayer"
 
     outputs  [list of strings with substitutions]
         Valid for: Linker and compiler tools (required)
@@ -702,7 +706,8 @@
         Shared libraries packaged as framework bundle. This is principally
         used on Apple's platforms (macOS and iOS). All name must be ending
         with ".framework" suffix; the suffix will be stripped when expanding
-        {{frameworks}} and each item will be preceded by "-framework".
+        {{frameworks}} and each item will be preceded by "-framework" or
+        "-weak_framework".
 
 )"  // String break to prevent overflowing the 16K max VC string length.
     R"(  The static library ("alink") tool allows {{arflags}} plus the common tool
diff --git a/src/gn/ninja_binary_target_writer.cc b/src/gn/ninja_binary_target_writer.cc
index cde6be6..2afb3db 100644
--- a/src/gn/ninja_binary_target_writer.cc
+++ b/src/gn/ninja_binary_target_writer.cc
@@ -341,11 +341,16 @@
 
 void NinjaBinaryTargetWriter::WriteFrameworks(std::ostream& out,
                                               const Tool* tool) {
-  FrameworksWriter writer(tool->framework_switch());
-
   // Frameworks that have been recursively pushed through the dependency tree.
+  FrameworksWriter writer(tool->framework_switch());
   const auto& all_frameworks = target_->all_frameworks();
   for (size_t i = 0; i < all_frameworks.size(); i++) {
     writer(all_frameworks[i], out);
   }
+
+  FrameworksWriter weak_writer(tool->weak_framework_switch());
+  const auto& all_weak_frameworks = target_->all_weak_frameworks();
+  for (size_t i = 0; i < all_weak_frameworks.size(); i++) {
+    weak_writer(all_weak_frameworks[i], out);
+  }
 }
diff --git a/src/gn/ninja_c_binary_target_writer_unittest.cc b/src/gn/ninja_c_binary_target_writer_unittest.cc
index 315232b..bc9907a 100644
--- a/src/gn/ninja_c_binary_target_writer_unittest.cc
+++ b/src/gn/ninja_c_binary_target_writer_unittest.cc
@@ -540,6 +540,7 @@
   Target target(setup.settings(), Label(SourceDir("//foo/"), "shlib"));
   target.set_output_type(Target::SHARED_LIBRARY);
   target.config_values().frameworks().push_back("System.framework");
+  target.config_values().weak_frameworks().push_back("Whizbang.framework");
   target.private_deps().push_back(LabelTargetPair(&framework));
   target.SetToolchain(setup.toolchain());
   ASSERT_TRUE(target.OnResolved(&err));
@@ -559,7 +560,8 @@
       "build ./libshlib.so: solink | obj/bar/framework.stamp\n"
       "  ldflags = -F.\n"
       "  libs =\n"
-      "  frameworks = -framework System -framework Bar\n"
+      "  frameworks = -framework System -framework Bar "
+      "-weak_framework Whizbang\n"
       "  output_extension = .so\n"
       "  output_dir = \n";
 
diff --git a/src/gn/target.cc b/src/gn/target.cc
index 115ab8e..1b26c06 100644
--- a/src/gn/target.cc
+++ b/src/gn/target.cc
@@ -372,6 +372,8 @@
     all_framework_dirs_.append(cur.framework_dirs().begin(),
                                cur.framework_dirs().end());
     all_frameworks_.append(cur.frameworks().begin(), cur.frameworks().end());
+    all_weak_frameworks_.append(cur.weak_frameworks().begin(),
+                                cur.weak_frameworks().end());
   }
 
   PullRecursiveBundleData();
@@ -701,6 +703,7 @@
 
     all_framework_dirs_.append(dep->all_framework_dirs());
     all_frameworks_.append(dep->all_frameworks());
+    all_weak_frameworks_.append(dep->all_weak_frameworks());
   }
 }
 
diff --git a/src/gn/target.h b/src/gn/target.h
index 44f7e17..6332880 100644
--- a/src/gn/target.h
+++ b/src/gn/target.h
@@ -283,6 +283,9 @@
   const OrderedSet<std::string>& all_frameworks() const {
     return all_frameworks_;
   }
+  const OrderedSet<std::string>& all_weak_frameworks() const {
+    return all_weak_frameworks_;
+  }
 
   const std::set<const Target*>& recursive_hard_deps() const {
     return recursive_hard_deps_;
@@ -451,6 +454,7 @@
   // all configs applying to this target.
   OrderedSet<SourceDir> all_framework_dirs_;
   OrderedSet<std::string> all_frameworks_;
+  OrderedSet<std::string> all_weak_frameworks_;
 
   // All hard deps from this target and all dependencies. Filled in when this
   // target is marked resolved. This will not include the current target.
diff --git a/src/gn/tool.h b/src/gn/tool.h
index 655e2b7..7fe6ed5 100644
--- a/src/gn/tool.h
+++ b/src/gn/tool.h
@@ -129,6 +129,14 @@
     framework_switch_ = std::move(s);
   }
 
+  const std::string& weak_framework_switch() const {
+    return weak_framework_switch_;
+  }
+  void set_weak_framework_switch(std::string s) {
+    DCHECK(!complete_);
+    weak_framework_switch_ = std::move(s);
+  }
+
   const std::string& framework_dir_switch() const {
     return framework_dir_switch_;
   }
@@ -259,6 +267,7 @@
   SubstitutionPattern depfile_;
   SubstitutionPattern description_;
   std::string framework_switch_;
+  std::string weak_framework_switch_;
   std::string framework_dir_switch_;
   std::string lib_switch_;
   std::string lib_dir_switch_;
diff --git a/src/gn/variables.cc b/src/gn/variables.cc
index 8a1ec2a..daabdb5 100644
--- a/src/gn/variables.cc
+++ b/src/gn/variables.cc
@@ -2140,6 +2140,27 @@
   See "gn help generated_file".
 )";
 
+const char kWeakFrameworks[] = "weak_frameworks";
+const char kWeakFrameworks_HelpShort[] =
+    "weak_frameworks: [name list] Name of frameworks that must be weak linked.";
+const char kWeakFrameworks_Help[] =
+    R"(weak_frameworks: [name list] Name of frameworks that must be weak linked.
+
+  A list of framework names.
+
+  The frameworks named in that list will be weak linked with any dynamic link
+  type target. Weak linking instructs the dynamic loader to attempt to load
+  the framework, but if it is not able to do so, it leaves any imported symbols
+  unresolved. This is typically used when a framework is present in a new
+  version of an SDK but not on older versions of the OS that the software runs
+  on.
+)" COMMON_ORDERING_HELP
+    R"(
+Example
+
+  weak_frameworks = [ "OnlyOnNewerOSes.framework" ]
+)";
+
 const char kWriteValueContents[] = "contents";
 const char kWriteValueContents_HelpShort[] =
     "contents: Contents to write to file.";
@@ -2296,6 +2317,7 @@
     INSERT_VARIABLE(Testonly)
     INSERT_VARIABLE(Visibility)
     INSERT_VARIABLE(WalkKeys)
+    INSERT_VARIABLE(WeakFrameworks)
     INSERT_VARIABLE(WriteOutputConversion)
     INSERT_VARIABLE(WriteValueContents)
     INSERT_VARIABLE(WriteRuntimeDeps)
diff --git a/src/gn/variables.h b/src/gn/variables.h
index dca4e34..acd78af 100644
--- a/src/gn/variables.h
+++ b/src/gn/variables.h
@@ -330,6 +330,10 @@
 extern const char kWalkKeys_HelpShort[];
 extern const char kWalkKeys_Help[];
 
+extern const char kWeakFrameworks[];
+extern const char kWeakFrameworks_HelpShort[];
+extern const char kWeakFrameworks_Help[];
+
 extern const char kWriteValueContents[];
 extern const char kWriteValueContents_HelpShort[];
 extern const char kWriteValueContents_Help[];