[common] Switch to std::filesystem

Deprecate unused ghc_filesystem external.

common/BUILD.bazel

@@ -338,18 +338,12 @@ drake_cc_library(
 )
 
 # The drake::filesystem support is intended ONLY for use within Drake's *.cc
-# files -- it is not a public dependency of Drake.  As such, we compile it with
-# hidden visibility and mark it internal.
+# files -- it is not a public dependency of Drake, so we mark it internal.
 drake_cc_library(
     name = "filesystem",
-    srcs = ["filesystem.cc"],
     hdrs = ["filesystem.h"],
-    copts = ["-fvisibility=hidden"],
     internal = True,
     visibility = ["//:__subpackages__"],
-    deps = [
-        "@ghc_filesystem",
-    ],
 )
 
 drake_cc_library(

common/filesystem.h

@@ -1,28 +1,17 @@
 #pragma once
 
-// Alias drake::filesystem to either std::filesystem or ghc::filesystem.
+// Alias drake::filesystem to std::filesystem.
 //
 // The drake::filesystem support is intended ONLY for use within Drake's *.cc
 // files -- it is not a public dependency of Drake; do not include this file
 // from Drake header files.
-//
-// Note that until Apple ships a working std::filesystem implementation, we
-// need to force-disable it. Similarly, GCC prior to 9 requires arcane linker
-// flags, so we need to exclude it as well.
-//
-// Keep this if-sequence in sync with drake/common/filesystem.cc.
-#if __has_include(<filesystem>) && !( \
-  defined(__APPLE__) || \
-  (!defined(__clang__) && defined(__GNUC__) && (__GNUC__ < 9)))
 
 #include <filesystem>
-namespace drake { namespace filesystem = std::filesystem; }
 
-#else
+namespace drake {
 
-#define GHC_FILESYSTEM_FWD
-#include "ghc/filesystem.hpp"
-#undef GHC_FILESYSTEM_FWD
-namespace drake { namespace filesystem = ghc::filesystem; }
+// For legacy reasons, we have an Anzu alias for std::filesystem.
+// In the future, we could port to `std` directly and remove this alias.
+namespace filesystem = std::filesystem;
 
-#endif
+}  // namespace drake

tools/workspace/BUILD.bazel

@@ -82,7 +82,6 @@ _DRAKE_EXTERNAL_PACKAGE_INSTALLS = ["@%s//:install" % p for p in [
     "commons_io",
     "fcl",
     "fmt",
-    "ghc_filesystem",
     "gz_math_internal",
     "gz_utils_internal",
     "lcm",

tools/workspace/ghc_filesystem/package.BUILD.bazel

@@ -13,6 +13,7 @@ cc_library(
     name = "ghc_filesystem",
     hdrs = ["include/ghc/filesystem.hpp"],
     strip_include_prefix = "include",
+    deprecation = "DRAKE DEPRECATED: The @ghc_filesystem repository is no longer used by Drake. If you still use it, you may copy the the repository rule into your own project. This target will be removed from Drake on or after 2023-01-01.",  # noqa
     linkstatic = 1,
 )