diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index cb9bf8772d..9e36a487ba 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -309,9 +309,6 @@ config("compiler") {
 
   configs += [
     # See the definitions below.
-    ":clang_revision",
-    ":rustc_revision",
-    ":compiler_cpu_abi",
     ":compiler_codegen",
     ":compiler_deterministic",
   ]
@@ -493,6 +490,9 @@ config("compiler") {
     ldflags += [ "-fPIC" ]
     rustflags += [ "-Crelocation-model=pic" ]
 
+    extra_rustflags = getenv("RUSTFLAGS")
+    rustflags += [ extra_rustflags ]
+
     if (!is_clang) {
       # Use pipes for communicating between sub-processes. Faster.
       # (This flag doesn't do anything with Clang.)
@@ -568,63 +568,6 @@ config("compiler") {
     ldflags += [ "-Wl,-z,keep-text-section-prefix" ]
   }
 
-  if (is_clang && !is_nacl) {
-    cflags += [ "-fcrash-diagnostics-dir=" + clang_diagnostic_dir ]
-    if (save_reproducers_on_lld_crash && use_lld) {
-      ldflags += [
-        "-fcrash-diagnostics=all",
-        "-fcrash-diagnostics-dir=" + clang_diagnostic_dir,
-      ]
-    }
-
-    # TODO(hans): Remove this once Clang generates better optimized debug info
-    # by default. https://crbug.com/765793
-    cflags += [
-      "-mllvm",
-      "-instcombine-lower-dbg-declare=0",
-    ]
-    if (!is_debug && use_thin_lto && is_a_target_toolchain) {
-      if (is_win) {
-        ldflags += [ "-mllvm:-instcombine-lower-dbg-declare=0" ]
-      } else {
-        ldflags += [ "-Wl,-mllvm,-instcombine-lower-dbg-declare=0" ]
-      }
-    }
-
-    # TODO(crbug.com/40283598): This causes binary size growth and potentially
-    # other problems.
-    # TODO(crbug.com/40284925): This isn't supported by Cronet's mainline llvm version.
-    if (default_toolchain != "//build/toolchain/cros:target" &&
-        !llvm_android_mainline) {
-      cflags += [
-        "-mllvm",
-        "-split-threshold-for-reg-with-hint=0",
-      ]
-      if (use_thin_lto && is_a_target_toolchain) {
-        if (is_win) {
-          ldflags += [ "-mllvm:-split-threshold-for-reg-with-hint=0" ]
-        } else {
-          ldflags += [ "-Wl,-mllvm,-split-threshold-for-reg-with-hint=0" ]
-        }
-      }
-    }
-
-    # TODO(crbug.com/40192287): Investigate why/if this should be needed.
-    if (is_win) {
-      cflags += [ "/clang:-ffp-contract=off" ]
-    } else {
-      cflags += [ "-ffp-contract=off" ]
-    }
-
-    # Enable ELF CREL (see crbug.com/357878242) for all platforms that use ELF
-    # (excluding toolchains that use an older version of LLVM).
-    # TODO(crbug.com/376278218): This causes segfault on Linux ARM builds.
-    if (is_linux && !llvm_android_mainline && current_cpu != "arm" &&
-        default_toolchain != "//build/toolchain/cros:target") {
-      cflags += [ "-Wa,--crel,--allow-experimental-crel" ]
-    }
-  }
-
   # C11/C++11 compiler flags setup.
   # ---------------------------
   if (is_linux || is_chromeos || is_android || (is_nacl && is_clang) ||
@@ -1045,20 +988,6 @@ config("compiler") {
     # `-nodefaultlibs` from the linker invocation from Rust, which would be used
     # to compile dylibs on Android, such as for constructing unit test APKs.
     "-Cdefault-linker-libraries",
-
-    # To make Rust .d files compatible with ninja
-    "-Zdep-info-omit-d-target",
-
-    # If a macro panics during compilation, show which macro and where it is
-    # defined.
-    "-Zmacro-backtrace",
-
-    # For deterministic builds, keep the local machine's current working
-    # directory from appearing in build outputs.
-    "-Zremap-cwd-prefix=.",
-
-    # We use clang-rt sanitizer runtimes.
-    "-Zexternal-clangrt",
   ]
 
   if (!is_win || force_rustc_color_output) {
@@ -1107,7 +1036,7 @@ config("compiler") {
 # Don't allow unstable features to be enabled by `#![feature()]` without
 # additional command line flags.
 config("disallow_unstable_features") {
-  rustflags = [ "-Zallow-features=" ]
+  #rustflags = [ "-Zallow-features=" ]
 }
 
 config("libcxx_hardening") {
@@ -1583,52 +1512,6 @@ config("compiler_deterministic") {
     }
   }
 
-  # Makes builds independent of absolute file path.
-  if (is_clang && strip_absolute_paths_from_debug_symbols) {
-    # If debug option is given, clang includes $cwd in debug info by default.
-    # For such build, this flag generates reproducible obj files even we use
-    # different build directory like "out/feature_a" and "out/feature_b" if
-    # we build same files with same compile flag.
-    # Other paths are already given in relative, no need to normalize them.
-    if (is_nacl) {
-      # TODO(https://crbug.com/1231236): Use -ffile-compilation-dir= here.
-      cflags += [
-        "-Xclang",
-        "-fdebug-compilation-dir",
-        "-Xclang",
-        ".",
-      ]
-    } else {
-      # -ffile-compilation-dir is an alias for both -fdebug-compilation-dir=
-      # and -fcoverage-compilation-dir=.
-      cflags += [ "-ffile-compilation-dir=." ]
-
-      # Convert absolute paths to relative paths. Expands to, for example:
-      #   -file-prefix-map /path/to/chromium/src=../..
-      swiftflags += [
-        "-file-prefix-map",
-        rebase_path("//.", "") + "=" + rebase_path("//.", root_build_dir),
-      ]
-    }
-    if (!is_win) {
-      # We don't use clang -cc1as on Windows (yet? https://crbug.com/762167)
-      asmflags = [ "-Wa,-fdebug-compilation-dir,." ]
-    }
-
-    if (is_win && use_lld) {
-      if (symbol_level == 2 || (is_clang && using_sanitizer)) {
-        # Absolutize source file paths for PDB. Pass the real build directory
-        # if the pdb contains source-level debug information and if linker
-        # reproducibility is not critical.
-        ldflags += [ "/PDBSourcePath:" + rebase_path(root_build_dir) ]
-      } else {
-        # Use a fake fixed base directory for paths in the pdb to make the pdb
-        # output fully deterministic and independent of the build directory.
-        ldflags += [ "/PDBSourcePath:o:\fake\prefix" ]
-      }
-    }
-  }
-
   # Tells the compiler not to use absolute paths when passing the default
   # paths to the tools it invokes. We don't want this because we don't
   # really need it and it can mess up the RBE cache entries.
@@ -1647,40 +1530,6 @@ config("compiler_deterministic") {
   }
 }
 
-config("clang_revision") {
-  if (is_clang && clang_base_path == default_clang_base_path &&
-      current_os != "zos") {
-    update_args = [
-      "--print-revision",
-      "--verify-version=$clang_version",
-    ]
-    if (llvm_force_head_revision) {
-      update_args += [ "--llvm-force-head-revision" ]
-    }
-    clang_revision = exec_script("//tools/clang/scripts/update.py",
-                                 update_args,
-                                 "trim string")
-
-    # This is here so that all files get recompiled after a clang roll and
-    # when turning clang on or off. (defines are passed via the command line,
-    # and build system rebuild things when their commandline changes). Nothing
-    # should ever read this define.
-    defines = [ "CR_CLANG_REVISION=\"$clang_revision\"" ]
-  }
-}
-
-config("rustc_revision") {
-  if (rustc_revision != "") {
-    # Similar to the above config, this is here so that all files get recompiled
-    # after a rustc roll. Nothing should ever read this cfg. This will not be
-    # set if a custom toolchain is used.
-    rustflags = [
-      "--cfg",
-      "cr_rustc_revision=\"$rustc_revision\"",
-    ]
-  }
-}
-
 config("compiler_arm_fpu") {
   if (current_cpu == "arm" && !is_ios && !is_nacl) {
     cflags = [ "-mfpu=$arm_fpu" ]
@@ -2086,11 +1935,7 @@ config("chromium_code") {
       defines = [ "_HAS_NODISCARD" ]
     }
   } else {
-    cflags = [ "-Wall" ]
-    if (is_clang) {
-      # Enable extra warnings for chromium_code when we control the compiler.
-      cflags += [ "-Wextra" ]
-    }
+    cflags = [ ]
 
     # In Chromium code, we define __STDC_foo_MACROS in order to get the
     # C99 macros on Mac and Linux.
@@ -2099,24 +1944,6 @@ config("chromium_code") {
       "__STDC_FORMAT_MACROS",
     ]
 
-    if (!is_debug && !using_sanitizer && current_cpu != "s390x" &&
-        current_cpu != "s390" && current_cpu != "ppc64" &&
-        current_cpu != "mips" && current_cpu != "mips64" &&
-        current_cpu != "riscv64" && current_cpu != "loong64") {
-      # Non-chromium code is not guaranteed to compile cleanly with
-      # _FORTIFY_SOURCE. Also, fortified build may fail when optimizations are
-      # disabled, so only do that for Release build.
-      fortify_level = "2"
-
-      # ChromeOS's toolchain supports a high-quality _FORTIFY_SOURCE=3
-      # implementation with a few custom glibc patches. Use that if it's
-      # available.
-      if (is_chromeos_device && !lacros_use_chromium_toolchain) {
-        fortify_level = "3"
-      }
-      defines += [ "_FORTIFY_SOURCE=" + fortify_level ]
-    }
-
     if (is_apple) {
       cflags_objc = [ "-Wimplicit-retain-self" ]
       cflags_objcc = [ "-Wimplicit-retain-self" ]
@@ -2544,7 +2371,8 @@ config("default_stack_frames") {
 # [0]: https://pinpoint-dot-chromeperf.appspot.com/job/147634a8be0000
 # [1]: https://pinpoint-dot-chromeperf.appspot.com/job/132bc772be0000
 # [2]: https://crrev.com/c/5447532
-config("optimize") {
+config("optimize") { }
+config("xoptimize") {
   if (is_win) {
     # clang-cl's /O2 corresponds to clang's -O3, and really want -O2 for
     # consistency with the other platforms.
@@ -2593,7 +2421,8 @@ config("optimize") {
 }
 
 # Turn off optimizations.
-config("no_optimize") {
+config("no_optimize") { }
+config("xno_optimize") {
   if (is_win) {
     cflags = [
       "/Od",  # Disable optimization.
@@ -2633,7 +2462,8 @@ config("no_optimize") {
 # Turns up the optimization level. Used to explicitly enable -O2 instead of
 # -Os for select targets on platforms that use optimize_for_size. No-op
 # elsewhere.
-config("optimize_max") {
+config("optimize_max") { }
+config("xoptimize_max") {
   if (is_nacl && is_nacl_irt) {
     # The NaCl IRT is a special case and always wants its own config.
     # Various components do:
@@ -2666,7 +2496,8 @@ config("optimize_max") {
 #
 # TODO(crbug.com/41259697) - rework how all of these configs are related
 # so that we don't need this disclaimer.
-config("optimize_speed") {
+config("optimize_speed") { }
+config("xoptimize_speed") {
   if (is_nacl && is_nacl_irt) {
     # The NaCl IRT is a special case and always wants its own config.
     # Various components do:
@@ -2695,7 +2526,8 @@ config("optimize_speed") {
   }
 }
 
-config("optimize_fuzzing") {
+config("optimize_fuzzing") { }
+config("xoptimize_fuzzing") {
   cflags = [ "-O1" ] + common_optimize_on_cflags
   rustflags = [ "-Copt-level=1" ]
   ldflags = common_optimize_on_ldflags
@@ -2826,7 +2658,8 @@ config("win_pdbaltpath") {
 }
 
 # Full symbols.
-config("symbols") {
+config("symbols") { }
+config("xsymbols") {
   rustflags = []
   configs = []
   if (is_win) {
@@ -2988,7 +2821,8 @@ config("symbols") {
 # Minimal symbols.
 # This config guarantees to hold symbol for stack trace which are shown to user
 # when crash happens in unittests running on buildbot.
-config("minimal_symbols") {
+config("minimal_symbols") { }
+config("xminimal_symbols") {
   rustflags = []
   if (is_win) {
     # Functions, files, and line tables only.
@@ -3073,7 +2907,8 @@ config("minimal_symbols") {
 # This configuration contains function names only. That is, the compiler is
 # told to not generate debug information and the linker then just puts function
 # names in the final debug information.
-config("no_symbols") {
+config("no_symbols") { }
+config("xno_symbols") {
   if (is_win) {
     ldflags = [ "/DEBUG" ]
 
diff --git a/build/config/gcc/BUILD.gn b/build/config/gcc/BUILD.gn
index a659210b19..bde0e1b273 100644
--- a/build/config/gcc/BUILD.gn
+++ b/build/config/gcc/BUILD.gn
@@ -32,7 +32,7 @@ declare_args() {
 # See http://gcc.gnu.org/wiki/Visibility
 config("symbol_visibility_hidden") {
   cflags = [ "-fvisibility=hidden" ]
-  rustflags = [ "-Zdefault-visibility=hidden" ]
+  # rustflags = [ "-Zdefault-visibility=hidden" ]
 
   # Visibility attribute is not supported on AIX.
   if (current_os != "aix") {
diff --git a/build/config/rust.gni b/build/config/rust.gni
index 5b7807a1b8..4b3acd300e 100644
--- a/build/config/rust.gni
+++ b/build/config/rust.gni
@@ -79,7 +79,8 @@ declare_args() {
   #
   # TODO(crbug.com/40281834): Re-enable ThinLTO for Rust on LaCrOS
   # TODO(b/300937673): Re-enable ThinLTO for Rust on ash-chrome
-  toolchain_supports_rust_thin_lto = !is_chromeos
+  # toolchain_supports_rust_thin_lto = !is_chromeos
+  toolchain_supports_rust_thin_lto = false
 
   # Any extra std rlibs in your Rust toolchain, relative to the standard
   # Rust toolchain. Typically used with 'rust_sysroot_absolute'