Build FreeBSD as a Linux variant in GN (series 0003): use the base
system clang (freebsd_clang_dir, default /usr/bin) instead of the
prebuilt Linux toolchain from CIPD, skip the CIPD toolchain and sysroot
stamps, and keep current_os "freebsd" in secondary toolchains.
FreeBSD-specific, not submitted upstream yet.
--- build/toolchain/linux/BUILD.gn.orig	2026-09-29 08:00:52 UTC
+++ build/toolchain/linux/BUILD.gn
@@ -15,6 +15,11 @@ declare_args() {
   arm64_toolchain_prefix = ""
   riscv32_toolchain_prefix = ""
   riscv64_toolchain_prefix = ""
+
+  # Directory holding clang and the llvm-* tools when building on FreeBSD.
+  # The base system provides them in /usr/bin; a newer toolchain from ports
+  # can be selected with e.g. "/usr/local/llvm21/bin".
+  freebsd_clang_dir = "/usr/bin"
 }
 
 if (use_rbe) {
@@ -44,8 +49,10 @@ if (use_rbe) {
   gcc_compiler_prefix = compiler_prefix
 }
 
-# Google's clang does not work for alpine, use alpine's system clang
-if (dart_sysroot == "alpine") {
+# Google's clang does not work for alpine or FreeBSD, use the system clang
+if (host_os == "freebsd") {
+  rebased_clang_dir = freebsd_clang_dir
+} else if (dart_sysroot == "alpine") {
   rebased_clang_dir = "/usr/bin"
 } else if (host_cpu == "arm64") {
   rebased_clang_dir =
@@ -55,6 +62,14 @@ if (dart_sysroot == "alpine") {
       rebase_path("//buildtools/linux-x64/clang/bin", root_build_dir)
 }
 
+# The toolchains in this file also build for FreeBSD, which is a Linux variant
+# (see //build/config/BUILDCONFIG.gn).
+if (target_os == "freebsd") {
+  linux_toolchain_os = "freebsd"
+} else {
+  linux_toolchain_os = "linux"
+}
+
 template("gcc_toolchain_suite") {
   toolchain_suite(target_name) {
     toolchain_template = "gcc_toolchain"
@@ -79,7 +94,7 @@ gcc_toolchain_suite("arm") {
   strip = "${prefix}strip"
 
   toolchain_cpu = "arm"
-  toolchain_os = "linux"
+  toolchain_os = linux_toolchain_os
   is_clang = false
 }
 
@@ -95,7 +110,7 @@ gcc_toolchain_suite("clang_arm") {
   llvm_objcopy = "${prefix}/llvm-objcopy"
 
   toolchain_cpu = "arm"
-  toolchain_os = "linux"
+  toolchain_os = linux_toolchain_os
   is_clang = true
 }
 
@@ -116,7 +131,7 @@ gcc_toolchain_suite("arm64") {
   strip = "${prefix}strip"
 
   toolchain_cpu = "arm64"
-  toolchain_os = "linux"
+  toolchain_os = linux_toolchain_os
   is_clang = false
 }
 
@@ -132,7 +147,7 @@ gcc_toolchain_suite("clang_arm64") {
   llvm_objcopy = "${prefix}/llvm-objcopy"
 
   toolchain_cpu = "arm64"
-  toolchain_os = "linux"
+  toolchain_os = linux_toolchain_os
   is_clang = true
 }
 
@@ -148,7 +163,7 @@ gcc_toolchain_suite("clang_x86") {
   llvm_objcopy = "${prefix}/llvm-objcopy"
 
   toolchain_cpu = "x86"
-  toolchain_os = "linux"
+  toolchain_os = linux_toolchain_os
   is_clang = true
 }
 
@@ -168,7 +183,7 @@ gcc_toolchain_suite("x86") {
   strip = "${prefix}strip"
 
   toolchain_cpu = "x86"
-  toolchain_os = "linux"
+  toolchain_os = linux_toolchain_os
   is_clang = false
 }
 
@@ -184,7 +199,7 @@ gcc_toolchain_suite("clang_x64") {
   llvm_objcopy = "${prefix}/llvm-objcopy"
 
   toolchain_cpu = "x64"
-  toolchain_os = "linux"
+  toolchain_os = linux_toolchain_os
   is_clang = true
 }
 
@@ -204,7 +219,7 @@ gcc_toolchain_suite("x64") {
   strip = "${prefix}strip"
 
   toolchain_cpu = "x64"
-  toolchain_os = "linux"
+  toolchain_os = linux_toolchain_os
   is_clang = false
 }
 
@@ -225,7 +240,7 @@ gcc_toolchain_suite("riscv32") {
   strip = "${prefix}strip"
 
   toolchain_cpu = "riscv32"
-  toolchain_os = "linux"
+  toolchain_os = linux_toolchain_os
   is_clang = false
 }
 
@@ -241,7 +256,7 @@ gcc_toolchain_suite("clang_riscv32") {
   llvm_objcopy = "${prefix}/llvm-objcopy"
 
   toolchain_cpu = "riscv32"
-  toolchain_os = "linux"
+  toolchain_os = linux_toolchain_os
   is_clang = true
 }
 
@@ -262,7 +277,7 @@ gcc_toolchain_suite("riscv64") {
   strip = "${prefix}strip"
 
   toolchain_cpu = "riscv64"
-  toolchain_os = "linux"
+  toolchain_os = linux_toolchain_os
   is_clang = false
 }
 
@@ -278,6 +293,6 @@ gcc_toolchain_suite("clang_riscv64") {
   llvm_objcopy = "${prefix}/llvm-objcopy"
 
   toolchain_cpu = "riscv64"
-  toolchain_os = "linux"
+  toolchain_os = linux_toolchain_os
   is_clang = true
 }