[rust] Use rustc --print=cfg to emulate CARGO_CFG_TARGET_... env.

Before this CL we were trying to simulate Cargo behavior in
`rust/run_build_script.py` and the results were inaccurate and didn't
match Cargo (e.g. https://crbug.com/401427004 reported that
`CARGO_CFG_TARGET_ARCH` was set to `i686` instead of `x86`).

After this CL we instead run `rustc --print=cfg` to tell us all the
`target_foo` properties.  This matches `rustc` and `cargo` behavior.

Fixed: 401427004
Fixed: 364904747
Change-Id: I4ce8b0a988fa951974fdbf21f896a3ff9375f50d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6336767
Reviewed-by: Collin Baker <collinbaker@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1430501}

build/rust/cargo_crate.gni

@@ -449,20 +449,6 @@ template("cargo_crate") {
           cargo_target_abi,
         ]
       }
-      if (current_cpu == "arm64" || current_cpu == "x64" ||
-          current_cpu == "loong64" || current_cpu == "riscv64") {
-        args += [
-          "--pointer-width",
-          "64",
-        ]
-      } else if (current_cpu == "arm" || current_cpu == "x86") {
-        args += [
-          "--pointer-width",
-          "32",
-        ]
-      } else {
-        assert(false, "Architecture not supported")
-      }
       if (defined(invoker.features)) {
         args += [ "--features" ]
         args += invoker.features

build/rust/run_build_script.py

@@ -62,6 +62,30 @@ def host_triple(rustc_path):
   return known_vars["host"]
 
 
+def set_cargo_cfg_target_env_variables(rustc_path, env):
+  """ Sets CARGO_CFG_TARGET_... based on output from rustc. """
+  target_triple = env["TARGET"]
+  assert target_triple
+
+  # TODO(lukasza): Check if command-line flags other `--target` may affect the
+  # output of `--print-cfg`.  If so, then consider also passing extra `args`
+  # (derived from `rustflags` maybe?).
+  args = [rustc_path, "--print=cfg", f"--target={target_triple}"]
+
+  proc = subprocess.Popen(args, stdout=subprocess.PIPE)
+  for line in io.TextIOWrapper(proc.stdout, encoding="utf-8"):
+    line = line.strip()
+    if "=" not in line: continue
+    key, value = line.split("=")
+    if key.startswith("target_"):
+      key = "CARGO_CFG_" + key.upper()
+      value = value.strip('"')
+      if key in env:
+        env[key] = env[key] + f",{value}"
+      else:
+        env[key] = value
+
+
 # Before 1.77, the format was `cargo:rustc-cfg=`. As of 1.77 the format is now
 # `cargo::rustc-cfg=`.
 RUSTC_CFG_LINE = re.compile("cargo::?rustc-cfg=(.*)")
@@ -77,7 +101,6 @@ def main():
                       help='where to write output rustc flags')
   parser.add_argument('--target', help='rust target triple')
   parser.add_argument('--target-abi', help='rust target_abi')
-  parser.add_argument('--pointer-width', help='rust target pointer width')
   parser.add_argument('--features', help='features', nargs='+')
   parser.add_argument('--env', help='environment variable', nargs='+')
   parser.add_argument('--rustflags',
@@ -108,43 +131,11 @@ def main():
     env["OUT_DIR"] = tempdir
     env["CARGO_MANIFEST_DIR"] = os.path.abspath(args.src_dir)
     env["HOST"] = host_triple(rustc_path)
-    env["CARGO_CFG_TARGET_POINTER_WIDTH"] = args.pointer_width
     if args.target is None:
       env["TARGET"] = env["HOST"]
     else:
       env["TARGET"] = args.target
-    target_components = env["TARGET"].split("-")
-    if len(target_components) == 2:
-      env["CARGO_CFG_TARGET_ARCH"] = target_components[0]
-      env["CARGO_CFG_TARGET_VENDOR"] = ''
-      env["CARGO_CFG_TARGET_OS"] = target_components[1]
-      env["CARGO_CFG_TARGET_ENV"] = ''
-    elif len(target_components) == 3:
-      env["CARGO_CFG_TARGET_ARCH"] = target_components[0]
-      env["CARGO_CFG_TARGET_VENDOR"] = target_components[1]
-      env["CARGO_CFG_TARGET_OS"] = target_components[2]
-      env["CARGO_CFG_TARGET_ENV"] = ''
-    elif len(target_components) == 4:
-      env["CARGO_CFG_TARGET_ARCH"] = target_components[0]
-      env["CARGO_CFG_TARGET_VENDOR"] = target_components[1]
-      env["CARGO_CFG_TARGET_OS"] = target_components[2]
-      env["CARGO_CFG_TARGET_ENV"] = target_components[3]
-    else:
-      print(f'Invalid TARGET {env["TARGET"]}')
-      sys.exit(1)
-    # See https://crbug.com/325543500 for background.
-    # Cargo sets CARGO_CFG_TARGET_OS to "android" even when targeting
-    # *-androideabi.
-    if env["CARGO_CFG_TARGET_OS"].startswith("android"):
-      env["CARGO_CFG_TARGET_OS"] = "android"
-    elif env["CARGO_CFG_TARGET_OS"] == "darwin":
-      env["CARGO_CFG_TARGET_OS"] = "macos"
-    env["CARGO_CFG_TARGET_ENDIAN"] = "little"
-    if env["CARGO_CFG_TARGET_OS"] == "windows":
-      env["CARGO_CFG_TARGET_FAMILY"] = "windows"
-    else:
-      env["CARGO_CFG_TARGET_FAMILY"] = "unix"
-    env["CARGO_CFG_TARGET_ABI"] = args.target_abi if args.target_abi else ""
+    set_cargo_cfg_target_env_variables(rustc_path, env)
     if args.features:
       for f in args.features:
         feature_name = f.upper().replace("-", "_")

build/rust/tests/BUILD.gn

@@ -45,6 +45,15 @@ group("deps") {
       #"test_rs_bindings_from_cc:test_rs_bindings_from_cc",
     ]
 
+    if (target_cpu == "x86") {
+      # Some `target_os`s do not support 32-bit platforms anymore.  The
+      # selection below is _loosely_ based on `is_valid_x86_target` from
+      # the top-level `//BUILD.gn`.
+      if (is_win || is_android) {
+        deps += [ "//build/rust/tests/test_build_rs_target_arch_x86" ]
+      }
+    }
+
     if (!(is_apple && is_official_build)) {
       # TODO: crbug.com/372055517 - Apple lld has an ordering dependency bug
       # when LTO is enabled, and this particular build target runs into that.

build/rust/tests/test_build_rs_target_arch_x86/BUILD.gn

@@ -0,0 +1,14 @@
+# Copyright 2021 The Chromium Authors
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/rust/cargo_crate.gni")
+
+cargo_crate("test_build_rs_target_arch_x86") {
+  crate_name = "test_build_rs_target_arch_x86"
+  crate_root = "lib.rs"
+  sources = [ "lib.rs" ]
+  build_sources = [ "build.rs" ]
+  build_root = "build.rs"
+  epoch = "0.1"
+}

build/rust/tests/test_build_rs_target_arch_x86/build.rs

@@ -0,0 +1,25 @@
+// Copyright 2025 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+use std::env;
+
+// This is a regression test against https://crbug.com/401427004 which
+// reported that Chromium's build infrastructure (`run_build_script.py`)
+// sets `CARGO_CFG_TARGET_ARCH` environment variable incorrectly.
+fn main() {
+    println!("cargo:rustc-cfg=build_script_ran");
+
+    // https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-build-scripts
+    // documents how `CARGO_CFG_TARGET_ARCH` should be set.
+    let target = env::var("CARGO_CFG_TARGET_ARCH").unwrap();
+
+    // https://doc.rust-lang.org/reference/conditional-compilation.html#target_arch lists the
+    // following example values: "x86", "x86_64", "mips", "powerpc", "powerpc64",
+    // "arm", "aarch64".
+    //
+    // Before https://crbug.com/401427004 was fixed `run_build_script.py` would incorrectly
+    // take the first component of a target triple (e.g. `i686` from
+    // `i686-pc-windows-msvc`) and set that as `CARGO_CFG_TARGET_ARCH`.
+    assert_eq!(target, "x86");
+}

build/rust/tests/test_build_rs_target_arch_x86/lib.rs

@@ -0,0 +1,6 @@
+// Copyright 2025 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Empty `lib.rs` file - the real test is whether `build.rs` gets the expected
+// `CARGO_CFG_TARGET_ARCH` value (it will panic otherwise).