Suppress misleading rustc warning when running gn gen

rustc prints the warning:
  `-C save-temps` might not produce all requested temporary
  products when incremental compilation is enabled.

However, incremental compilation isn't even enabled.

A look at the Rust source code confirms that this warning is indeed
printed unconditionally when the `-C save-temps` flag is specified:
https://github.com/rust-lang/rust/blob/5f2b325f64ed6caa7179f3e04913db437656ec7e/src/librustc/session/config.rs#L1015-L1018

build_extra/rust/get_rust_ldflags.py

@@ -29,6 +29,7 @@
 import sys
 import os
 from os import path
+import re
 import subprocess
 import tempfile
 
@@ -77,10 +78,41 @@ def main():
  rustc_env["ARGSFILE_PATH"] = argsfile_path
 
  try:
- # Spawn rustc, and make it use this very script as its "linker".
- rustc_args = ["-Clinker=" + rustc_linker, "-Csave-temps"
- ] + sys.argv[1:]
- subprocess.check_call(["rustc"] + rustc_args, env=rustc_env)
+ # Build the rustc command line.
+ # * `-Clinker=` tells rustc to use our fake linker.
+ # * `-Csave-temps` prevents rustc from deleting object files after
+ # linking. We need to preserve the file `xx.crate.allocator.rcgu.o`.
+ rustc_cmd = [
+ "rustc",
+ "-Clinker=" + rustc_linker,
+ "-Csave-temps",
+ ] + sys.argv[1:]
+
+ # Spawn the rust compiler.
+ rustc_proc = subprocess.Popen(
+ rustc_cmd,
+ env=rustc_env,
+ stdout=subprocess.PIPE,
+ stderr=subprocess.STDOUT)
+
+ # Forward rustc's output to stderr.
+ for line in rustc_proc.stdout:
+ # Suppress the warning:
+ # `-C save-temps` might not produce all requested temporary
+ # products when incremental compilation is enabled.
+ # It's pointless, because incremental compilation is disabled.
+ if re.match(r"^warning:.*save-temps.*incremental compilation",
+ line):
+ continue
+ # Also, do not write completely blank lines to stderr.
+ if line.strip() == "":
+ continue
+ sys.stderr.write(line)
+
+ # The rustc process should return zero. If not, raise an exception.
+ rustc_retcode = rustc_proc.wait()
+ if rustc_retcode != 0:
+ raise subprocess.CalledProcessError(rustc_retcode, rustc_cmd)
 
  # Read captured linker arguments from argsfile.
  argsfile_size = os.fstat(argsfile_fd).st_size