Enable LTO in cargo2android.py.

The latest compiler roll also enabled LTO.  This broke
cargo2android.py, as the test prebuit uses LTO and so cannot be linked
with code built without.  We fix this by enabling it in
cargo2android.py (and ensuring it doesn't add the flag into
Android.bp).

Test: Run
Change-Id: I6c6a813ff58a9e5064752aa59b78247c4af77a26

scripts/cargo2android.py

@@ -352,12 +352,14 @@ class Crate(object):
     # 'prefer-dynamic' does not work with common flag -C lto
     # 'embed-bitcode' is ignored; we might control LTO with other .bp flag
     # 'codegen-units' is set in Android global config or by default
+    # 'lto' is used in Android, but it's set by the build system
     if not (flag.startswith('codegen-units=') or
             flag.startswith('debuginfo=') or
             flag.startswith('embed-bitcode=') or
             flag.startswith('extra-filename=') or
             flag.startswith('incremental=') or
             flag.startswith('metadata=') or
+            flag.startswith('lto=') or
             flag == 'prefer-dynamic'):
       self.codegens.append(flag)
 
@@ -1342,6 +1344,9 @@ class Runner(object):
     # set up search PATH for cargo to find the correct rustc
     saved_path = os.environ['PATH']
     os.environ['PATH'] = os.path.dirname(self.cargo_path) + ':' + saved_path
+    # We need to enable lto since our test prebuilts use it
+    saved_rustflags = os.environ.get('RUSTFLAGS', '')
+    os.environ['RUSTFLAGS'] = '-C lto=thin -C embed-bitcode=yes ' + saved_rustflags
     # Add [workspace] to Cargo.toml if it is not there.
     added_workspace = False
     if self.args.add_workspace:
@@ -1367,7 +1372,7 @@ class Runner(object):
       cmd = self.cargo_path + cmd_v_flag
       cmd += c + features + cmd_tail_target + cmd_tail_redir
       if self.args.rustflags and c != 'clean':
-        cmd = 'RUSTFLAGS="' + self.args.rustflags + '" ' + cmd
+        cmd = 'RUSTFLAGS="' + os.environ['RUSTFLAGS'] + ' ' + self.args.rustflags + '" ' + cmd
       self.run_cmd(cmd, cargo_out)
     if self.args.tests:
       cmd = self.cargo_path + ' test' + features + cmd_tail_target + ' -- --list' + cmd_tail_redir
@@ -1378,6 +1383,7 @@ class Runner(object):
       if self.args.verbose:
         print('### INFO: restored original Cargo.toml')
     os.environ['PATH'] = saved_path
+    os.environ['RUSTFLAGS'] = saved_rustflags
     if not self.dry_run:
       if not had_cargo_lock:  # restore to no Cargo.lock state
         os.remove(cargo_lock)