meson: Don't override RUSTFLAGS in the env

Meson does not add RUSTFLAGS to rustc.cmd_array(), so the effect of
this code is to override that value in the env.

This is hacky, since the env has to match during setup and compile
now, and rust_args added in the cross / native file will be ignored.
But at least it fixes this regression:

Fixes gstreamer/gst-plugins-rs#372

Part-of: <gstreamer/gst-plugins-rs!1330>

cargo_wrapper.py

@@ -7,6 +7,7 @@ import os
 import shutil
 import subprocess
 import sys
+import shlex
 from argparse import ArgumentParser
 from pathlib import Path as P
 
@@ -83,14 +84,16 @@ if __name__ == "__main__":
     env['PKG_CONFIG_PATH'] = os.pathsep.join(pkg_config_path)
 
     rustc_target = None
-    if 'RUSTFLAGS' in env:
+    if 'RUSTC' in env:
+        rustc_cmdline = shlex.split(env['RUSTC'])
         # grab target from RUSTFLAGS
-        rust_flags = env['RUSTFLAGS'].split()
+        rust_flags = rustc_cmdline[1:] + shlex.split(env.get('RUSTFLAGS', ''))
         if '--target' in rust_flags:
             rustc_target_idx = rust_flags.index('--target')
             _ = rust_flags.pop(rustc_target_idx)  # drop '--target'
             rustc_target = rust_flags.pop(rustc_target_idx)
-        env['RUSTFLAGS'] = ' '.join(rust_flags)
+        env['RUSTFLAGS'] = shlex.join(rust_flags)
+        env['RUSTC'] = rustc_cmdline[0]
 
     features = opts.features
     if opts.command == 'build':

meson.build

@@ -386,17 +386,8 @@ if pkg_config_path.length() > 0
   extra_env += {'PKG_CONFIG_PATH': pathsep.join(pkg_config_path)}
 endif
 
-# get compiler and flags for rust (if any)
-rustc_cmd = ''
-rust_flags = []
-foreach rustc_arg : rustc.cmd_array()
-  if rustc_cmd == ''
-    rustc_cmd = rustc_arg
-  else
-    rust_flags += [rustc_arg]
-  endif
-endforeach
-extra_env += {'RUSTC': rustc_cmd, 'RUSTFLAGS': ' '.join(rust_flags)}
+# get cmdline for rust
+extra_env += {'RUSTC': ' '.join(rustc.cmd_array())}
 
 rs_plugins = custom_target('gst-plugins-rs',
   build_by_default: true,