meson: support rust cross-compiling with cargo wrapper

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1189>

cargo_wrapper.py

@@ -82,6 +82,16 @@ if __name__ == "__main__":
     pkg_config_path.append(str(opts.root_dir / 'meson-uninstalled'))
     env['PKG_CONFIG_PATH'] = os.pathsep.join(pkg_config_path)
 
+    rustc_target = None
+    if 'RUSTFLAGS' in env:
+        # grab target from RUSTFLAGS
+        rust_flags = env['RUSTFLAGS'].split()
+        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)
+
     features = opts.features
     if opts.command == 'build':
         cargo_cmd = ['cargo']
@@ -100,6 +110,8 @@ if __name__ == "__main__":
         print("Unknown command:", opts.command, file=logfile)
         sys.exit(1)
 
+    if rustc_target:
+        cargo_cmd += ['--target', rustc_target]
     if features:
         cargo_cmd += ['--features', ','.join(features)]
     cargo_cmd += ['--target-dir', cargo_target_dir]

meson.build

@@ -21,13 +21,14 @@ endif
 cargo = find_program('cargo', version:'>=1.40')
 cargo_wrapper = find_program('cargo_wrapper.py')
 cargo_c = find_program('cargo-cbuild', version:'>=0.9.3', required: false)
-rustc = find_program('rustc', version:'>=1.52')
+
+rustc = meson.get_compiler('rust')
 
 if not cargo_c.found()
   error('cargo-c missing, install it with: \'cargo install cargo-c\'')
 endif
 
-system = build_machine.system()
+system = host_machine.system()
 exe_suffix = ''
 if system == 'windows'
   exe_suffix = '.exe'
@@ -365,7 +366,7 @@ pkgconfig_install_dir = get_option('libdir') / 'pkgconfig'
 
 extra_args = []
 if get_option('doc').disabled()
-  extra_args = ['--disable-doc']
+  extra_args += ['--disable-doc']
 endif
 
 # 'pkgconfig' is the entry in the machine file, if specified
@@ -383,6 +384,18 @@ 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)}
+
 rs_plugins = custom_target('gst-plugins-rs',
   build_by_default: true,
   output: output,