Add support for rust cfg response file

Add support to write the rustc_cfg response file via
write_rustcfg. This is equivalent to the rust response file used for
rust support in the kernel as introduced in
2f7ab1267dc9b2d1f29695aff3211c87483480f3 in the kernel tree.

Link: https://lore.kernel.org/rust-for-linux/20220212130410.6901-17-ojeda@kernel.org/#Z31scripts:kconfig:confdata.c

kconfiglib.py

@@ -1729,6 +1729,50 @@ def _min_config_contents(self, header):
 
         return "".join(chunks)
 
+    def write_rustcfg(self, filename=None):
+        r"""
+        Write out rustcfg flags as a response file, matching the format used
+        by include/generated/rustc_cfg in the kernel.
+
+        filename (default: None):
+          Path to write respone file to.
+
+          If None (the default), the path in the environment variable
+          KCONFIG_RUSTCCFG is used if set, and "include/generated/rustc_cfg"
+          otherwise. This is compatible with the C tools.
+        """
+        if filename is None:
+            filename = os.getenv("KCONFIG_RUSTCCFG",
+                                 "include/generated/rustc_cfg")
+
+        if self._write_if_changed(filename, self._rustcfg_contents()):
+            return "Kconfig cfg saved to '{}".format(filename)
+        return "No changes to Kconfig cfg in '{}'".format(filename)
+
+    def _rustcfg_contents(self):
+        chunks = []
+        add = chunks.append
+
+        for sym in self.unique_defined_syms:
+            if not sym.choice and \
+               sym.visibility <= expr_value(sym.rev_dep):
+                continue
+
+            val = sym.str_value
+            if sym.orig_type in _BOOL_TRISTATE:
+                # We do not care about disabled ones, would be a comment
+                if val == "n":
+                    continue
+                add("--cfg={}{}\n"
+                    .format(self.config_prefix, sym.name))
+            elif sym.orig_type == HEX:
+                if not val.lower().startswith("0x"):
+                    val = "0x{}".format(val);
+            add("--cfg={}{}={}\n"
+                .format(self.config_prefix, sym.name, escape(val)))
+
+        return "".join(chunks)
+
     def sync_deps(self, path):
         """
         Creates or updates a directory structure that can be used to avoid

testsuite.py

@@ -1987,6 +1987,18 @@ def verify_file_contents(fname, contents):
 #define CONFIG_I 5
 #define CONFIG_N 6
 #define CONFIG_G 7
+"""[1:])
+
+    c.write_rustcfg(config_test_file)
+    verify_file_contents(config_test_file, """
+--cfg=CONFIG_O=0
+--cfg=CONFIG_R=1
+--cfg=CONFIG_D=2
+--cfg=CONFIG_E=3
+--cfg=CONFIG_R2=4
+--cfg=CONFIG_I=5
+--cfg=CONFIG_N=6
+--cfg=CONFIG_G=7
 """[1:])
 
     # Differs from defaults