Expose host environment variables to C++ compiler

cxx_env.py

#!/usr/bin/env python3

import os
import json
import string
import tempfile
import subprocess
import sys

cxx_template = string.Template('''#ifndef COMPILE_ENV_INCLUDED
#define COMPILE_ENV_INCLUDED

#include <array>
#include <algorithm>
#include <string_view>
#include <utility>

namespace CompileEnv
{
	constexpr std::array<std::pair<std::string_view, char const*>, $NUMVARS> Variables{$VARIABLES};

	consteval char const* get(std::string_view key)
	{
		return std::find_if(std::begin(Variables), std::end(Variables), [key](auto const item) {
			return item.first == key;
		})->second;
	}
}

#endif
''')

def escape_string(str):
	ret = '';
	for ch in str:
		if (ord(ch) >= 0 and ord(ch) < 32) or ch in('\\', '"'):
			ret+='\\%03o' % ord(ch)
		else:
			ret+=ch
	return ret;

subst = dict()
vars = []
for key, value in os.environ.items():
	vars.append('std::pair{std::string_view{"%s"}, "%s"}'%(escape_string(key), escape_string(value)))
subst['VARIABLES'] = ', '.join(vars)
subst['NUMVARS'] = len(vars)

fd, path = tempfile.mkstemp()
try:
	with os.fdopen(fd, 'w') as include:
		include.write(cxx_template.substitute(subst))
	cmdline = ['g++-10', '-std=c++20', '-include%s'%path]
	cmdline.extend(sys.argv[1:])
	exit(subprocess.run(cmdline).returncode)
finally:
	os.remove(path)