scan

    pub fn scan(environ: *Environ, allocator: Allocator) void {
        if (is_windows) {
            // This value expires with any call that modifies the environment,
            // which is outside of this Io implementation's control, so references
            // must be short-lived.
            const peb = windows.peb();
            assert(windows.ntdll.RtlEnterCriticalSection(peb.FastPebLock) == .SUCCESS);
            defer assert(windows.ntdll.RtlLeaveCriticalSection(peb.FastPebLock) == .SUCCESS);
            const ptr = peb.ProcessParameters.Environment;

            var i: usize = 0;
            while (ptr[i] != 0) {
                // There are some special environment variables that start with =,
                // so we need a special case to not treat = as a key/value separator
                // if it's the first character.
                // https://devblogs.microsoft.com/oldnewthing/20100506-00/?p=14133
                const key_start = i;
                if (ptr[i] == '=') i += 1;
                while (ptr[i] != 0 and ptr[i] != '=') : (i += 1) {}
                const key_w = ptr[key_start..i];

                const value_start = i + 1;
                while (ptr[i] != 0) : (i += 1) {} // skip over '=' and value
                const value_w = ptr[value_start..i];
                i += 1; // skip over null byte

                if (windows.eqlIgnoreCaseWtf16(key_w, &.{ 'N', 'O', '_', 'C', 'O', 'L', 'O', 'R' })) {
                    environ.exist.NO_COLOR = true;
                } else if (windows.eqlIgnoreCaseWtf16(key_w, &.{ 'C', 'L', 'I', 'C', 'O', 'L', 'O', 'R', '_', 'F', 'O', 'R', 'C', 'E' })) {
                    environ.exist.CLICOLOR_FORCE = true;
                } else if (windows.eqlIgnoreCaseWtf16(key_w, &.{ 'Z', 'I', 'G', '_', 'P', 'R', 'O', 'G', 'R', 'E', 'S', 'S' })) {
                    environ.zig_progress_file = file: {
                        var value_buf: [std.fmt.count("{d}", .{std.math.maxInt(usize)})]u8 = undefined;
                        const len = std.unicode.calcWtf8Len(value_w);
                        if (len > value_buf.len) break :file error.UnrecognizedFormat;
                        assert(std.unicode.wtf16LeToWtf8(&value_buf, value_w) == len);
                        break :file .{
                            .handle = @ptrFromInt(std.fmt.parseInt(usize, value_buf[0..len], 10) catch
                                break :file error.UnrecognizedFormat),
                            .flags = .{ .nonblocking = true },
                        };
                    };
                }
                comptime assert(@sizeOf(String) == 0);
            }
        } else if (native_os == .wasi and !builtin.link_libc) {
            var environ_size: usize = undefined;
            var environ_buf_size: usize = undefined;

            switch (std.os.wasi.environ_sizes_get(&environ_size, &environ_buf_size)) {
                .SUCCESS => {},
                else => |err| {
                    environ.err = posix.unexpectedErrno(err);
                    return;
                },
            }
            if (environ_size == 0) return;

            const wasi_environ = allocator.alloc([*:0]u8, environ_size) catch |err| {
                environ.err = err;
                return;
            };
            defer allocator.free(wasi_environ);
            const wasi_environ_buf = allocator.alloc(u8, environ_buf_size) catch |err| {
                environ.err = err;
                return;
            };
            defer allocator.free(wasi_environ_buf);

            switch (std.os.wasi.environ_get(wasi_environ.ptr, wasi_environ_buf.ptr)) {
                .SUCCESS => {},
                else => |err| {
                    environ.err = posix.unexpectedErrno(err);
                    return;
                },
            }

            for (wasi_environ) |env| {
                const pair = std.mem.sliceTo(env, 0);
                var parts = std.mem.splitScalar(u8, pair, '=');
                const key = parts.first();
                if (std.mem.eql(u8, key, "NO_COLOR")) {
                    environ.exist.NO_COLOR = true;
                } else if (std.mem.eql(u8, key, "CLICOLOR_FORCE")) {
                    environ.exist.CLICOLOR_FORCE = true;
                }
                comptime assert(@sizeOf(String) == 0);
            }
        } else {
            for (environ.process_environ.block.slice) |opt_entry| {
                const entry = opt_entry.?;
                var entry_i: usize = 0;
                while (entry[entry_i] != 0 and entry[entry_i] != '=') : (entry_i += 1) {}
                const key = entry[0..entry_i];

                var end_i: usize = entry_i;
                while (entry[end_i] != 0) : (end_i += 1) {}
                const value = entry[entry_i + 1 .. end_i :0];

                if (std.mem.eql(u8, key, "NO_COLOR")) {
                    environ.exist.NO_COLOR = true;
                } else if (std.mem.eql(u8, key, "CLICOLOR_FORCE")) {
                    environ.exist.CLICOLOR_FORCE = true;
                } else if (std.mem.eql(u8, key, "ZIG_PROGRESS")) {
                    environ.zig_progress_file = file: {
                        break :file .{
                            .handle = std.fmt.parseInt(u31, value, 10) catch
                                break :file error.UnrecognizedFormat,
                            .flags = .{ .nonblocking = true },
                        };
                    };
                } else inline for (@typeInfo(String).@"struct".field_names) |field_name| {
                    if (std.mem.eql(u8, key, field_name)) @field(environ.string, field_name) = value;
                }
            }
        }
    }