It is generally hard to do what you request. Config files are read to find values that override default ones. To detect extra-numerous keys requires to have a schema of the configuration and validate any input against this schema. But a INI is « open » by nature.

Seems to be a limit of toml.DecodeFile then.

OK: how about make general.domain a mandatory setting and give a clearer error if it's missing? (e.g. "Required setting 'domain' in section '[general]' is missing")

Aha, it is possible: see "Example (StrictDecoding)" at https://godocs.io/github.com/BurntSushi/toml

Something like this should do the trick:

        md, err := toml.DecodeFile(fname, &conf)
        if err != nil {
                // Return with config file parsing errors from toml package
                return conf, err
        }
        undecoded := md.Undecoded()
        if len(undecoded) > 0 {
                return conf, fmt.Errorf("Unexpected keys: %v", undecoded)
        }