UnmarshalExact() returns misleading error message

[ardrabczyk]

Preflight Checklist

• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

Example:

package main

import (
        "bytes"
        "log"

        "github.com/spf13/viper"
)

type settings struct {
        URL      string
        Username string
        Password string
}

func main() {
        viper.SetConfigType("toml")

        confContents := []byte(`
url = 'http://localhost'
username = 'my-user'
password = 'my-password'
extra = 'fake'
`)

        _ = viper.ReadConfig(bytes.NewBuffer(confContents))
        settings := settings{}
        err := viper.UnmarshalExact(&settings)
        if err != nil {
                log.Fatal(err)
        }
}

Output:

$ go run .
2023/12/20 10:52:40 1 error(s) decoding:

* '' has invalid keys: extra
exit status 1

Can this * '' has invalid keys: extra error get more readable? I especially don't like '' part, it's very misleading for users and isn't helpful for them.

I know it comes straight from https://github.com/mitchellh/mapstructure/blob/main/mapstructure.go#L417 but can it maybe be made more readable here?

Proposed Solution

I would like the error message to be more readable and do not mention a suspicious '' part.

Alternatives Considered

No response

Additional Information

No response

[github-actions]

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[ardrabczyk]

I found I can always do

var md mapstructure.Metadata
err = viper.Unmarshal(&settings, func(config *mapstructure.DecoderConfig) {
                config.Metadata = &md
        })

and then loop through md.Unused but UnmarshalExact() doesn't set it, I don't yet know why.

EDIT:

decodeStructFromMap() returns early when ErrorUnused is set https://github.com/mitchellh/mapstructure/blob/main/mapstructure.go#L1459