Document how the env tag works when using structs-in-structs

[sweepies]

Suppose I have the following code:

type Config struct {
	S3 S3Configuration
}

type S3Configuration struct {
	Endpoint string `env:"S3_ENDPOINT"`
	Region   string `env:"S3_REGION"`
	Bucket   string `env:"S3_BUCKET"`
}

The env tags in the S3Configuration struct won't work properly because there is no env tag specified on the S3 field in the Config struct. What you have to do instead is specify an env tag with an S3 namespace and then remove the S3_ prefix from the sub-struct, like so:

type Config struct {
	S3 S3Configuration `env:"S3"
}

type S3Configuration struct {
	Endpoint string `env:"ENDPOINT"`
	Region   string `env:"REGION"`
	Bucket   string `env:"BUCKET"`
}

This was unintuitive to me and caused some headache, so I feel it would help if it was documented 😅