Struct tags

1. Tag reference
2. env-* tag aliases

---

type Config struct {
    Port     int           `env:"PORT" default:"8080" desc:"listen port"`
    Host     string        `env:"HOST,required"`
    Tags     []string      `env:"TAGS" separator:","`
    Labels   map[string]int `env:"LABELS"`               // KEY:VALUE pairs, comma-separated
    Started  time.Time     `env:"STARTED" layout:"2006-01-02"`
    Password string        `env:"PASSWORD,file"`          // value read from the file at this path
    Token    string        `env:"TOKEN,notEmpty"`         // present but empty is an error
    Ignored  string        `env:"-"`                      // never populated
    DB       DBConfig      `envPrefix:"DB_"`              // nested struct
}

Tag reference

Tag	Applies to	Meaning
env:"NAME"	any field	Environment key. Defaults to the Go field name if omitted. env:"-" skips the field.
env:"NAME,required"	any field	Fail if the value is absent from every source.
env:"NAME,notEmpty"	any field	Fail if the value is present but empty.
env:"NAME,file"	string-ish	Treat the resolved value as a path and read the file’s contents as the real value.
env:"NAME,init"	pointer / slice / map	Allocate a non-nil zero value even when no value is supplied.
env:"NAME,unset"	any field	Remove the variable from the process environment after reading it.
env:"NAME,secret"	any field	Mask the value in Redacted / RedactedMap output (plain Marshal keeps it).
default:"..."	any field	Fallback value when nothing else provides one.
separator:","	slice / map	Element separator. envSeparator is accepted as an alias.
layout:"..."	time.Time	time.Parse layout (default time.RFC3339).
envPrefix:"DB_"	nested struct	Prefix applied to every key inside the nested struct.
desc:"..."	any field	Human description, surfaced by Usage.

Multiple options combine: env:"TOKEN,required,file" reads a required secret file.

env-* tag aliases

Every configuration tag also has an env-* spelling, so an env-prefixed convention can be used throughout:

Native	env-* alias
default:"8080"	env-default:"8080"
separator:"," (or envSeparator)	env-separator:","
desc:"..."	env-description:"..."
layout:"..."	env-layout:"..."
envPrefix:"DB_"	env-prefix:"DB_"
env:"NAME,required"	env-required:"true"
env:"NAME,notEmpty"	env-notempty:"true"
env:"NAME,file"	env-file:"true"
env:"NAME,init"	env-init:"true"
env:"NAME,unset"	env-unset:"true"

type Config struct {
    Port int      `env:"PORT" env-default:"8080" env-description:"listen port"`
    Tags []string `env:"TAGS" env-separator:";"`
    Host string   `env:"HOST" env-required:"true"`
    DB   DBConfig `env-prefix:"DB_"`
}

Priority when both spellings are present: the env-* form always wins; the native tag is the fallback. A boolean env-* tag like env-required:"false" explicitly turns the option off.