How do you unmarshal environment variables _N_ directly to slice?

[space88man]

Problem statement: I am trying to bind environment variables to struct with nested slice
X_Y_0_HOST, X_Y_0_POST, X_Y_1_HOST, X_Y_1_PORT to something like

type HostPort struct {
 Host string
 Port int
}
struct {
  Y []HostPost
}

Sidenote: from a Java/Spring Boot background this is how it unmarshals environment variables to field types of List/Array. I am trying to emulate this behaviour so that the environment can be reused across Spring Boot/Go

---

While using YAML lists

y:
  - host: localhost
    port: 12345
  - host: abcdefg
    port: 12346

we can bind to a slice

struct {
    Y: []HostPort `mapstructure:"y"`
}

Can this be done by environment variables with the notation:

# set prefix = X_
X_Y_0_HOST=localhost
X_Y_0_PORT=12345
X_Y_1_HOST=abcdefg
X_Y_1_PORT=12346

A bit of tinkering shows that the internal map for y is: map[int]HostPost instead of []HostPort; I always end up with an empty slice. I am using ExperimentalBindStruct + pure environment variables (no YAML schema preloaded).

Preloading with SetDefault("y.0.host"..), SetDefault("y.0.port"...), ... "sorta" works with map[int]HostPort but the number of items is not fixed. Also it never seems to work with []HostPort.