vendor/github.com/hashicorp/terraform/terraform/eval_interpolate.go

package terraform

import (
	"log"

	"github.com/hashicorp/terraform/config"
)

// EvalInterpolate is an EvalNode implementation that takes a raw
// configuration and interpolates it.
type EvalInterpolate struct {
	Config        *config.RawConfig
	Resource      *Resource
	Output        **ResourceConfig
	ContinueOnErr bool
}

func (n *EvalInterpolate) Eval(ctx EvalContext) (interface{}, error) {
	rc, err := ctx.Interpolate(n.Config, n.Resource)
	if err != nil {
		if n.ContinueOnErr {
			log.Printf("[WARN] Interpolation %q failed: %s", n.Config.Key, err)
			return nil, EvalEarlyExitError{}
		}
		return nil, err
	}

	if n.Output != nil {
		*n.Output = rc
	}

	return nil, nil
}

// EvalInterpolateProvider is an EvalNode implementation that takes a
// ProviderConfig and interpolates it. Provider configurations are the only
// "inherited" type of configuration we have, and the original raw config may
// have a different interpolation scope.
type EvalInterpolateProvider struct {
	Config   *config.ProviderConfig
	Resource *Resource
	Output   **ResourceConfig
}

func (n *EvalInterpolateProvider) Eval(ctx EvalContext) (interface{}, error) {
	rc, err := ctx.InterpolateProvider(n.Config, n.Resource)
	if err != nil {
		return nil, err
	}

	if n.Output != nil {
		*n.Output = rc
	}

	return nil, nil
}