Revert "Convert env management to a resolver."

This reverts commit fbe8d48e4b.

env.go

@@ -1,21 +0,0 @@
-package kong
-
-import (
-	"os"
-)
-
-// EnvarResolver resolves values from environment variables.
-//
-// It is installed by default. Use ClearResolvers() to disable this.
-func EnvarResolver() Resolver {
-	return ResolverFunc(func(context *Context, parent *Path, flag *Flag) (interface{}, error) {
-		if flag.Tag.Env == "" {
-			return nil, nil
-		}
-		envar := os.Getenv(flag.Tag.Env)
-		if envar != "" {
-			return envar, nil
-		}
-		return nil, nil
-	})
-}

kong.go

@@ -71,7 +71,6 @@ func New(grammar interface{}, options ...Option) (*Kong, error) {
 		vars:          Vars{},
 		bindings:      bindings{},
 		helpFormatter: DefaultHelpValueFormatter,
-		resolvers:     []Resolver{EnvarResolver()},
 	}
 
 	options = append(options, Bind(k))

kong_test.go

@@ -675,19 +675,6 @@ func TestHooksCalledForDefault(t *testing.T) {
 	require.Equal(t, []string{"before:default", "after:default"}, ctx.values)
 }
 
-func TestHooksCalledForEnv(t *testing.T) {
-	var cli struct {
-		Flag hookValue `env:"FLAG"`
-	}
-	defer tempEnv(envMap{"FLAG": "flagged"})()
-
-	ctx := &hookContext{}
-	_, err := mustNew(t, &cli, kong.Bind(ctx)).Parse(nil)
-	require.NoError(t, err)
-	require.Equal(t, "flagged", string(cli.Flag))
-	require.Equal(t, []string{"before:", "after:flagged"}, ctx.values)
-}
-
 func TestEnum(t *testing.T) {
 	var cli struct {
 		Flag string `enum:"a,b,c"`

model.go

@@ -3,6 +3,7 @@ package kong
 import (
 	"fmt"
 	"math"
+	"os"
 	"reflect"
 	"strconv"
 	"strings"
@@ -325,6 +326,16 @@ func (v *Value) ApplyDefault() error {
 // Does not include resolvers.
 func (v *Value) Reset() error {
 	v.Target.Set(reflect.Zero(v.Target.Type()))
+	if v.Tag.Env != "" {
+		envar := os.Getenv(v.Tag.Env)
+		if envar != "" {
+			err := v.Parse(ScanFromTokens(Token{Type: FlagValueToken, Value: envar}), v.Target)
+			if err != nil {
+				return fmt.Errorf("%s (from envar %s=%q)", err, v.Tag.Env, envar)
+			}
+			return nil
+		}
+	}
 	if v.Default != "" {
 		return v.Parse(ScanFromTokens(Token{Type: FlagValueToken, Value: v.Default}), v.Target)
 	}