Drop ExtractDefaults functionality

**:warning: Breaking!** Remove `ExtractDefaults` function. It was rarely used and resulted in extraneous processing.

envconfig.go

@@ -242,28 +242,6 @@ func ProcessWith(ctx context.Context, i any, l Lookuper, mus ...Mutator) error {
 	return processWith(ctx, i, l, false, mus...)
 }
 
-// ExtractDefaults is a helper that returns a fully-populated struct with the
-// default values resolved. This is helpful when you want to return a constant
-// "default" configuration that is not affected by the user's environment.
-//
-//	type Config struct {
-//	  Port string `env:"PORT,default=8080"`
-//	}
-//
-//	func DefaultConfig() *Config {
-//	  var cfg Config
-//	  if err := envconfig.ExtractDefaults(ctx, &cfg); err != nil {
-//	    panic("failed to extract default config: %s" + err.Error())
-//	  }
-//	  return &cfg
-//	}
-//
-// This is effectively the same as calling [ProcessWith] with an empty
-// [MapLookuper].
-func ExtractDefaults(ctx context.Context, i any, mus ...Mutator) error {
-	return processWith(ctx, i, MapLookuper(nil), false, mus...)
-}
-
 // processWith is a helper that captures whether the parent wanted
 // initialization.
 func processWith(ctx context.Context, i any, l Lookuper, parentNoInit bool, mus ...Mutator) error {

envconfig_test.go

@@ -2682,79 +2682,3 @@ func TestValidateEnvName(t *testing.T) {
 			}
 		})
 	}
-}
-
-func TestExtractDefaults(t *testing.T) {
-	t.Parallel()
-
-	cases := []struct {
-		name   string
-		input  any
-		exp    any
-		err    error
-		errMsg string
-	}{
-		{
-			name:  "nil",
-			input: (*Electron)(nil),
-			err:   ErrNotStruct,
-		},
-		{
-			name: "extracts",
-			input: &struct {
-				Field1 string `env:"FIELD, default=8080"`
-				Field2 string `env:"FIELD"`
-			}{},
-			exp: &struct {
-				Field1 string `env:"FIELD, default=8080"`
-				Field2 string `env:"FIELD"`
-			}{
-				Field1: "8080",
-				Field2: "",
-			},
-		},
-	}
-
-	for _, tc := range cases {
-		tc := tc
-
-		t.Run(tc.name, func(t *testing.T) {
-			t.Parallel()
-
-			ctx := context.Background()
-			if err := ExtractDefaults(ctx, tc.input); err != nil {
-				if tc.err == nil && tc.errMsg == "" {
-					t.Fatal(err)
-				}
-
-				if tc.err != nil && !errors.Is(err, tc.err) {
-					t.Fatalf("expected \n%+v\n to be \n%+v\n", err, tc.err)
-				}
-
-				if got, want := err.Error(), tc.errMsg; want != "" && !strings.Contains(got, want) {
-					t.Fatalf("expected \n%+v\n to match \n%+v\n", got, want)
-				}
-
-				// There's an error, but it passed all our tests, so return now.
-				return
-			}
-
-			opts := cmp.AllowUnexported(
-				// Custom decoder type
-				CustomDecoderType{},
-
-				// Custom standard library interfaces decoder type
-				CustomStdLibDecodingType{},
-
-				// Custom decoder type that returns an error
-				CustomTypeError{},
-
-				// Anonymous struct with private fields
-				struct{ field string }{},
-			)
-			if diff := cmp.Diff(tc.exp, tc.input, opts); diff != "" {
-				t.Fatalf("mismatch (-want, +got):\n%s", diff)
-			}
-		})
-	}
-}