Handle embedded and ignored fields in toml env override

One of the closed source copies of applyEnvOverrides needed this
behavior.

toml/toml.go

@@ -180,9 +180,24 @@ func applyEnvOverrides(getenv func(string) string, prefix string, spec reflect.V
 				continue
 			}
 
-			fieldName := typeOfSpec.Field(i).Name
+			structField := typeOfSpec.Field(i)
+			fieldName := structField.Name
+
+			configName := structField.Tag.Get("toml")
+			if configName == "-" {
+				// Skip fields with tag `toml:"-"`.
+				continue
+			}
+
+			if configName == "" && structField.Anonymous {
+				// Embedded field without a toml tag.
+				// Don't modify prefix.
+				if err := applyEnvOverrides(getenv, prefix, field, fieldName); err != nil {
+					return err
+				}
+				continue
+			}
 
-			configName := typeOfSpec.Field(i).Tag.Get("toml")
 			// Replace hyphens with underscores to avoid issues with shells
 			configName = strings.Replace(configName, "-", "_", -1)
 

toml/toml_test.go

@@ -94,6 +94,8 @@ func TestEnvOverride_Builtins(t *testing.T) {
 		"X_FLOAT64":       "12.5",
 		"X_NESTED_STRING": "a nested string",
 		"X_NESTED_INT":    "13",
+		"X_ES":            "an embedded string",
+		"X__":             "-1", // This value should not be applied to the "ignored" field with toml tag -.
 	}
 
 	env := func(s string) string {
@@ -104,6 +106,9 @@ func TestEnvOverride_Builtins(t *testing.T) {
 		Str string `toml:"string"`
 		Int int    `toml:"int"`
 	}
+	type Embedded struct {
+		ES string `toml:"es"`
+	}
 	type all struct {
 		Str     string         `toml:"string"`
 		Dur     itoml.Duration `toml:"duration"`
@@ -121,6 +126,10 @@ func TestEnvOverride_Builtins(t *testing.T) {
 		Float32 float32        `toml:"float32"`
 		Float64 float64        `toml:"float64"`
 		Nested  nested         `toml:"nested"`
+
+		Embedded
+
+		Ignored int `toml:"-"`
 	}
 
 	var got all
@@ -148,6 +157,10 @@ func TestEnvOverride_Builtins(t *testing.T) {
 			Str: "a nested string",
 			Int: 13,
 		},
+		Embedded: Embedded{
+			ES: "an embedded string",
+		},
+		Ignored: 0,
 	}
 
 	if diff := cmp.Diff(got, exp); diff != "" {