feat: Support map types in flag, tag and env loaders

Adds support for loading configuration into map[string]string and
map[string]int (and related) types.
The string encoding used is the same as the one used in pflag: elements
are delimited by comma's (like for slices) and key values are split by
equals sign (=). For example:
key1=val1,key2=val2

While updating the string parser, I have updated the error messages that
are returned. They now leverage error wrapping and include more
information about the field and value that contained the error.

env_test.go

@@ -1,11 +1,11 @@
 package multiconfig
 
 import (
-	"os"
 	"strings"
 	"testing"
 
 	"github.com/fatih/structs"
+	"github.com/stretchr/testify/require"
 )
 
 func TestENV(t *testing.T) {
@@ -128,9 +128,7 @@ func setEnvVars(t *testing.T, structName, prefix string) {
 
 	for key, val := range env {
 		env := prefix + "_" + key
-		if err := os.Setenv(env, val); err != nil {
-			t.Fatal(err)
-		}
+		t.Setenv(env, val)
 	}
 }
 
@@ -152,3 +150,24 @@ func TestENVgetPrefix(t *testing.T) {
 		t.Errorf("Prefix is wrong: %s, want: %s", p, prefix)
 	}
 }
+
+func TestMapEnvSupport(t *testing.T) {
+	m := &EnvironmentLoader{CamelCase: true}
+
+	env := map[string]string{
+		"_MAP_STRING_INT":    "key1=1234,key2=456",
+		"_MAP_STRING_STRING": "key1=val1,key2=val2",
+	}
+	for key, val := range env {
+		t.Setenv(key, val)
+	}
+
+	var e struct {
+		MapStringInt    map[string]int
+		MapStringString map[string]string
+	}
+
+	require.NoError(t, m.Load(&e))
+	require.EqualValues(t, map[string]int{"key1": 1234, "key2": 456}, e.MapStringInt)
+	require.EqualValues(t, map[string]string{"key1": "val1", "key2": "val2"}, e.MapStringString)
+}

flag_test.go

@@ -219,6 +219,7 @@ func TestFlagValueSupport(t *testing.T) {
 		t.Fatalf("got %q, want %q", e.Public, m.Args[3])
 	}
 }
+
 func TestCustomUsageTag(t *testing.T) {
 	const usageMsg = "foobar help"
 	strt := struct {
@@ -239,6 +240,24 @@ func TestCustomUsageTag(t *testing.T) {
 	}
 }
 
+func TestMapFlagSupport(t *testing.T) {
+	m := &FlagLoader{CamelCase: true}
+
+	m.Args = []string{
+		"-map-string-int", "key1=1234,key2=456",
+		"-map-string-string", "key1=val1,key2=val2",
+	}
+
+	var e struct {
+		MapStringInt    map[string]int
+		MapStringString map[string]string
+	}
+
+	require.NoError(t, m.Load(&e))
+	require.EqualValues(t, map[string]int{"key1": 1234, "key2": 456}, e.MapStringInt)
+	require.EqualValues(t, map[string]string{"key1": "val1", "key2": "val2"}, e.MapStringString)
+}
+
 // getFlags returns a slice of arguments that can be passed to flag.Parse()
 func getFlags(t *testing.T, structName, prefix string) []string {
 	t.Helper()