Run Go 1.16 in CI

The only reason I retained backwards-compatibility is because Debian 11
is still on Go 1.15, but Debian 12 should be released this week with Go
1.19 so we can fairly safely drop it.

.github/workflows/test.yml

@@ -5,7 +5,7 @@
     "test": {
       "strategy": {
         "matrix": {
-          "go-version": ["1.13.x", "1.20.x"],
+          "go-version": ["1.16.x", "1.20.x"],
           "os":         ["ubuntu-latest", "macos-latest", "windows-latest"]
         }
       },

bench_test.go

@@ -1,12 +1,8 @@
-//go:build go1.16
-// +build go1.16
-
 package toml_test
 
 import (
 	"bytes"
 	"io/fs"
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"sort"
@@ -132,7 +128,7 @@ func BenchmarkEncode(b *testing.B) {
 }
 
 func BenchmarkExample(b *testing.B) {
-	d, err := ioutil.ReadFile("_example/example.toml")
+	d, err := os.ReadFile("_example/example.toml")
 	if err != nil {
 		b.Fatal(err)
 	}

decode.go

@@ -6,7 +6,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
-	"io/ioutil"
+	"io/fs"
 	"math"
 	"os"
 	"reflect"
@@ -46,6 +46,17 @@ func DecodeFile(path string, v interface{}) (MetaData, error) {
 	return NewDecoder(fp).Decode(v)
 }
 
+// DecodeFS reads the contents of a file from [fs.FS] and decodes it with
+// [Decode].
+func DecodeFS(fsys fs.FS, path string, v interface{}) (MetaData, error) {
+	fp, err := fsys.Open(path)
+	if err != nil {
+		return MetaData{}, err
+	}
+	defer fp.Close()
+	return NewDecoder(fp).Decode(v)
+}
+
 // Primitive is a TOML value that hasn't been decoded into a Go value.
 //
 // This type can be used for any value, which will cause decoding to be delayed.
@@ -148,7 +159,7 @@ func (dec *Decoder) Decode(v interface{}) (MetaData, error) {
 
 	// TODO: parser should read from io.Reader? Or at the very least, make it
 	// read from []byte rather than string
-	data, err := ioutil.ReadAll(dec.r)
+	data, err := io.ReadAll(dec.r)
 	if err != nil {
 		return MetaData{}, err
 	}

decode_test.go

@@ -5,14 +5,14 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"io/ioutil"
 	"math"
 	"os"
 	"reflect"
 	"strconv"
 	"strings"
 	"sync"
 	"testing"
+	"testing/fstest"
 	"time"
 
 	"github.com/BurntSushi/toml/internal"
@@ -38,7 +38,7 @@ func TestDecodeReader(t *testing.T) {
 }
 
 func TestDecodeFile(t *testing.T) {
-	tmp, err := ioutil.TempFile("", "toml-")
+	tmp, err := os.CreateTemp("", "toml-")
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -63,6 +63,25 @@ func TestDecodeFile(t *testing.T) {
 	}
 }
 
+func TestDecodeFS(t *testing.T) {
+	fsys := fstest.MapFS{
+		"test.toml": &fstest.MapFile{
+			Data: []byte("a = 42"),
+		},
+	}
+
+	var i struct{ A int }
+	meta, err := DecodeFS(fsys, "test.toml", &i)
+	if err != nil {
+		t.Fatal(err)
+	}
+	have := fmt.Sprintf("%v %v %v", i, meta.Keys(), meta.Type("a"))
+	want := "{42} [a] Integer"
+	if have != want {
+		t.Errorf("\nhave: %s\nwant: %s", have, want)
+	}
+}
+
 func TestDecodeBOM(t *testing.T) {
 	for _, tt := range [][]byte{
 		[]byte("\xff\xfea = \"b\""),

error_test.go

@@ -1,6 +1,3 @@
-//go:build go1.16
-// +build go1.16
-
 package toml_test
 
 import (

internal/toml-test/json.go

@@ -1,6 +1,3 @@
-//go:build go1.16
-// +build go1.16
-
 package tomltest
 
 import (

internal/toml-test/runner.go

@@ -1,8 +1,5 @@
 //go:generate ./gen-multi.py
 
-//go:build go1.16
-// +build go1.16
-
 package tomltest
 
 import (

internal/toml-test/toml.go

@@ -1,6 +1,3 @@
-//go:build go1.16
-// +build go1.16
-
 package tomltest
 
 import (

internal/toml-test/version.go

@@ -1,6 +1,3 @@
-//go:build go1.16
-// +build go1.16
-
 package tomltest
 
 type versionSpec struct {

toml_test.go

@@ -1,6 +1,3 @@
-//go:build go1.16
-// +build go1.16
-
 package toml_test
 
 import (