gofumpt: Add lang-version option + update linter

Without the Go language version input, `gofumpt` cannot apply some rules
which are version dependent.

Defaulting `lang-version` to the Go version of the running environment
if not specified.

Signed-off-by: Kailun Qin <[email protected]>

Author: kailun-qin

.golangci.example.yml

@@ -259,6 +259,10 @@ linters-settings:
     simplify: true
 
   gofumpt:
+    # Select the Go language version to target.
+    # Default is the Go version installed in the running environment.
+    lang-version: "1.15"
+
     # Choose whether or not to use the extra rules that are disabled
     # by default
     extra-rules: false

pkg/config/linters_settings.go

@@ -59,7 +59,8 @@ var defaultLintersSettings = LintersSettings{
 		DefaultSignifiesExhaustive: false,
 	},
 	Gofumpt: GofumptSettings{
-		ExtraRules: false,
+		LangVersion: "",
+		ExtraRules:  false,
 	},
 	ErrorLint: ErrorLintSettings{
 		Errorf:     true,
@@ -232,7 +233,8 @@ type GoFmtSettings struct {
 }
 
 type GofumptSettings struct {
-	ExtraRules bool `mapstructure:"extra-rules"`
+	LangVersion string `mapstructure:"lang-version"`
+	ExtraRules  bool   `mapstructure:"extra-rules"`
 }
 
 type GoHeaderSettings struct {

pkg/golinters/gofumpt.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"fmt"
 	"io/ioutil"
+	"os/exec"
 	"sync"
 
 	"github.com/pkg/errors"
@@ -46,8 +47,21 @@ func NewGofumpt() *goanalysis.Linter {
 				if err != nil {
 					return nil, fmt.Errorf("unable to open file %s: %w", f, err)
 				}
+
+				// Get the target Go language version to be used in gofumpt.
+				// Reuse logic from `https://github.com/mvdan/gofumpt/blob/master/gofmt.go`.
+				langVersion := lintCtx.Settings().Gofumpt.LangVersion
+				if langVersion == "" {
+					outGoVersion, errGoVersion := exec.Command("go", "list", "-m", "-f", "{{.GoVersion}}").Output()
+					outGoVersion = bytes.TrimSpace(outGoVersion)
+					if errGoVersion == nil && len(outGoVersion) > 0 {
+						langVersion = string(outGoVersion)
+					}
+				}
+
 				output, err := format.Source(input, format.Options{
-					ExtraRules: lintCtx.Settings().Gofumpt.ExtraRules,
+					LangVersion: langVersion,
+					ExtraRules:  lintCtx.Settings().Gofumpt.ExtraRules,
 				})
 				if err != nil {
 					return nil, fmt.Errorf("error while running gofumpt: %w", err)