require Go 1.22 and use go/version for Options.LangVersion

This is a Go API breaking change; where existing users could use
strings such as "1.16" or "v1.16", they should now use "go1.16".
Note that the "go" prefix is mandatory, following go/version.

I considered a softer landing to manipulate the older string formats
to conform to the new format, but given how go/version is strict
and Go versions are now clearly defined differently from SemVer,
it seems best to draw a line and follow go/version's notion of validity.

Supporting "go1.16" alongside "v1.16" could also cause confusion in the
long run, as Go versions are subtly different from SemVer in terms
of validity and order. Requiring the "go" prefix avoids footguns.

Most gofumpt CLI users shouldn't be using the -lang CLI flag directly,
as by default it grabs its value from the go directive in go.mod.
Hence, most gofumpt CLI users should not be affected by this breakage.
Go API users, such as tool or IDE integration authors, are more likely
to get broken as the API falls back to assuming a conservative Go 1.0.
Fixing the breakage is rather simple, as shown by the changes below.

While here, suggest `go list -m` rather than `go mod edit -json`,
as the latter requires using `jq` to obtain just one field.
We still mention the latter command as an alternative.

Author: mvdan

Diff for: README.md

@@ -651,7 +651,7 @@ package p
 $ gofumpt f.go
 package p
 
-//gofumpt:diagnose v0.1.1-0.20211103104632-bdfa3b02e50a -lang=v1.16
+//gofumpt:diagnose v0.1.1-0.20211103104632-bdfa3b02e50a -lang=go1.16
 ```
 
 ### License

Diff for: format/format.go

@@ -12,6 +12,7 @@ import (
 	"go/ast"
 	"go/parser"
 	"go/token"
+	goversion "go/version"
 	"os"
 	"reflect"
 	"regexp"
@@ -23,7 +24,6 @@ import (
 	"unicode/utf8"
 
 	"github.com/google/go-cmp/cmp"
-	"golang.org/x/mod/semver"
 	"golang.org/x/tools/go/ast/astutil"
 
 	"mvdan.cc/gofumpt/internal/govendor/go/format"
@@ -32,26 +32,25 @@ import (
 
 // Options is the set of formatting options which affect gofumpt.
 type Options struct {
-	// TODO: link to the go/version docs once Go 1.22 is out.
-	// The old semver docs said:
-	//
-	// LangVersion is treated as a semantic version, which may start with a "v"
-	// prefix. Like Go versions, it may also be incomplete; "1.14" is equivalent
-	// to "1.14.0". When empty, it is equivalent to "v1", to not use language
-	// features which could break programs.
-
 	// LangVersion is the Go version a piece of code is written in.
 	// The version is used to decide whether to apply formatting
 	// rules which require new language features.
-	// When inside a Go module, LangVersion should typically be:
+	// When empty, a default of go1 is assumed.
+	// Otherwise, the version must satisfy [go/version.IsValid].
+	//
+	// When formatting a Go module, LangVersion should typically be
 	//
-	//     go mod edit -json | jq -r '.Go'
+	//     go list -m -f {{.GoVersion}}
+	//
+	// with a "go" prefix, or the equivalent from `go mod edit -json`.
 	LangVersion string
 
 	// ModulePath corresponds to the Go module path which contains the source
-	// code being formatted. When inside a Go module, ModulePath should be:
+	// code being formatted. When formatting a Go module, ModulePath should be
+	//
+	//     go list -m -f {{.Path}}
 	//
-	//     go mod edit -json | jq -r '.Module.Path'
+	// or the equivalent from `go mod edit -json`.
 	//
 	// ModulePath is used for formatting decisions like what import paths are
 	// considered to be not part of the standard library. When empty, the source
@@ -86,26 +85,21 @@ func Source(src []byte, opts Options) ([]byte, error) {
 	return buf.Bytes(), nil
 }
 
-var rxGoVersionMajorMinor = regexp.MustCompile(`^(v|go)?([1-9]+)\.([0-9]+)`)
-
 // File modifies a file and fset in place to follow gofumpt's format. The
 // changes might include manipulating adding or removing newlines in fset,
 // modifying the position of nodes, or modifying literal values.
 func File(fset *token.FileSet, file *ast.File, opts Options) {
 	simplify(file)
 
-	// TODO: replace this hacky mess with go/version once we can rely on Go 1.22,
-	// as well as replacing our uses of the semver package.
-	// In particular, we likely want to allow any of 1.21, 1.21.2, or go1.21rc3,
-	// but we can rely on go/version.Lang to validate and normalize.
 	if opts.LangVersion == "" {
-		opts.LangVersion = "v1.0"
-	}
-	m := rxGoVersionMajorMinor.FindStringSubmatch(opts.LangVersion)
-	if m == nil {
-		panic(fmt.Sprintf("invalid Go version: %q", opts.LangVersion))
+		opts.LangVersion = "go1"
+	} else {
+		lang := goversion.Lang(opts.LangVersion)
+		if lang == "" {
+			panic(fmt.Sprintf("invalid Go version: %q", opts.LangVersion))
+		}
+		opts.LangVersion = lang
 	}
-	opts.LangVersion = "v" + m[2] + "." + m[3]
 	f := &fumpter{
 		file:    fset.File(file.Pos()),
 		fset:    fset,
@@ -680,8 +674,8 @@ func (f *fumpter) applyPre(c *astutil.Cursor) {
 		}
 
 	case *ast.BasicLit:
-		// Octal number literals were introduced in 1.13.
-		if semver.Compare(f.LangVersion, "v1.13") >= 0 {
+		// Octal number literals were introduced in Go 1.13.
+		if goversion.Compare(f.LangVersion, "go1.13") >= 0 {
 			if node.Kind == token.INT && rxOctalInteger.MatchString(node.Value) {
 				node.Value = "0o" + node.Value[1:]
 				c.Replace(node)

Diff for: format/fuzz_test.go

@@ -26,9 +26,9 @@ func FuzzFormat(f *testing.F) {
 		for _, file := range archive.Files {
 			f.Logf("adding %s from %s", file.Name, path)
 			if strings.HasSuffix(file.Name, ".go") || strings.Contains(file.Name, ".go.") {
-				f.Add(string(file.Data), int8(18), false) // -lang=1.18
-				f.Add(string(file.Data), int8(1), false)  // -lang=1.1
-				f.Add(string(file.Data), int8(18), true)  // -lang=1.18 -extra
+				f.Add(string(file.Data), int8(18), false) // -lang=go1.18
+				f.Add(string(file.Data), int8(1), false)  // -lang=go1.1
+				f.Add(string(file.Data), int8(18), true)  // -lang=go1.18 -extra
 			}
 		}
 	}
@@ -40,7 +40,7 @@ func FuzzFormat(f *testing.F) {
 		// TODO: also fuzz Options.ModulePath
 		opts := Options{ExtraRules: extraRules}
 		if majorVersion >= 0 {
-			opts.LangVersion = fmt.Sprintf("1.%d", majorVersion)
+			opts.LangVersion = fmt.Sprintf("go1.%d", majorVersion)
 		}
 
 		orig := []byte(src)

Diff for: go.mod

@@ -1,6 +1,6 @@
 module mvdan.cc/gofumpt
 
-go 1.21
+go 1.22
 
 require (
 	github.com/go-quicktest/qt v1.101.0

Diff for: gofmt.go

@@ -96,7 +96,7 @@ func usage() {
 	-w        write result to (source) file instead of stdout
 	-extra    enable extra rules which should be vetted by a human
 
-	-lang       str    target Go version in the form "1.X" (default from go.mod)
+	-lang       str    target Go version in the form "go1.X" (default from go.mod)
 	-modpath    str    Go module path containing the source file (default from go.mod)
 `)
 }
@@ -298,7 +298,7 @@ func processFile(filename string, info fs.FileInfo, in io.Reader, r *reporter, e
 		if ok && mod != nil {
 			mod := mod.(*module)
 			if lang == "" {
-				lang = mod.Go
+				lang = "go" + mod.Go
 			}
 			if modpath == "" {
 				modpath = mod.Module.Path

Diff for: testdata/script/diagnose.txtar

@@ -14,7 +14,7 @@ cmp stdout foo.go.golden
 exec gofumpt -extra foo.go
 cmp stdout foo.go.golden-extra
 
-exec gofumpt -lang=1.0 foo.go
+exec gofumpt -lang=go1 foo.go
 cmp stdout foo.go.golden-lang
 
 exec gofumpt -d nochange.go
@@ -79,24 +79,24 @@ package p
 -- foo.go.golden --
 package p
 
-//gofumpt:diagnose version: v0.0.0-20220727155840-8dda8068d9f3 (go1.18.29) flags: -lang=v1.16 -modpath=test
+//gofumpt:diagnose version: v0.0.0-20220727155840-8dda8068d9f3 (go1.18.29) flags: -lang=go1.16 -modpath=test
 -- foo.go.golden-devel --
 package p
 
-//gofumpt:diagnose version: (devel) (go1.18.29) flags: -lang=v1.16 -modpath=test
+//gofumpt:diagnose version: (devel) (go1.18.29) flags: -lang=go1.16 -modpath=test
 -- foo.go.golden-extra --
 package p
 
-//gofumpt:diagnose version: v0.0.0-20220727155840-8dda8068d9f3 (go1.18.29) flags: -lang=v1.16 -modpath=test -extra
+//gofumpt:diagnose version: v0.0.0-20220727155840-8dda8068d9f3 (go1.18.29) flags: -lang=go1.16 -modpath=test -extra
 -- foo.go.golden-lang --
 package p
 
-//gofumpt:diagnose version: v0.0.0-20220727155840-8dda8068d9f3 (go1.18.29) flags: -lang=v1.0 -modpath=test
+//gofumpt:diagnose version: v0.0.0-20220727155840-8dda8068d9f3 (go1.18.29) flags: -lang=go1 -modpath=test
 -- foo.go.golden-released --
 package p
 
-//gofumpt:diagnose v0.3.2-0.20220627183521-8dda8068d9f3 -lang=v1.16 -modpath=test
+//gofumpt:diagnose v0.3.2-0.20220627183521-8dda8068d9f3 -lang=go1.16 -modpath=test
 -- foo.go.golden-external --
 package p
 
-//gofumpt:diagnose (devel) -lang=v1.16 -modpath=
+//gofumpt:diagnose (devel) -lang=go1.16 -modpath=

Diff for: testdata/script/gomod.txtar

@@ -1,10 +1,10 @@
 # Test various edge cases with go.mod files.
 
 exec gofumpt toolchain-stable/a.go
-stdout '//gofumpt:diagnose.* -lang=v1.21'
+stdout '//gofumpt:diagnose.* -lang=go1.21'
 
 exec gofumpt toolchain-unstable/a.go
-stdout '//gofumpt:diagnose.* -lang=v1.21'
+stdout '//gofumpt:diagnose.* -lang=go1.21'
 
 -- toolchain-stable/go.mod --
 module a

Diff for: testdata/script/octal-literals.txtar

@@ -3,7 +3,7 @@ exec gofumpt -l .
 ! stdout .
 
 # We can give an explicitly newer version.
-exec gofumpt -lang=1.13 -l .
+exec gofumpt -lang=go1.13 -l .
 stdout -count=1 'foo\.go'
 stdout -count=1 'nested[/\\]nested\.go'
 
@@ -20,7 +20,7 @@ exec gofumpt -d foo.go.golden
 ! stdout .
 
 # We can give an explicitly older version, too
-exec gofumpt -lang=1.0 -l .
+exec gofumpt -lang=go1.0 -l .
 ! stdout .
 
 -- go.mod --