format: support Go versions from newer go.mod files

The 'go' directive in go.mod files can now contain strings such as
"go1.22rc1" or "go1.21.1", and our old semver logic didn't support
some of those.

Upstream is providing new API via go/version in Go 1.22,
but until that is out, we implement our own workaround to avoid panics.

Fixes #280.

format/format.go

@@ -31,17 +31,20 @@ import (
 
 // Options is the set of formatting options which affect gofumpt.
 type Options struct {
-	// LangVersion corresponds to the Go language 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 be:
-	//
-	//     go mod edit -json | jq -r '.Go'
+	// 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:
+	//
+	//     go mod edit -json | jq -r '.Go'
 	LangVersion string
 
 	// ModulePath corresponds to the Go module path which contains the source
@@ -82,20 +85,26 @@ 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"
-	} else if opts.LangVersion[0] != 'v' {
-		opts.LangVersion = "v" + opts.LangVersion
+		opts.LangVersion = "v1.0"
 	}
-	if !semver.IsValid(opts.LangVersion) {
-		panic(fmt.Sprintf("invalid semver string: %q", opts.LangVersion))
+	m := rxGoVersionMajorMinor.FindStringSubmatch(opts.LangVersion)
+	if m == nil {
+		panic(fmt.Sprintf("invalid Go version: %q", opts.LangVersion))
 	}
+	opts.LangVersion = "v" + m[2] + "." + m[3]
 	f := &fumpter{
 		File:    fset.File(file.Pos()),
 		fset:    fset,

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=v1 foo.go
+exec gofumpt -lang=1.0 foo.go
 cmp stdout foo.go.golden-lang
 
 exec gofumpt -d nochange.go
@@ -91,7 +91,7 @@ package p
 -- foo.go.golden-lang --
 package p
 
-//gofumpt:diagnose version: v0.0.0-20220727155840-8dda8068d9f3 (go1.18.29) flags: -lang=v1 -modpath=test
+//gofumpt:diagnose version: v0.0.0-20220727155840-8dda8068d9f3 (go1.18.29) flags: -lang=v1.0 -modpath=test
 -- foo.go.golden-released --
 package p
 

testdata/script/gomod.txtar

@@ -0,0 +1,25 @@
+# Test various edge cases with go.mod files.
+
+exec gofumpt toolchain-stable/a.go
+stdout '//gofumpt:diagnose.* -lang=v1.21'
+
+exec gofumpt toolchain-unstable/a.go
+stdout '//gofumpt:diagnose.* -lang=v1.21'
+
+-- toolchain-stable/go.mod --
+module a
+
+go 1.21.2
+-- toolchain-stable/a.go --
+package a
+
+//gofumpt:diagnose
+
+-- toolchain-unstable/go.mod --
+module a
+
+go 1.21rc3
+-- toolchain-unstable/a.go --
+package a
+
+//gofumpt:diagnose

testdata/script/octal-literals.txtar

@@ -20,7 +20,7 @@ exec gofumpt -d foo.go.golden
 ! stdout .
 
 # We can give an explicitly older version, too
-exec gofumpt -lang=v1 -l .
+exec gofumpt -lang=1.0 -l .
 ! stdout .
 
 -- go.mod --