use go1.21 ast.IsGenerated when available

comment.go

@@ -3,42 +3,9 @@ package exhaustive
 import (
 	"go/ast"
 	"go/token"
-	"regexp"
 	"strings"
 )
 
-// For definition of generated file see:
-// http://golang.org/s/generatedcode
-
-var generatedCodeRe = regexp.MustCompile(`^// Code generated .* DO NOT EDIT\.$`)
-
-func isGeneratedFile(file *ast.File) bool {
-	// NOTE: file.Comments includes file.Doc as well, so no need
-	// to separately check file.Doc.
-	for _, c := range file.Comments {
-		for _, cc := range c.List {
-			// This check handles the "must appear before the first
-			// non-comment, non-blank text in the file" requirement.
-			//
-			// According to https://golang.org/ref/spec#Source_file_organization
-			// the package clause is the first element in a file, which
-			// should make it the first non-comment, non-blank text.
-			if c.Pos() >= file.Package {
-				return false
-			}
-			// According to the docs:
-			//   '\r' has been removed.
-			//   '\n' has been removed for //-style comments
-			// This has also been manually verified.
-			if generatedCodeRe.MatchString(cc.Text) {
-				return true
-			}
-		}
-	}
-
-	return false
-}
-
 const (
 	ignoreComment  = "//exhaustive:ignore"
 	enforceComment = "//exhaustive:enforce"

comment_go121.go

@@ -0,0 +1,11 @@
+//go:build go1.21
+
+package exhaustive
+
+import (
+	"go/ast"
+)
+
+func isGeneratedFile(file *ast.File) bool {
+	return ast.IsGenerated(file)
+}

comment_pre_go121.go

@@ -0,0 +1,27 @@
+//go:build !go1.21
+
+package exhaustive
+
+import (
+	"go/ast"
+	"regexp"
+)
+
+// For definition of generated file see:
+// http://golang.org/s/generatedcode
+
+var generatedCodeRe = regexp.MustCompile(`^// Code generated .* DO NOT EDIT\.$`)
+
+func isGeneratedFile(file *ast.File) bool {
+	for _, c := range file.Comments {
+		for _, cc := range c.List {
+			if cc.Pos() > file.Package {
+				break
+			}
+			if generatedCodeRe.MatchString(cc.Text) {
+				return true
+			}
+		}
+	}
+	return false
+}