go/ast: add IsGenerated(*File) predicate

See https://go.dev/s/generatedcode for spec.

Fixes #28089

Change-Id: Ic9bb138bdd180f136f9e8e74e187319acca5dbac
Reviewed-on: https://go-review.googlesource.com/c/go/+/487935
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Author: adonovan

api/next/28089.txt

@@ -0,0 +1 @@
+pkg go/ast, func IsGenerated(*File) bool #28089

src/go/ast/ast.go

@@ -1073,3 +1073,40 @@ type Package struct {
 
 func (p *Package) Pos() token.Pos { return token.NoPos }
 func (p *Package) End() token.Pos { return token.NoPos }
+
+// IsGenerated reports whether the file was generated by a program,
+// not handwritten, by detecting the special comment described
+// at https://go.dev/s/generatedcode.
+//
+// The syntax tree must have been parsed with the ParseComments flag.
+// Example:
+//
+//	f, err := parser.ParseFile(fset, filename, src, parser.ParseComments|parser.PackageClauseOnly)
+//	if err != nil { ... }
+//	gen := ast.IsGenerated(f)
+func IsGenerated(file *File) bool {
+	_, ok := generator(file)
+	return ok
+}
+
+func generator(file *File) (string, bool) {
+	for _, group := range file.Comments {
+		for _, comment := range group.List {
+			if comment.Pos() > file.Package {
+				break // after package declaration
+			}
+			// opt: check Contains first to avoid unnecessary array allocation in Split.
+			const prefix = "// Code generated "
+			if strings.Contains(comment.Text, prefix) {
+				for _, line := range strings.Split(comment.Text, "\n") {
+					if rest, ok := strings.CutPrefix(line, prefix); ok {
+						if gen, ok := strings.CutSuffix(rest, " DO NOT EDIT."); ok {
+							return gen, true
+						}
+					}
+				}
+			}
+		}
+	}
+	return "", false
+}

src/go/ast/issues_test.go

@@ -40,3 +40,100 @@ func TestIssue33649(t *testing.T) {
 		}
 	}
 }
+
+// TestIssue28089 exercises the IsGenerated function.
+func TestIssue28089(t *testing.T) {
+	for i, test := range []struct {
+		src  string
+		want bool
+	}{
+		// No file comments.
+		{`package p`, false},
+		// Irrelevant file comments.
+		{`// Package p doc.
+package p`, false},
+		// Special comment misplaced after package decl.
+		{`// Package p doc.
+package p
+// Code generated by gen. DO NOT EDIT.
+`, false},
+		// Special comment appears inside string literal.
+		{`// Package p doc.
+package p
+const c = "` + "`" + `
+// Code generated by gen. DO NOT EDIT.
+` + "`" + `
+`, false},
+		// Special comment appears properly.
+		{`// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package p doc comment goes here.
+//
+// Code generated by gen. DO NOT EDIT.
+package p
+
+... `, true},
+		// Special comment is indented.
+		//
+		// Strictly, the indent should cause IsGenerated to
+		// yield false, but we cannot detect the indent
+		// without either source text or a token.File.
+		// In other words, the function signature cannot
+		// implement the spec. Let's brush this under the
+		// rug since well-formatted code has no indent.
+		{`// Package p doc comment goes here.
+//
+	// Code generated by gen. DO NOT EDIT.
+package p
+
+... `, true},
+		// Special comment has unwanted spaces after "DO NOT EDIT."
+		{`// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package p doc comment goes here.
+//
+// Code generated by gen. DO NOT EDIT. 
+package p
+
+... `, false},
+		// Special comment has rogue interior space.
+		{`//     Code generated by gen. DO NOT EDIT.
+package p
+`, false},
+		// Special comment lacks the middle portion.
+		{`// Code generated DO NOT EDIT.
+package p
+`, false},
+		// Special comment (incl. "//") appears within a /* block */ comment,
+		// an obscure corner case of the spec.
+		{`/* start of a general comment
+
+// Code generated by tool; DO NOT EDIT.
+
+end of a general comment */
+
+// +build !dev
+
+// Package comment.
+package p
+
+// Does match even though it's inside general comment (/*-style).
+`, true},
+	} {
+		fset := token.NewFileSet()
+		f, err := parser.ParseFile(fset, "", test.src, parser.PackageClauseOnly|parser.ParseComments)
+		if f == nil {
+			t.Fatalf("parse %d failed to return AST: %v", i, err)
+		}
+
+		got := ast.IsGenerated(f)
+		if got != test.want {
+			t.Errorf("%d: IsGenerated on <<%s>> returned %t", i, test.src, got)
+		}
+	}
+
+}