1 files changed, 216 insertions, 0 deletions
diff --git a/go/analysis/passes/directive/directive.go b/go/analysis/passes/directive/directive.go
new file mode 100644
index 000000000..76d852cd0
--- /dev/null
+++ b/go/analysis/passes/directive/directive.go
@@ -0,0 +1,216 @@
+// Copyright 2023 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 directive defines an Analyzer that checks known Go toolchain directives.
+package directive
+
+import (
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+
+	"golang.org/x/tools/go/analysis"
+	"golang.org/x/tools/go/analysis/passes/internal/analysisutil"
+)
+
+const Doc = `check Go toolchain directives such as //go:debug
+
+This analyzer checks for problems with known Go toolchain directives
+in all Go source files in a package directory, even those excluded by
+//go:build constraints, and all non-Go source files too.
+
+For //go:debug (see https://go.dev/doc/godebug), the analyzer checks
+that the directives are placed only in Go source files, only above the
+package comment, and only in package main or *_test.go files.
+
+Support for other known directives may be added in the future.
+
+This analyzer does not check //go:build, which is handled by the
+buildtag analyzer.
+`
+
+var Analyzer = &analysis.Analyzer{
+	Name: "directive",
+	Doc:  Doc,
+	Run:  runDirective,
+}
+
+func runDirective(pass *analysis.Pass) (interface{}, error) {
+	for _, f := range pass.Files {
+		checkGoFile(pass, f)
+	}
+	for _, name := range pass.OtherFiles {
+		if err := checkOtherFile(pass, name); err != nil {
+			return nil, err
+		}
+	}
+	for _, name := range pass.IgnoredFiles {
+		if strings.HasSuffix(name, ".go") {
+			f, err := parser.ParseFile(pass.Fset, name, nil, parser.ParseComments)
+			if err != nil {
+				// Not valid Go source code - not our job to diagnose, so ignore.
+				continue
+			}
+			checkGoFile(pass, f)
+		} else {
+			if err := checkOtherFile(pass, name); err != nil {
+				return nil, err
+			}
+		}
+	}
+	return nil, nil
+}
+
+func checkGoFile(pass *analysis.Pass, f *ast.File) {
+	check := newChecker(pass, pass.Fset.File(f.Package).Name(), f)
+
+	for _, group := range f.Comments {
+		// A +build comment is ignored after or adjoining the package declaration.
+		if group.End()+1 >= f.Package {
+			check.inHeader = false
+		}
+		// A //go:build comment is ignored after the package declaration
+		// (but adjoining it is OK, in contrast to +build comments).
+		if group.Pos() >= f.Package {
+			check.inHeader = false
+		}
+
+		// Check each line of a //-comment.
+		for _, c := range group.List {
+			check.comment(c.Slash, c.Text)
+		}
+	}
+}
+
+func checkOtherFile(pass *analysis.Pass, filename string) error {
+	// We cannot use the Go parser, since is not a Go source file.
+	// Read the raw bytes instead.
+	content, tf, err := analysisutil.ReadFile(pass.Fset, filename)
+	if err != nil {
+		return err
+	}
+
+	check := newChecker(pass, filename, nil)
+	check.nonGoFile(token.Pos(tf.Base()), string(content))
+	return nil
+}
+
+type checker struct {
+	pass     *analysis.Pass
+	filename string
+	file     *ast.File // nil for non-Go file
+	inHeader bool      // in file header (before package declaration)
+	inStar   bool      // currently in a /* */ comment
+}
+
+func newChecker(pass *analysis.Pass, filename string, file *ast.File) *checker {
+	return &checker{
+		pass:     pass,
+		filename: filename,
+		file:     file,
+		inHeader: true,
+	}
+}
+
+func (check *checker) nonGoFile(pos token.Pos, fullText string) {
+	// Process each line.
+	text := fullText
+	inStar := false
+	for text != "" {
+		offset := len(fullText) - len(text)
+		var line string
+		line, text, _ = stringsCut(text, "\n")
+
+		if !inStar && strings.HasPrefix(line, "//") {
+			check.comment(pos+token.Pos(offset), line)
+			continue
+		}
+
+		// Skip over, cut out any /* */ comments,
+		// to avoid being confused by a commented-out // comment.
+		for {
+			line = strings.TrimSpace(line)
+			if inStar {
+				var ok bool
+				_, line, ok = stringsCut(line, "*/")
+				if !ok {
+					break
+				}
+				inStar = false
+				continue
+			}
+			line, inStar = stringsCutPrefix(line, "/*")
+			if !inStar {
+				break
+			}
+		}
+		if line != "" {
+			// Found non-comment non-blank line.
+			// Ends space for valid //go:build comments,
+			// but also ends the fraction of the file we can
+			// reliably parse. From this point on we might
+			// incorrectly flag "comments" inside multiline
+			// string constants or anything else (this might
+			// not even be a Go program). So stop.
+			break
+		}
+	}
+}
+
+func (check *checker) comment(pos token.Pos, line string) {
+	if !strings.HasPrefix(line, "//go:") {
+		return
+	}
+	// testing hack: stop at // ERROR
+	if i := strings.Index(line, " // ERROR "); i >= 0 {
+		line = line[:i]
+	}
+
+	verb := line
+	if i := strings.IndexFunc(verb, unicode.IsSpace); i >= 0 {
+		verb = verb[:i]
+		if line[i] != ' ' && line[i] != '\t' && line[i] != '\n' {
+			r, _ := utf8.DecodeRuneInString(line[i:])
+			check.pass.Reportf(pos, "invalid space %#q in %s directive", r, verb)
+		}
+	}
+
+	switch verb {
+	default:
+		// TODO: Use the go language version for the file.
+		// If that version is not newer than us, then we can
+		// report unknown directives.
+
+	case "//go:build":
+		// Ignore. The buildtag analyzer reports misplaced comments.
+
+	case "//go:debug":
+		if check.file == nil {
+			check.pass.Reportf(pos, "//go:debug directive only valid in Go source files")
+		} else if check.file.Name.Name != "main" && !strings.HasSuffix(check.filename, "_test.go") {
+			check.pass.Reportf(pos, "//go:debug directive only valid in package main or test")
+		} else if !check.inHeader {
+			check.pass.Reportf(pos, "//go:debug directive only valid before package declaration")
+		}
+	}
+}
+
+// Go 1.18 strings.Cut.
+func stringsCut(s, sep string) (before, after string, found bool) {
+	if i := strings.Index(s, sep); i >= 0 {
+		return s[:i], s[i+len(sep):], true
+	}
+	return s, "", false
+}
+
+// Go 1.20 strings.CutPrefix.
+func stringsCutPrefix(s, prefix string) (after string, found bool) {
+	if !strings.HasPrefix(s, prefix) {
+		return s, false
+	}
+	return s[len(prefix):], true
+}