go/ast: drop //directive comments from doc.Text

This allows writing

	// F does a thing.
	//go:noinline
	func F()

without the //go:noinline or other directive (such as //line)
ending up looking like extra words in the doc comment.

Fixes #37974.

Change-Id: Ic738d72802cc2fa448f7633915e7126d2f76d8ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/224737
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/ast/ast.go

@@ -87,10 +87,10 @@ func stripTrailingWhitespace(s string) string {
 
 // Text returns the text of the comment.
 // Comment markers (//, /*, and */), the first space of a line comment, and
-// leading and trailing empty lines are removed. Multiple empty lines are
-// reduced to one, and trailing space on lines is trimmed. Unless the result
-// is empty, it is newline-terminated.
-//
+// leading and trailing empty lines are removed.
+// Comment directives like "//line" and "//go:noinline" are also removed.
+// Multiple empty lines are reduced to one, and trailing space on lines is trimmed.
+// Unless the result is empty, it is newline-terminated.
 func (g *CommentGroup) Text() string {
 	if g == nil {
 		return ""
@@ -108,9 +108,18 @@ func (g *CommentGroup) Text() string {
 		case '/':
 			//-style comment (no newline at the end)
 			c = c[2:]
-			// strip first space - required for Example tests
-			if len(c) > 0 && c[0] == ' ' {
+			if len(c) == 0 {
+				// empty line
+				break
+			}
+			if c[0] == ' ' {
+				// strip first space - required for Example tests
 				c = c[1:]
+				break
+			}
+			if isDirective(c) {
+				// Ignore //go:noinline, //line, and so on.
+				continue
 			}
 		case '*':
 			/*-style comment */
@@ -145,6 +154,32 @@ func (g *CommentGroup) Text() string {
 	return strings.Join(lines, "\n")
 }
 
+// isDirective reports whether c is a comment directive.
+func isDirective(c string) bool {
+	// "//line " is a line directive.
+	// (The // has been removed.)
+	if strings.HasPrefix(c, "line ") {
+		return true
+	}
+
+	// "//[a-z0-9]+:[a-z0-9]"
+	// (The // has been removed.)
+	colon := strings.Index(c, ":")
+	if colon <= 0 || colon+1 >= len(c) {
+		return false
+	}
+	for i := 0; i <= colon+1; i++ {
+		if i == colon {
+			continue
+		}
+		b := c[i]
+		if !('a' <= b && b <= 'z' || '0' <= b && b <= '9') {
+			return false
+		}
+	}
+	return true
+}
+
 // ----------------------------------------------------------------------------
 // Expressions and types
 

src/go/ast/ast_test.go

@@ -33,6 +33,9 @@ var comments = []struct {
 	{[]string{"/* Foo*/", "/*\n*/", "//", "/*\n*/", "// Bar"}, " Foo\n\nBar\n"},
 	{[]string{"/* Foo*/", "// Bar"}, " Foo\nBar\n"},
 	{[]string{"/* Foo\n Bar*/"}, " Foo\n Bar\n"},
+
+	{[]string{"// foo", "//go:noinline", "// bar", "//:baz"}, "foo\nbar\n:baz\n"},
+	{[]string{"// foo", "//lint123:ignore", "// bar"}, "foo\nbar\n"},
 }
 
 func TestCommentText(t *testing.T) {
@@ -48,3 +51,29 @@ func TestCommentText(t *testing.T) {
 		}
 	}
 }
+
+var isDirectiveTests = []struct {
+	in string
+	ok bool
+}{
+	{"abc", false},
+	{"go:inline", true},
+	{"Go:inline", false},
+	{"go:Inline", false},
+	{":inline", false},
+	{"lint:ignore", true},
+	{"lint:1234", true},
+	{"1234:lint", true},
+	{"go: inline", false},
+	{"go:", false},
+	{"go:*", false},
+	{"go:x*", true},
+}
+
+func TestIsDirective(t *testing.T) {
+	for _, tt := range isDirectiveTests {
+		if ok := isDirective(tt.in); ok != tt.ok {
+			t.Errorf("isDirective(%q) = %v, want %v", tt.in, ok, tt.ok)
+		}
+	}
+}