go/ast, go/printer: recognize export and extern line directives

Now that gofmt is reformatting these, we can't get away with
not knowing about directives such as //export and //extern (for gccgo).
Otherwise "//export foo" and "//extern foo" turn into "// export foo",
and "// extern foo", which are completely different meanings.

For golang#51082.

Change-Id: Id0970331fa0b52ab5fa621631b5fa460767068bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/399734
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Russ Cox <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

src/go/ast/ast.go

@@ -161,8 +161,10 @@ func (g *CommentGroup) Text() string {
 // This code is also in go/printer.
 func isDirective(c string) bool {
 	// "//line " is a line directive.
+	// "//extern " is for gccgo.
+	// "//export " is for cgo.
 	// (The // has been removed.)
-	if strings.HasPrefix(c, "line ") {
+	if strings.HasPrefix(c, "line ") || strings.HasPrefix(c, "extern ") || strings.HasPrefix(c, "export ") {
 		return true
 	}
 

src/go/ast/ast_test.go

@@ -68,6 +68,9 @@ var isDirectiveTests = []struct {
 	{"go:", false},
 	{"go:*", false},
 	{"go:x*", true},
+	{"export foo", true},
+	{"extern foo", true},
+	{"expert foo", false},
 }
 
 func TestIsDirective(t *testing.T) {

src/go/printer/comment.go

@@ -111,8 +111,10 @@ func formatDocComment(list []*ast.Comment) []*ast.Comment {
 // This code is also in go/ast.
 func isDirective(c string) bool {
 	// "//line " is a line directive.
+	// "//extern " is for gccgo.
+	// "//export " is for cgo.
 	// (The // has been removed.)
-	if strings.HasPrefix(c, "line ") {
+	if strings.HasPrefix(c, "line ") || strings.HasPrefix(c, "extern ") || strings.HasPrefix(c, "export ") {
 		return true
 	}
 

src/go/printer/testdata/comments.golden

@@ -692,6 +692,12 @@ func _() {
 	}
 }
 
+//extern foo
+func foo()	{}
+
+//export bar
+func bar()	{}
+
 // Print line directives correctly.
 
 // The following is a legal line directive.

src/go/printer/testdata/comments.input

@@ -691,6 +691,12 @@ func _() {
 	}
 }
 
+//extern foo
+func foo() {}
+
+//export bar
+func bar() {}
+
 // Print line directives correctly.
 
 // The following is a legal line directive.