godoc: show comments in various filtered views

Fixes #3454.

R=rsc
CC=golang-dev
https://golang.org/cl/6305069

src/cmd/godoc/godoc.go

@@ -866,6 +866,19 @@ func inList(name string, list []string) bool {
 	return false
 }
 
+// packageExports is a local implementation of ast.PackageExports
+// which correctly updates each package file's comment list.
+// (The ast.PackageExports signature is frozen, hence the local
+// implementation).
+//
+func packageExports(fset *token.FileSet, pkg *ast.Package) {
+	for _, src := range pkg.Files {
+		cmap := ast.NewCommentMap(fset, src)
+		ast.FileExports(src)
+		src.Comments = cmap.Filter(src).Comments()
+	}
+}
+
 // getPageInfo returns the PageInfo for a package directory abspath. If the
 // parameter genAST is set, an AST containing only the package exports is
 // computed (PageInfo.PAst), otherwise package documentation (PageInfo.Doc)
@@ -1012,9 +1025,9 @@ func (h *docServer) getPageInfo(abspath, relpath, pkgname string, mode PageInfoM
 			// TODO(gri) Consider eliminating export filtering in this mode,
 			//           or perhaps eliminating the mode altogether.
 			if mode&noFiltering == 0 {
-				ast.PackageExports(pkg)
+				packageExports(fset, pkg)
 			}
-			past = ast.MergePackageFiles(pkg, ast.FilterUnassociatedComments)
+			past = ast.MergePackageFiles(pkg, 0)
 		}
 	}
 

src/cmd/godoc/main.go

@@ -36,6 +36,7 @@ import (
 	"fmt"
 	"go/ast"
 	"go/build"
+	"go/printer"
 	"io"
 	"log"
 	"net/http"
@@ -424,20 +425,24 @@ func main() {
 		filter := func(s string) bool { return rx.MatchString(s) }
 		switch {
 		case info.PAst != nil:
+			cmap := ast.NewCommentMap(info.FSet, info.PAst)
 			ast.FilterFile(info.PAst, filter)
 			// Special case: Don't use templates for printing
 			// so we only get the filtered declarations without
 			// package clause or extra whitespace.
 			for i, d := range info.PAst.Decls {
+				// determine the comments associated with d only
+				comments := cmap.Filter(d).Comments()
+				cn := &printer.CommentedNode{Node: d, Comments: comments}
 				if i > 0 {
 					fmt.Println()
 				}
 				if *html {
 					var buf bytes.Buffer
-					writeNode(&buf, info.FSet, d)
+					writeNode(&buf, info.FSet, cn)
 					FormatText(os.Stdout, buf.Bytes(), -1, true, "", nil)
 				} else {
-					writeNode(os.Stdout, info.FSet, d)
+					writeNode(os.Stdout, info.FSet, cn)
 				}
 				fmt.Println()
 			}