go/doc: fix identifier blank import handling for examples

Replacement CL for 6813061; thanks to minux for prototyping.

Fixes #4300.

R=minux.ma
CC=golang-dev
https://golang.org/cl/6782082

src/cmd/godoc/godoc.go

@@ -356,6 +356,7 @@ func example_htmlFunc(funcName string, examples []*doc.Example, fset *token.File
 		// (use tabs, no comment highlight, etc).
 		play := ""
 		if eg.Play != nil && *showPlayground {
+			ast.SortImports(fset, eg.Play)
 			var buf bytes.Buffer
 			err := (&printer.Config{Mode: printer.TabIndent, Tabwidth: 8}).Fprint(&buf, fset, eg.Play)
 			if err != nil {

src/pkg/go/doc/example.go

@@ -145,8 +145,9 @@ func playExample(file *ast.File, body *ast.BlockStmt) *ast.File {
 
 	// Use unresolved identifiers to determine the imports used by this
 	// example. The heuristic assumes package names match base import
-	// paths. (Should be good enough most of the time.)
-	imports := make(map[string]string) // [name]path
+	// paths for imports w/o renames (should be good enough most of the time).
+	namedImports := make(map[string]string) // [name]path
+	var blankImports []ast.Spec             // _ imports
 	for _, s := range file.Imports {
 		p, err := strconv.Unquote(s.Path.Value)
 		if err != nil {
@@ -154,14 +155,18 @@ func playExample(file *ast.File, body *ast.BlockStmt) *ast.File {
 		}
 		n := path.Base(p)
 		if s.Name != nil {
-			if s.Name.Name == "." {
+			n = s.Name.Name
+			switch n {
+			case "_":
+				blankImports = append(blankImports, s)
+				continue
+			case ".":
 				// We can't resolve dot imports (yet).
 				return nil
 			}
-			n = s.Name.Name
 		}
 		if unresolved[n] {
-			imports[n] = p
+			namedImports[n] = p
 			delete(unresolved, n)
 		}
 	}
@@ -172,13 +177,19 @@ func playExample(file *ast.File, body *ast.BlockStmt) *ast.File {
 		return nil
 	}
 
-	// Filter out comments that are outside the function body.
+	// Include documentation belonging to blank imports.
 	var comments []*ast.CommentGroup
+	for _, s := range blankImports {
+		if c := s.(*ast.ImportSpec).Doc; c != nil {
+			comments = append(comments, c)
+		}
+	}
+
+	// Include comments that are inside the function body.
 	for _, c := range file.Comments {
-		if c.Pos() < body.Pos() || c.Pos() >= body.End() {
-			continue
+		if body.Pos() <= c.Pos() && c.End() <= body.End() {
+			comments = append(comments, c)
 		}
-		comments = append(comments, c)
 	}
 
 	// Strip "Output:" commment and adjust body end position.
@@ -190,13 +201,14 @@ func playExample(file *ast.File, body *ast.BlockStmt) *ast.File {
 		Lparen: 1, // Need non-zero Lparen and Rparen so that printer
 		Rparen: 1, // treats this as a factored import.
 	}
-	for n, p := range imports {
+	for n, p := range namedImports {
 		s := &ast.ImportSpec{Path: &ast.BasicLit{Value: strconv.Quote(p)}}
 		if path.Base(p) != n {
 			s.Name = ast.NewIdent(n)
 		}
 		importDecl.Specs = append(importDecl.Specs, s)
 	}
+	importDecl.Specs = append(importDecl.Specs, blankImports...)
 
 	// Synthesize main function.
 	funcDecl := &ast.FuncDecl{
@@ -213,7 +225,7 @@ func playExample(file *ast.File, body *ast.BlockStmt) *ast.File {
 	}
 }
 
-// playExample takes a whole file example and synthesizes a new *ast.File
+// playExampleFile takes a whole file example and synthesizes a new *ast.File
 // such that the example is function main in package main.
 func playExampleFile(file *ast.File) *ast.File {
 	// Strip copyright comment if present.