cmd/guru/guru.go - tools - Git at Google

 // Copyright 2014 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 main

 // TODO(adonovan): new queries
 // - show all statements that may update the selected lvalue
 //   (local, global, field, etc).
 // - show all places where an object of type T is created
 //   (&T{}, var t T, new(T), new(struct{array [3]T}), etc.

 import (
 	"encoding/json"
 	"fmt"
 	"go/ast"
 	"go/build"
 	"go/parser"
 	"go/token"
 	"go/types"
 	"io"
 	"log"
 	"path/filepath"
 	"strings"

 	"golang.org/x/tools/go/ast/astutil"
 	"golang.org/x/tools/go/buildutil"
 	"golang.org/x/tools/go/loader"
 	"golang.org/x/tools/go/pointer"
 	"golang.org/x/tools/go/ssa"
 )

 type printfFunc func(pos interface{}, format string, args ...interface{})

 // A QueryResult is an item of output.  Each query produces a stream of
 // query results, calling Query.Output for each one.
 type QueryResult interface {
 	// JSON returns the QueryResult in JSON form.
 	JSON(fset *token.FileSet) []byte

 	// PrintPlain prints the QueryResult in plain text form.
 	// The implementation calls printfFunc to print each line of output.
 	PrintPlain(printf printfFunc)
 }

 // A QueryPos represents the position provided as input to a query:
 // a textual extent in the program's source code, the AST node it
 // corresponds to, and the package to which it belongs.
 // Instances are created by parseQueryPos.
 type queryPos struct {
 	fset       *token.FileSet
 	start, end token.Pos           // source extent of query
 	path       []ast.Node          // AST path from query node to root of ast.File
 	exact      bool                // 2nd result of PathEnclosingInterval
 	info       *loader.PackageInfo // type info for the queried package (nil for fastQueryPos)
 }

 // TypeString prints type T relative to the query position.
 func (qpos *queryPos) typeString(T types.Type) string {
 	return types.TypeString(T, types.RelativeTo(qpos.info.Pkg))
 }

 // ObjectString prints object obj relative to the query position.
 func (qpos *queryPos) objectString(obj types.Object) string {
 	return types.ObjectString(obj, types.RelativeTo(qpos.info.Pkg))
 }

 // SelectionString prints selection sel relative to the query position.
 func (qpos *queryPos) selectionString(sel *types.Selection) string {
 	return types.SelectionString(sel, types.RelativeTo(qpos.info.Pkg))
 }

 // A Query specifies a single guru query.
 type Query struct {
 	Pos   string         // query position
 	Build *build.Context // package loading configuration

 	// pointer analysis options
 	Scope      []string  // main packages in (*loader.Config).FromArgs syntax
 	PTALog     io.Writer // (optional) pointer-analysis log file
 	Reflection bool      // model reflection soundly (currently slow).

 	// result-printing function
 	Output func(*token.FileSet, QueryResult)
 }

 // Run runs an guru query and populates its Fset and Result.
 func Run(mode string, q *Query) error {
 	switch mode {
 	case "callees":
 		return callees(q)
 	case "callers":
 		return callers(q)
 	case "callstack":
 		return callstack(q)
 	case "peers":
 		return peers(q)
 	case "pointsto":
 		return pointsto(q)
 	case "whicherrs":
 		return whicherrs(q)
 	case "definition":
 		return definition(q)
 	case "describe":
 		return describe(q)
 	case "freevars":
 		return freevars(q)
 	case "implements":
 		return implements(q)
 	case "referrers":
 		return referrers(q)
 	case "what":
 		return what(q)
 	default:
 		return fmt.Errorf("invalid mode: %q", mode)
 	}
 }

 func setPTAScope(lconf *loader.Config, scope []string) error {
 	pkgs := buildutil.ExpandPatterns(lconf.Build, scope)
 	if len(pkgs) == 0 {
 		return fmt.Errorf("no packages specified for pointer analysis scope")
 	}
 	// The value of each entry in pkgs is true,
 	// giving ImportWithTests (not Import) semantics.
 	lconf.ImportPkgs = pkgs
 	return nil
 }

 // Create a pointer.Config whose scope is the initial packages of lprog
 // and their dependencies.
 func setupPTA(prog *ssa.Program, lprog *loader.Program, ptaLog io.Writer, reflection bool) (*pointer.Config, error) {
 	// For each initial package (specified on the command line),
 	// if it has a main function, analyze that,
 	// otherwise analyze its tests, if any.
 	var mains []*ssa.Package
 	for _, info := range lprog.InitialPackages() {
 		p := prog.Package(info.Pkg)

 		// Add package to the pointer analysis scope.
 		if p.Pkg.Name() == "main" && p.Func("main") != nil {
 			mains = append(mains, p)
 		} else if main := prog.CreateTestMainPackage(p); main != nil {
 			mains = append(mains, main)
 		}
 	}
 	if mains == nil {
 		return nil, fmt.Errorf("analysis scope has no main and no tests")
 	}
 	return &pointer.Config{
 		Log:        ptaLog,
 		Reflection: reflection,
 		Mains:      mains,
 	}, nil
 }

 // importQueryPackage finds the package P containing the
 // query position and tells conf to import it.
 // It returns the package's path.
 func importQueryPackage(pos string, conf *loader.Config) (string, error) {
 	fqpos, err := fastQueryPos(conf.Build, pos)
 	if err != nil {
 		return "", err // bad query
 	}
 	filename := fqpos.fset.File(fqpos.start).Name()

 	_, importPath, err := guessImportPath(filename, conf.Build)
 	if err != nil {
 		// Can't find GOPATH dir.
 		// Treat the query file as its own package.
 		importPath = "command-line-arguments"
 		conf.CreateFromFilenames(importPath, filename)
 	} else {
 		// Check that it's possible to load the queried package.
 		// (e.g. guru tests contain different 'package' decls in same dir.)
 		// Keep consistent with logic in loader/util.go!
 		cfg2 := *conf.Build
 		cfg2.CgoEnabled = false
 		bp, err := cfg2.Import(importPath, "", 0)
 		if err != nil {
 			return "", err // no files for package
 		}

 		switch pkgContainsFile(bp, filename) {
 		case 'T':
 			conf.ImportWithTests(importPath)
 		case 'X':
 			conf.ImportWithTests(importPath)
 			importPath += "_test" // for TypeCheckFuncBodies
 		case 'G':
 			conf.Import(importPath)
 		default:
 			// This happens for ad-hoc packages like
 			// $GOROOT/src/net/http/triv.go.
 			return "", fmt.Errorf("package %q doesn't contain file %s",
 				importPath, filename)
 		}
 	}

 	conf.TypeCheckFuncBodies = func(p string) bool { return p == importPath }

 	return importPath, nil
 }

 // pkgContainsFile reports whether file was among the packages Go
 // files, Test files, eXternal test files, or not found.
 func pkgContainsFile(bp *build.Package, filename string) byte {
 	for i, files := range [][]string{bp.GoFiles, bp.TestGoFiles, bp.XTestGoFiles} {
 		for _, file := range files {
 			if sameFile(filepath.Join(bp.Dir, file), filename) {
 				return "GTX"[i]
 			}
 		}
 	}
 	return 0 // not found
 }

 // ParseQueryPos parses the source query position pos and returns the
 // AST node of the loaded program lprog that it identifies.
 // If needExact, it must identify a single AST subtree;
 // this is appropriate for queries that allow fairly arbitrary syntax,
 // e.g. "describe".
 //
 func parseQueryPos(lprog *loader.Program, pos string, needExact bool) (*queryPos, error) {
 	filename, startOffset, endOffset, err := parsePos(pos)
 	if err != nil {
 		return nil, err
 	}

 	// Find the named file among those in the loaded program.
 	var file *token.File
 	lprog.Fset.Iterate(func(f *token.File) bool {
 		if sameFile(filename, f.Name()) {
 			file = f
 			return false // done
 		}
 		return true // continue
 	})
 	if file == nil {
 		return nil, fmt.Errorf("file %s not found in loaded program", filename)
 	}

 	start, end, err := fileOffsetToPos(file, startOffset, endOffset)
 	if err != nil {
 		return nil, err
 	}
 	info, path, exact := lprog.PathEnclosingInterval(start, end)
 	if path == nil {
 		return nil, fmt.Errorf("no syntax here")
 	}
 	if needExact && !exact {
 		return nil, fmt.Errorf("ambiguous selection within %s", astutil.NodeDescription(path[0]))
 	}
 	return &queryPos{lprog.Fset, start, end, path, exact, info}, nil
 }

 // ---------- Utilities ----------

 // loadWithSoftErrors calls lconf.Load, suppressing "soft" errors.  (See Go issue 16530.)
 // TODO(adonovan): Once the loader has an option to allow soft errors,
 // replace calls to loadWithSoftErrors with loader calls with that parameter.
 func loadWithSoftErrors(lconf *loader.Config) (*loader.Program, error) {
 	lconf.AllowErrors = true

 	// Ideally we would just return conf.Load() here, but go/types
 	// reports certain "soft" errors that gc does not (Go issue 14596).
 	// As a workaround, we set AllowErrors=true and then duplicate
 	// the loader's error checking but allow soft errors.
 	// It would be nice if the loader API permitted "AllowErrors: soft".
 	prog, err := lconf.Load()
 	if err != nil {
 		return nil, err
 	}
 	var errpkgs []string
 	// Report hard errors in indirectly imported packages.
 	for _, info := range prog.AllPackages {
 		if containsHardErrors(info.Errors) {
 			errpkgs = append(errpkgs, info.Pkg.Path())
 		} else {
 			// Enable SSA construction for packages containing only soft errors.
 			info.TransitivelyErrorFree = true
 		}
 	}
 	if errpkgs != nil {
 		var more string
 		if len(errpkgs) > 3 {
 			more = fmt.Sprintf(" and %d more", len(errpkgs)-3)
 			errpkgs = errpkgs[:3]
 		}
 		return nil, fmt.Errorf("couldn't load packages due to errors: %s%s",
 			strings.Join(errpkgs, ", "), more)
 	}
 	return prog, err
 }

 func containsHardErrors(errors []error) bool {
 	for _, err := range errors {
 		if err, ok := err.(types.Error); ok && err.Soft {
 			continue
 		}
 		return true
 	}
 	return false
 }

 // allowErrors causes type errors to be silently ignored.
 // (Not suitable if SSA construction follows.)
 func allowErrors(lconf *loader.Config) {
 	ctxt := *lconf.Build // copy
 	ctxt.CgoEnabled = false
 	lconf.Build = &ctxt
 	lconf.AllowErrors = true
 	// AllErrors makes the parser always return an AST instead of
 	// bailing out after 10 errors and returning an empty ast.File.
 	lconf.ParserMode = parser.AllErrors
 	lconf.TypeChecker.Error = func(err error) {}
 }

 // ptrAnalysis runs the pointer analysis and returns its result.
 func ptrAnalysis(conf *pointer.Config) *pointer.Result {
 	result, err := pointer.Analyze(conf)
 	if err != nil {
 		panic(err) // pointer analysis internal error
 	}
 	return result
 }

 func unparen(e ast.Expr) ast.Expr { return astutil.Unparen(e) }

 // deref returns a pointer's element type; otherwise it returns typ.
 func deref(typ types.Type) types.Type {
 	if p, ok := typ.Underlying().(*types.Pointer); ok {
 		return p.Elem()
 	}
 	return typ
 }

 // fprintf prints to w a message of the form "location: message\n"
 // where location is derived from pos.
 //
 // pos must be one of:
 //    - a token.Pos, denoting a position
 //    - an ast.Node, denoting an interval
 //    - anything with a Pos() method:
 //         ssa.Member, ssa.Value, ssa.Instruction, types.Object, pointer.Label, etc.
 //    - a QueryPos, denoting the extent of the user's query.
 //    - nil, meaning no position at all.
 //
 // The output format is is compatible with the 'gnu'
 // compilation-error-regexp in Emacs' compilation mode.
 //
 func fprintf(w io.Writer, fset *token.FileSet, pos interface{}, format string, args ...interface{}) {
 	var start, end token.Pos
 	switch pos := pos.(type) {
 	case ast.Node:
 		start = pos.Pos()
 		end = pos.End()
 	case token.Pos:
 		start = pos
 		end = start
 	case *types.PkgName:
 		// The Pos of most PkgName objects does not coincide with an identifier,
 		// so we suppress the usual start+len(name) heuristic for types.Objects.
 		start = pos.Pos()
 		end = start
 	case types.Object:
 		start = pos.Pos()
 		end = start + token.Pos(len(pos.Name())) // heuristic
 	case interface {
 		Pos() token.Pos
 	}:
 		start = pos.Pos()
 		end = start
 	case *queryPos:
 		start = pos.start
 		end = pos.end
 	case nil:
 		// no-op
 	default:
 		panic(fmt.Sprintf("invalid pos: %T", pos))
 	}

 	if sp := fset.Position(start); start == end {
 		// (prints "-: " for token.NoPos)
 		fmt.Fprintf(w, "%s: ", sp)
 	} else {
 		ep := fset.Position(end)
 		// The -1 below is a concession to Emacs's broken use of
 		// inclusive (not half-open) intervals.
 		// Other editors may not want it.
 		// TODO(adonovan): add an -editor=vim|emacs|acme|auto
 		// flag; auto uses EMACS=t / VIM=... / etc env vars.
 		fmt.Fprintf(w, "%s:%d.%d-%d.%d: ",
 			sp.Filename, sp.Line, sp.Column, ep.Line, ep.Column-1)
 	}
 	fmt.Fprintf(w, format, args...)
 	io.WriteString(w, "\n")
 }

 func toJSON(x interface{}) []byte {
 	b, err := json.MarshalIndent(x, "", "\t")
 	if err != nil {
 		log.Fatalf("JSON error: %v", err)
 	}
 	return b
 }
	// Copyright 2014 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 main

	// TODO(adonovan): new queries
	// - show all statements that may update the selected lvalue
	// (local, global, field, etc).
	// - show all places where an object of type T is created
	// (&T{}, var t T, new(T), new(struct{array [3]T}), etc.

	import (
	"encoding/json"
	"fmt"
	"go/ast"
	"go/build"
	"go/parser"
	"go/token"
	"go/types"
	"io"
	"log"
	"path/filepath"
	"strings"

	"golang.org/x/tools/go/ast/astutil"
	"golang.org/x/tools/go/buildutil"
	"golang.org/x/tools/go/loader"
	"golang.org/x/tools/go/pointer"
	"golang.org/x/tools/go/ssa"
	)

	type printfFunc func(pos interface{}, format string, args ...interface{})

	// A QueryResult is an item of output. Each query produces a stream of
	// query results, calling Query.Output for each one.
	type QueryResult interface {
	// JSON returns the QueryResult in JSON form.
	JSON(fset *token.FileSet) []byte

	// PrintPlain prints the QueryResult in plain text form.
	// The implementation calls printfFunc to print each line of output.
	PrintPlain(printf printfFunc)
	}

	// A QueryPos represents the position provided as input to a query:
	// a textual extent in the program's source code, the AST node it
	// corresponds to, and the package to which it belongs.
	// Instances are created by parseQueryPos.
	type queryPos struct {
	fset *token.FileSet
	start, end token.Pos // source extent of query
	path []ast.Node // AST path from query node to root of ast.File
	exact bool // 2nd result of PathEnclosingInterval
	info *loader.PackageInfo // type info for the queried package (nil for fastQueryPos)
	}

	// TypeString prints type T relative to the query position.
	func (qpos *queryPos) typeString(T types.Type) string {
	return types.TypeString(T, types.RelativeTo(qpos.info.Pkg))
	}

	// ObjectString prints object obj relative to the query position.
	func (qpos *queryPos) objectString(obj types.Object) string {
	return types.ObjectString(obj, types.RelativeTo(qpos.info.Pkg))
	}

	// SelectionString prints selection sel relative to the query position.
	func (qpos queryPos) selectionString(sel types.Selection) string {
	return types.SelectionString(sel, types.RelativeTo(qpos.info.Pkg))
	}

	// A Query specifies a single guru query.
	type Query struct {
	Pos string // query position
	Build *build.Context // package loading configuration

	// pointer analysis options
	Scope []string // main packages in (*loader.Config).FromArgs syntax
	PTALog io.Writer // (optional) pointer-analysis log file
	Reflection bool // model reflection soundly (currently slow).

	// result-printing function
	Output func(*token.FileSet, QueryResult)
	}

	// Run runs an guru query and populates its Fset and Result.
	func Run(mode string, q *Query) error {
	switch mode {
	case "callees":
	return callees(q)
	case "callers":
	return callers(q)
	case "callstack":
	return callstack(q)
	case "peers":
	return peers(q)
	case "pointsto":
	return pointsto(q)
	case "whicherrs":
	return whicherrs(q)
	case "definition":
	return definition(q)
	case "describe":
	return describe(q)
	case "freevars":
	return freevars(q)
	case "implements":
	return implements(q)
	case "referrers":
	return referrers(q)
	case "what":
	return what(q)
	default:
	return fmt.Errorf("invalid mode: %q", mode)
	}
	}

	func setPTAScope(lconf *loader.Config, scope []string) error {
	pkgs := buildutil.ExpandPatterns(lconf.Build, scope)
	if len(pkgs) == 0 {
	return fmt.Errorf("no packages specified for pointer analysis scope")
	}
	// The value of each entry in pkgs is true,
	// giving ImportWithTests (not Import) semantics.
	lconf.ImportPkgs = pkgs
	return nil
	}

	// Create a pointer.Config whose scope is the initial packages of lprog
	// and their dependencies.
	func setupPTA(prog ssa.Program, lprog loader.Program, ptaLog io.Writer, reflection bool) (*pointer.Config, error) {
	// For each initial package (specified on the command line),
	// if it has a main function, analyze that,
	// otherwise analyze its tests, if any.
	var mains []*ssa.Package
	for _, info := range lprog.InitialPackages() {
	p := prog.Package(info.Pkg)

	// Add package to the pointer analysis scope.
	if p.Pkg.Name() == "main" && p.Func("main") != nil {
	mains = append(mains, p)
	} else if main := prog.CreateTestMainPackage(p); main != nil {
	mains = append(mains, main)
	}
	}
	if mains == nil {
	return nil, fmt.Errorf("analysis scope has no main and no tests")
	}
	return &pointer.Config{
	Log: ptaLog,
	Reflection: reflection,
	Mains: mains,
	}, nil
	}

	// importQueryPackage finds the package P containing the
	// query position and tells conf to import it.
	// It returns the package's path.
	func importQueryPackage(pos string, conf *loader.Config) (string, error) {
	fqpos, err := fastQueryPos(conf.Build, pos)
	if err != nil {
	return "", err // bad query
	}
	filename := fqpos.fset.File(fqpos.start).Name()

	_, importPath, err := guessImportPath(filename, conf.Build)
	if err != nil {
	// Can't find GOPATH dir.
	// Treat the query file as its own package.
	importPath = "command-line-arguments"
	conf.CreateFromFilenames(importPath, filename)
	} else {
	// Check that it's possible to load the queried package.
	// (e.g. guru tests contain different 'package' decls in same dir.)
	// Keep consistent with logic in loader/util.go!
	cfg2 := *conf.Build
	cfg2.CgoEnabled = false
	bp, err := cfg2.Import(importPath, "", 0)
	if err != nil {
	return "", err // no files for package
	}

	switch pkgContainsFile(bp, filename) {
	case 'T':
	conf.ImportWithTests(importPath)
	case 'X':
	conf.ImportWithTests(importPath)
	importPath += "_test" // for TypeCheckFuncBodies
	case 'G':
	conf.Import(importPath)
	default:
	// This happens for ad-hoc packages like
	// $GOROOT/src/net/http/triv.go.
	return "", fmt.Errorf("package %q doesn't contain file %s",
	importPath, filename)
	}
	}

	conf.TypeCheckFuncBodies = func(p string) bool { return p == importPath }

	return importPath, nil
	}

	// pkgContainsFile reports whether file was among the packages Go
	// files, Test files, eXternal test files, or not found.
	func pkgContainsFile(bp *build.Package, filename string) byte {
	for i, files := range [][]string{bp.GoFiles, bp.TestGoFiles, bp.XTestGoFiles} {
	for _, file := range files {
	if sameFile(filepath.Join(bp.Dir, file), filename) {
	return "GTX"[i]
	}
	}
	}
	return 0 // not found
	}

	// ParseQueryPos parses the source query position pos and returns the
	// AST node of the loaded program lprog that it identifies.
	// If needExact, it must identify a single AST subtree;
	// this is appropriate for queries that allow fairly arbitrary syntax,
	// e.g. "describe".
	//
	func parseQueryPos(lprog loader.Program, pos string, needExact bool) (queryPos, error) {
	filename, startOffset, endOffset, err := parsePos(pos)
	if err != nil {
	return nil, err
	}

	// Find the named file among those in the loaded program.
	var file *token.File
	lprog.Fset.Iterate(func(f *token.File) bool {
	if sameFile(filename, f.Name()) {
	file = f
	return false // done
	}
	return true // continue
	})
	if file == nil {
	return nil, fmt.Errorf("file %s not found in loaded program", filename)
	}

	start, end, err := fileOffsetToPos(file, startOffset, endOffset)
	if err != nil {
	return nil, err
	}
	info, path, exact := lprog.PathEnclosingInterval(start, end)
	if path == nil {
	return nil, fmt.Errorf("no syntax here")
	}
	if needExact && !exact {
	return nil, fmt.Errorf("ambiguous selection within %s", astutil.NodeDescription(path[0]))
	}
	return &queryPos{lprog.Fset, start, end, path, exact, info}, nil
	}

	// ---------- Utilities ----------

	// loadWithSoftErrors calls lconf.Load, suppressing "soft" errors. (See Go issue 16530.)
	// TODO(adonovan): Once the loader has an option to allow soft errors,
	// replace calls to loadWithSoftErrors with loader calls with that parameter.
	func loadWithSoftErrors(lconf loader.Config) (loader.Program, error) {
	lconf.AllowErrors = true

	// Ideally we would just return conf.Load() here, but go/types
	// reports certain "soft" errors that gc does not (Go issue 14596).
	// As a workaround, we set AllowErrors=true and then duplicate
	// the loader's error checking but allow soft errors.
	// It would be nice if the loader API permitted "AllowErrors: soft".
	prog, err := lconf.Load()
	if err != nil {
	return nil, err
	}
	var errpkgs []string
	// Report hard errors in indirectly imported packages.
	for _, info := range prog.AllPackages {
	if containsHardErrors(info.Errors) {
	errpkgs = append(errpkgs, info.Pkg.Path())
	} else {
	// Enable SSA construction for packages containing only soft errors.
	info.TransitivelyErrorFree = true
	}
	}
	if errpkgs != nil {
	var more string
	if len(errpkgs) > 3 {
	more = fmt.Sprintf(" and %d more", len(errpkgs)-3)
	errpkgs = errpkgs[:3]
	}
	return nil, fmt.Errorf("couldn't load packages due to errors: %s%s",
	strings.Join(errpkgs, ", "), more)
	}
	return prog, err
	}

	func containsHardErrors(errors []error) bool {
	for _, err := range errors {
	if err, ok := err.(types.Error); ok && err.Soft {
	continue
	}
	return true
	}
	return false
	}

	// allowErrors causes type errors to be silently ignored.
	// (Not suitable if SSA construction follows.)
	func allowErrors(lconf *loader.Config) {
	ctxt := *lconf.Build // copy
	ctxt.CgoEnabled = false
	lconf.Build = &ctxt
	lconf.AllowErrors = true
	// AllErrors makes the parser always return an AST instead of
	// bailing out after 10 errors and returning an empty ast.File.
	lconf.ParserMode = parser.AllErrors
	lconf.TypeChecker.Error = func(err error) {}
	}

	// ptrAnalysis runs the pointer analysis and returns its result.
	func ptrAnalysis(conf pointer.Config) pointer.Result {
	result, err := pointer.Analyze(conf)
	if err != nil {
	panic(err) // pointer analysis internal error
	}
	return result
	}

	func unparen(e ast.Expr) ast.Expr { return astutil.Unparen(e) }

	// deref returns a pointer's element type; otherwise it returns typ.
	func deref(typ types.Type) types.Type {
	if p, ok := typ.Underlying().(*types.Pointer); ok {
	return p.Elem()
	}
	return typ
	}

	// fprintf prints to w a message of the form "location: message\n"
	// where location is derived from pos.
	//
	// pos must be one of:
	// - a token.Pos, denoting a position
	// - an ast.Node, denoting an interval
	// - anything with a Pos() method:
	// ssa.Member, ssa.Value, ssa.Instruction, types.Object, pointer.Label, etc.
	// - a QueryPos, denoting the extent of the user's query.
	// - nil, meaning no position at all.
	//
	// The output format is is compatible with the 'gnu'
	// compilation-error-regexp in Emacs' compilation mode.
	//
	func fprintf(w io.Writer, fset *token.FileSet, pos interface{}, format string, args ...interface{}) {
	var start, end token.Pos
	switch pos := pos.(type) {
	case ast.Node:
	start = pos.Pos()
	end = pos.End()
	case token.Pos:
	start = pos
	end = start
	case *types.PkgName:
	// The Pos of most PkgName objects does not coincide with an identifier,
	// so we suppress the usual start+len(name) heuristic for types.Objects.
	start = pos.Pos()
	end = start
	case types.Object:
	start = pos.Pos()
	end = start + token.Pos(len(pos.Name())) // heuristic
	case interface {
	Pos() token.Pos
	}:
	start = pos.Pos()
	end = start
	case *queryPos:
	start = pos.start
	end = pos.end
	case nil:
	// no-op
	default:
	panic(fmt.Sprintf("invalid pos: %T", pos))
	}

	if sp := fset.Position(start); start == end {
	// (prints "-: " for token.NoPos)
	fmt.Fprintf(w, "%s: ", sp)
	} else {
	ep := fset.Position(end)
	// The -1 below is a concession to Emacs's broken use of
	// inclusive (not half-open) intervals.
	// Other editors may not want it.
	// TODO(adonovan): add an -editor=vim\|emacs\|acme\|auto
	// flag; auto uses EMACS=t / VIM=... / etc env vars.
	fmt.Fprintf(w, "%s:%d.%d-%d.%d: ",
	sp.Filename, sp.Line, sp.Column, ep.Line, ep.Column-1)
	}
	fmt.Fprintf(w, format, args...)
	io.WriteString(w, "\n")
	}

	func toJSON(x interface{}) []byte {
	b, err := json.MarshalIndent(x, "", "\t")
	if err != nil {
	log.Fatalf("JSON error: %v", err)
	}
	return b
	}