cmd/guru: make fastQueryPos indirect through the -modified map

This fixes a bug in which guru reports a "query position beyond end of
file" error when making a query from an unsaved editor buffer at an
offset greater than the length of the file on disk.

Change-Id: I908c024d8dd14baa657b4227e3231fa760443732
Reviewed-on: https://go-review.googlesource.com/20167
Reviewed-by: Daniel Morsing <daniel.morsing@gmail.com>

cmd/guru/definition.go

@@ -20,7 +20,7 @@ func definition(q *Query) error {
 	// (Extending this approach to all the files of the package,
 	// resolved using ast.NewPackage, was not worth the effort.)
 	{
-		qpos, err := fastQueryPos(q.Pos)
+		qpos, err := fastQueryPos(q.Build, q.Pos)
 		if err != nil {
 			return err
 		}

cmd/guru/guru.go

@@ -179,7 +179,7 @@ func setupPTA(prog *ssa.Program, lprog *loader.Program, ptaLog io.Writer, reflec
 // 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(pos)
+	fqpos, err := fastQueryPos(conf.Build, pos)
 	if err != nil {
 		return "", err // bad query
 	}

cmd/guru/pos.go

@@ -8,6 +8,7 @@ package main
 
 import (
 	"fmt"
+	"go/build"
 	"go/parser"
 	"go/token"
 	"os"
@@ -16,6 +17,7 @@ import (
 	"strings"
 
 	"golang.org/x/tools/go/ast/astutil"
+	"golang.org/x/tools/go/buildutil"
 )
 
 // parseOctothorpDecimal returns the numeric value if s matches "#%d",
@@ -106,14 +108,17 @@ func sameFile(x, y string) bool {
 
 // fastQueryPos parses the position string and returns a queryPos.
 // It parses only a single file and does not run the type checker.
-func fastQueryPos(pos string) (*queryPos, error) {
+func fastQueryPos(ctxt *build.Context, pos string) (*queryPos, error) {
 	filename, startOffset, endOffset, err := parsePos(pos)
 	if err != nil {
 		return nil, err
 	}
 
+	// Parse the file, opening it the file via the build.Context
+	// so that we observe the effects of the -modified flag.
 	fset := token.NewFileSet()
-	f, err := parser.ParseFile(fset, filename, nil, 0)
+	cwd, _ := os.Getwd()
+	f, err := buildutil.ParseFile(fset, ctxt, nil, cwd, filename, parser.Mode(0))
 	// ParseFile usually returns a partial file along with an error.
 	// Only fail if there is no file.
 	if f == nil {

cmd/guru/what.go

@@ -25,7 +25,7 @@ import (
 // the selected location.
 //
 func what(q *Query) error {
-	qpos, err := fastQueryPos(q.Pos)
+	qpos, err := fastQueryPos(q.Build, q.Pos)
 	if err != nil {
 		return err
 	}