gopls/internal/work: fix crash during go.work completion

Completing a use directive for a nonexistent directory crashed gopls.
Fix this, and switch the operation to use the more performant
filepath.WalkDir.

Fixes golang/go#64225

Change-Id: I11c463d2a19bfeefce552b6efdcc9c5892a722f9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/550376
Reviewed-by: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: findleyr

gopls/internal/test/integration/completion/completion_test.go

@@ -829,6 +829,7 @@ use ./a/ba
 use ./a/b/
 use ./dir/foo
 use ./dir/foobar/
+use ./missing/
 -- a/go.mod --
 -- go.mod --
 -- a/bar/go.mod --
@@ -853,6 +854,7 @@ use ./dir/foobar/
 			{`use ./a/ba()`, []string{"r"}},
 			{`use ./dir/foo()`, []string{"bar"}},
 			{`use ./dir/foobar/()`, []string{}},
+			{`use ./missing/()`, []string{}},
 		}
 		for _, tt := range tests {
 			completions := env.Completion(env.RegexpSearch("go.work", tt.re))

gopls/internal/work/completion.go

@@ -8,6 +8,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"sort"
@@ -74,7 +75,12 @@ func Completion(ctx context.Context, snapshot *cache.Snapshot, fh file.Handle, p
 	const numSeenBound = 10000
 	var numSeen int
 	stopWalking := errors.New("hit numSeenBound")
-	err = filepath.Walk(pathPrefixDir, func(wpath string, info os.FileInfo, err error) error {
+	err = filepath.WalkDir(pathPrefixDir, func(wpath string, entry fs.DirEntry, err error) error {
+		if err != nil {
+			// golang/go#64225: an error reading a dir is expected, as the user may
+			// be typing out a use directive for a directory that doesn't exist.
+			return nil
+		}
 		if numSeen > numSeenBound {
 			// Stop traversing if we hit bound.
 			return stopWalking
@@ -86,7 +92,7 @@ func Completion(ctx context.Context, snapshot *cache.Snapshot, fh file.Handle, p
 		// otherwise it won't match the beginning of the
 		// base component of the path the user typed in.
 		rel := strings.TrimPrefix(wpath[len(pathPrefixDir):], string(filepath.Separator))
-		if info.IsDir() && wpath != pathPrefixDir && !strings.HasPrefix(rel, pathPrefixBase) {
+		if entry.IsDir() && wpath != pathPrefixDir && !strings.HasPrefix(rel, pathPrefixBase) {
 			return filepath.SkipDir
 		}