gopls/internal/crash: don't crash in xrefs on out of bound nodes

As we've seen many times, it is currently (unfortunately) possible that
go/ast nodes exceed the bounds of the parsed file. Handle this
possibility correctly while building the xrefs index.

Updates golang/go#66683
Fixes golang/go#70446

Change-Id: If6364876eb7b8ed8ca11a058417aa028d6b55b41
Reviewed-on: https://go-review.googlesource.com/c/tools/+/630675
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

Author: findleyr

gopls/internal/cache/xrefs/xrefs.go

@@ -17,6 +17,7 @@ import (
 	"golang.org/x/tools/gopls/internal/cache/metadata"
 	"golang.org/x/tools/gopls/internal/cache/parsego"
 	"golang.org/x/tools/gopls/internal/protocol"
+	"golang.org/x/tools/gopls/internal/util/bug"
 	"golang.org/x/tools/gopls/internal/util/frob"
 )
 
@@ -43,15 +44,6 @@ func Index(files []*parsego.File, pkg *types.Package, info *types.Info) []byte {
 	objectpathFor := new(objectpath.Encoder).For
 
 	for fileIndex, pgf := range files {
-
-		nodeRange := func(n ast.Node) protocol.Range {
-			rng, err := pgf.PosRange(n.Pos(), n.End())
-			if err != nil {
-				panic(err) // can't fail
-			}
-			return rng
-		}
-
 		ast.Inspect(pgf.File, func(n ast.Node) bool {
 			switch n := n.(type) {
 			case *ast.Ident:
@@ -82,10 +74,15 @@ func Index(files []*parsego.File, pkg *types.Package, info *types.Info) []byte {
 							objects[obj] = gobObj
 						}
 
-						gobObj.Refs = append(gobObj.Refs, gobRef{
-							FileIndex: fileIndex,
-							Range:     nodeRange(n),
-						})
+						// golang/go#66683: nodes can under/overflow the file.
+						// For example, "var _ = x." creates a SelectorExpr(Sel=Ident("_"))
+						// that is beyond EOF. (Arguably Ident.Name should be "".)
+						if rng, err := pgf.NodeRange(n); err == nil {
+							gobObj.Refs = append(gobObj.Refs, gobRef{
+								FileIndex: fileIndex,
+								Range:     rng,
+							})
+						}
 					}
 				}
 
@@ -102,10 +99,15 @@ func Index(files []*parsego.File, pkg *types.Package, info *types.Info) []byte {
 					gobObj = &gobObject{Path: ""}
 					objects[nil] = gobObj
 				}
-				gobObj.Refs = append(gobObj.Refs, gobRef{
-					FileIndex: fileIndex,
-					Range:     nodeRange(n.Path),
-				})
+				// golang/go#66683: nodes can under/overflow the file.
+				if rng, err := pgf.NodeRange(n.Path); err == nil {
+					gobObj.Refs = append(gobObj.Refs, gobRef{
+						FileIndex: fileIndex,
+						Range:     rng,
+					})
+				} else {
+					bug.Reportf("out of bounds import spec %+v", n.Path)
+				}
 			}
 			return true
 		})