gopls/internal/golang: avoid crash in hover on field of non-struct

Avoid a crash where types.Info.TyepOf(n) is assumed to be a non-nil
Struct for a StructType expr: this is not guaranteed in the presence of
type errors.

Unfortunately, I was not able to reproduce the crash after ~20m of
trying. Nevertheless, the existing code is making invalid assumptions.

Fixes golang/go#69150

Change-Id: I48abe9c134722db5c43f1c7c382b59e7632e367f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/627135
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: findleyr

gopls/internal/golang/hover.go

@@ -1627,7 +1627,12 @@ func computeSizeOffsetInfo(pkg *cache.Package, path []ast.Node, obj types.Object
 		var tStruct *types.Struct
 		for _, n := range path {
 			if n, ok := n.(*ast.StructType); ok {
-				tStruct = pkg.TypesInfo().TypeOf(n).(*types.Struct)
+				t, ok := pkg.TypesInfo().TypeOf(n).(*types.Struct)
+				if ok {
+					// golang/go#69150: TypeOf(n) was observed not to be a Struct (likely
+					// nil) in some cases.
+					tStruct = t
+				}
 				break
 			}
 		}