internal/lsp: remove the TODO and add a test for the hover panic

Fixes golang/go#48249

Change-Id: I86da0f185f414848bf89243737668f1d427c3e4c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/348969
Trust: Rebecca Stambler <rstambler@golang.org>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>

gopls/internal/regtest/misc/hover_test.go

@@ -99,3 +99,17 @@ func main() {
 		}
 	})
 }
+
+// Tests that hovering does not trigger the panic in golang/go#48249.
+func TestPanicInHoverBrokenCode(t *testing.T) {
+	testenv.NeedsGo1Point(t, 13)
+	const source = `
+-- main.go --
+package main
+
+type Example struct`
+	Run(t, source, func(t *testing.T, env *Env) {
+		env.OpenFile("main.go")
+		env.Editor.Hover(env.Ctx, "main.go", env.RegexpSearch("main.go", "Example"))
+	})
+}

internal/lsp/source/hover.go

@@ -342,7 +342,6 @@ func HoverInfo(ctx context.Context, s Snapshot, pkg Package, obj types.Object, p
 			var spec ast.Spec
 			for _, s := range node.Specs {
 				// Avoid panics by guarding the calls to token.Offset (golang/go#48249).
-				// TODO(rstambler): Investigate this further and adjust if needed.
 				if InRange(tok2, s.Pos()) && InRange(tok2, s.End()) && tok2.Offset(s.Pos()) <= offset && offset <= tok2.Offset(s.End()) {
 					spec = s
 					break