go/types, types2: dump position stack for non-bailout panics

We make sure to dump to stderr since that's where the panic information
ends up. Long traces get truncated with a "..." in the middle. We pick
an arbitrary limit of 10 positions, but this could be changed.

For #51603

Change-Id: I02326a93181e94e1c48afc05684240540c2c90ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/676815
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Mark Freeman <mark@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/types2/check.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"go/constant"
 	. "internal/types/errors"
+	"os"
 	"sync/atomic"
 )
 
@@ -419,7 +420,24 @@ func (check *Checker) handleBailout(err *error) {
 		// normal return or early exit
 		*err = check.firstErr
 	default:
-		// TODO(markfreeman): dump posStack if available
+		if len(check.posStack) > 0 {
+			doPrint := func(ps []syntax.Pos) {
+				for i := len(ps) - 1; i >= 0; i-- {
+					fmt.Fprintf(os.Stderr, "\t%v\n", ps[i])
+				}
+			}
+
+			fmt.Fprintln(os.Stderr, "The following panic happened checking types near:")
+			if len(check.posStack) <= 10 {
+				doPrint(check.posStack)
+			} else {
+				// if it's long, truncate the middle; it's least likely to help
+				doPrint(check.posStack[len(check.posStack)-5:])
+				fmt.Fprintln(os.Stderr, "\t...")
+				doPrint(check.posStack[:5])
+			}
+		}
+
 		// re-panic
 		panic(p)
 	}

src/go/types/check.go

@@ -13,6 +13,7 @@ import (
 	"go/token"
 	"internal/godebug"
 	. "internal/types/errors"
+	"os"
 	"sync/atomic"
 )
 
@@ -444,7 +445,24 @@ func (check *Checker) handleBailout(err *error) {
 		// normal return or early exit
 		*err = check.firstErr
 	default:
-		// TODO(markfreeman): dump posStack if available
+		if len(check.posStack) > 0 {
+			doPrint := func(ps []positioner) {
+				for i := len(ps) - 1; i >= 0; i-- {
+					fmt.Fprintf(os.Stderr, "\t%v\n", check.fset.Position(ps[i].Pos()))
+				}
+			}
+
+			fmt.Fprintln(os.Stderr, "The following panic happened checking types near:")
+			if len(check.posStack) <= 10 {
+				doPrint(check.posStack)
+			} else {
+				// if it's long, truncate the middle; it's least likely to help
+				doPrint(check.posStack[len(check.posStack)-5:])
+				fmt.Fprintln(os.Stderr, "\t...")
+				doPrint(check.posStack[:5])
+			}
+		}
+
 		// re-panic
 		panic(p)
 	}