Catch nil Mapper funcs

bind.go

@@ -215,12 +215,16 @@ func (ml *maybeLineage) checkLensesOrder() error {
 }
 
 func (ml *maybeLineage) checkGoLensCompleteness() error {
+	// TODO(sdboyer) it'd be nice to consolidate all the errors so that the user always sees a complete set of problems
 	all := make(map[lensID]bool)
 	for _, lens := range ml.implens {
 		id := lid(lens.To, lens.From)
 		if all[id] {
 			return fmt.Errorf("duplicate Go migration %s", id)
 		}
+		if lens.Mapper == nil {
+			return fmt.Errorf("nil Go migration func for %s", id)
+		}
 		all[id] = true
 	}
 

gomig_test.go

@@ -285,7 +285,6 @@ schemas: [{
 			}
 			tex, tname := ex, name
 			t.Run(tname, func(t *testing.T) {
-				t.Log(start.Version(), end.Version())
 				tinst, lacunas, err := tex.Translate(end.Version())
 				require.NoError(t, err)
 				assert.Nil(t, lacunas, "pure go migrations cannot emit lacunas")
@@ -360,6 +359,12 @@ schemas: [{
 			Mapper: func(inst *Instance, to Schema) (*Instance, error) { return nil, nil },
 		})...))
 		assert.Error(t, err, "expected error when providing Go migration for minor version upgrade")
+
+		_, err = BindLineage(linval, rt, ImperativeLenses(append(correctLenses[:1], ImperativeLens{
+			To:   SV(2, 0),
+			From: SV(1, 1),
+		})...))
+		assert.Error(t, err, "expected error when Mapper func is nil")
 	})
 }