Return error rather than panic

errors.go

@@ -2,6 +2,7 @@ package ipld
 
 import (
 	"fmt"
+	"strings"
 )
 
 // ErrWrongKind may be returned from functions on the Node interface when
@@ -152,6 +153,15 @@ func (e ErrIteratorOverread) Error() string {
 
 type ErrCannotBeNull struct{} // Review: arguably either ErrInvalidKindForNodePrototype.
 
-type ErrMissingRequiredField struct{}     // only possible for typed nodes -- specifically, struct types.
+// ErrMissingRequiredField is returned when calling 'Finish' on a NodeAssembler
+// for a Struct that has not has all required fields set.
+type ErrMissingRequiredField struct {
+	Missing []string
+}
+
+func (e ErrMissingRequiredField) Error() string {
+	return "missing required fields: " + strings.Join(e.Missing, ",")
+}
+
 type ErrListOverrun struct{}              // only possible for typed nodes -- specifically, struct types with list (aka tuple) representations.
 type ErrInvalidUnionDiscriminant struct{} // only possible for typed nodes -- specifically, union types.

schema/gen/go/genStruct.go

@@ -508,7 +508,15 @@ func (g structBuilderGenerator) emitMapAssemblerMethods(w io.Writer) {
 				panic("invalid state: Finish cannot be called on an assembler that's already finished")
 			}
 			if ma.s & fieldBits__{{ $type | TypeSymbol }}_sufficient != fieldBits__{{ $type | TypeSymbol }}_sufficient {
-				panic("invalid state: Not all required fields set")
+				err := ipld.ErrMissingRequiredField{Missing: make([]string, 0)}
+				{{- range $i, $field := .Type.Fields }}
+				{{- if not $field.IsMaybe}}
+				if ma.s & fieldBit__{{ $type | TypeSymbol }}_{{ $field | FieldSymbolUpper }} == 0 {
+					err.Missing = append(err.Missing, "{{ $field.Name }}")
+				}
+				{{- end}}
+				{{- end}}
+				return err
 			}
 			ma.state = maState_finished
 			*ma.m = schema.Maybe_Value

schema/gen/go/testStructsContainingMaybe_test.go

@@ -187,15 +187,10 @@ func TestStructsContainingMaybe(t *testing.T) {
 				}
 				v.AssignString("v1")
 
-				shouldPanicOnBuild := func(t *testing.T) {
-					defer func() {
-						if r := recover(); r == nil {
-							t.Errorf("The code did not panic")
-						}
-					}()
-					_ = b.Build()
+				err = mb.Finish()
+				if _, ok := err.(ipld.ErrMissingRequiredField); !ok {
+					t.Fatalf("Expected error for missing field, got %v", err)
 				}
-				shouldPanicOnBuild(t)
 			})
 		})
 	})