refactor: params extra types are zero values not nil pointers by default

libevm/hookstest/stub.go

@@ -32,7 +32,7 @@ type Stub struct {
 // Register is a convenience wrapper for registering s as both the
 // [params.ChainConfigHooks] and [params.RulesHooks] via [Register].
 func (s *Stub) Register(tb testing.TB) {
-	Register(tb, params.Extras[Stub, Stub]{
+	Register(tb, params.Extras[*Stub, *Stub]{
 		NewRules: func(_ *params.ChainConfig, _ *params.Rules, _ *Stub, blockNum *big.Int, isMerge bool, timestamp uint64) *Stub {
 			return s
 		},

libevm/pseudo/type.go

@@ -60,6 +60,27 @@ func Zero[T any]() *Pseudo[T] {
 	return From[T](x)
 }
 
+// PointerTo is equivalent to [From] called with a pointer to the payload
+// carried by `t`. It first confirms that the payload is of type `T`.
+func PointerTo[T any](t *Type) (*Pseudo[*T], error) {
+	c, ok := t.val.(*concrete[T])
+	if !ok {
+		var want *T
+		return nil, fmt.Errorf("cannot create *Pseudo[%T] from *Type carrying %T", want, t.val.get())
+	}
+	return From(&c.val), nil
+}
+
+// MustPointerTo is equivalent to [PointerTo] except that it panics instead of
+// returning an error.
+func MustPointerTo[T any](t *Type) *Pseudo[*T] {
+	p, err := PointerTo[T](t)
+	if err != nil {
+		panic(err)
+	}
+	return p
+}
+
 // Interface returns the wrapped value as an `any`, equivalent to
 // [reflect.Value.Interface]. Prefer [Value.Get].
 func (t *Type) Interface() any { return t.val.get() }

libevm/pseudo/type_test.go

@@ -77,3 +77,26 @@ func ExamplePseudo_TypeAndValue() {
 	_ = typ
 	_ = val
 }
+
+func TestPointer(t *testing.T) {
+	type carrier struct {
+		payload int
+	}
+
+	typ, val := From(carrier{42}).TypeAndValue()
+
+	t.Run("invalid type", func(t *testing.T) {
+		_, err := PointerTo[int](typ)
+		require.Errorf(t, err, "PointerTo[int](%T)", carrier{})
+	})
+
+	t.Run("valid type", func(t *testing.T) {
+		ptrVal := MustPointerTo[carrier](typ).Value
+
+		assert.Equal(t, 42, val.Get().payload, "before setting via pointer")
+		var ptr *carrier = ptrVal.Get()
+		ptr.payload = 314159
+		assert.Equal(t, 314159, val.Get().payload, "after setting via pointer")
+	})
+
+}

params/config.libevm.go

@@ -25,25 +25,27 @@ type Extras[C ChainConfigHooks, R RulesHooks] struct {
 	// NewRules, if non-nil is called at the end of [ChainConfig.Rules] with the
 	// newly created [Rules] and other context from the method call. Its
 	// returned value will be the extra payload of the [Rules]. If NewRules is
-	// nil then so too will the [Rules] extra payload be a nil `*R`.
+	// nil then so too will the [Rules] extra payload be a zero-value `R`.
 	//
 	// NewRules MAY modify the [Rules] but MUST NOT modify the [ChainConfig].
-	NewRules func(_ *ChainConfig, _ *Rules, _ *C, blockNum *big.Int, isMerge bool, timestamp uint64) *R
+	// TODO(arr4n): add the [Rules] to the return signature to make it clearer
+	// that the caller can modify the generated Rules.
+	NewRules func(_ *ChainConfig, _ *Rules, _ C, blockNum *big.Int, isMerge bool, timestamp uint64) R
 }
 
 // RegisterExtras registers the types `C` and `R` such that they are carried as
 // extra payloads in [ChainConfig] and [Rules] structs, respectively. It is
 // expected to be called in an `init()` function and MUST NOT be called more
-// than once. Both `C` and `R` MUST be structs.
+// than once. Both `C` and `R` MUST be structs or pointers to structs.
 //
 // After registration, JSON unmarshalling of a [ChainConfig] will create a new
-// `*C` and unmarshal the JSON key "extra" into it. Conversely, JSON marshalling
-// will populate the "extra" key with the contents of the `*C`. Both the
+// `C` and unmarshal the JSON key "extra" into it. Conversely, JSON marshalling
+// will populate the "extra" key with the contents of the `C`. Both the
 // [json.Marshaler] and [json.Unmarshaler] interfaces are honoured if
 // implemented by `C` and/or `R.`
 //
 // Calls to [ChainConfig.Rules] will call the `NewRules` function of the
-// registered [Extras] to create a new `*R`.
+// registered [Extras] to create a new `R`.
 //
 // The payloads can be accessed via the [ExtraPayloadGetter.FromChainConfig] and
 // [ExtraPayloadGetter.FromRules] methods of the getter returned by
@@ -54,16 +56,16 @@ func RegisterExtras[C ChainConfigHooks, R RulesHooks](e Extras[C, R]) ExtraPaylo
 	if registeredExtras != nil {
 		panic("re-registration of Extras")
 	}
-	mustBeStruct[C]()
-	mustBeStruct[R]()
+	mustBeStructOrPointerToOne[C]()
+	mustBeStructOrPointerToOne[R]()
 
 	getter := e.getter()
 	registeredExtras = &extraConstructors{
-		chainConfig:   pseudo.NewConstructor[C](),
-		rules:         pseudo.NewConstructor[R](),
-		reuseJSONRoot: e.ReuseJSONRoot,
-		newForRules:   e.newForRules,
-		getter:        getter,
+		newChainConfig: pseudo.NewConstructor[C]().Zero,
+		newRules:       pseudo.NewConstructor[R]().Zero,
+		reuseJSONRoot:  e.ReuseJSONRoot,
+		newForRules:    e.newForRules,
+		getter:         getter,
 	}
 	return getter
 }
@@ -95,9 +97,9 @@ func TestOnlyClearRegisteredExtras() {
 var registeredExtras *extraConstructors
 
 type extraConstructors struct {
-	chainConfig, rules pseudo.Constructor
-	reuseJSONRoot      bool
-	newForRules        func(_ *ChainConfig, _ *Rules, blockNum *big.Int, isMerge bool, timestamp uint64) *pseudo.Type
+	newChainConfig, newRules func() *pseudo.Type
+	reuseJSONRoot            bool
+	newForRules              func(_ *ChainConfig, _ *Rules, blockNum *big.Int, isMerge bool, timestamp uint64) *pseudo.Type
 	// use top-level hooksFrom<X>() functions instead of these as they handle
 	// instances where no [Extras] were registered.
 	getter interface {
@@ -108,27 +110,34 @@ type extraConstructors struct {
 
 func (e *Extras[C, R]) newForRules(c *ChainConfig, r *Rules, blockNum *big.Int, isMerge bool, timestamp uint64) *pseudo.Type {
 	if e.NewRules == nil {
-		return registeredExtras.rules.NilPointer()
+		return registeredExtras.newRules()
 	}
 	rExtra := e.NewRules(c, r, e.getter().FromChainConfig(c), blockNum, isMerge, timestamp)
 	return pseudo.From(rExtra).Type
 }
 
 func (*Extras[C, R]) getter() (g ExtraPayloadGetter[C, R]) { return }
 
-// mustBeStruct panics if `T` isn't a struct.
-func mustBeStruct[T any]() {
+// mustBeStructOrPointerToOne panics if `T` isn't a struct or a *struct.
+func mustBeStructOrPointerToOne[T any]() {
 	var x T
-	if k := reflect.TypeOf(x).Kind(); k != reflect.Struct {
-		panic(notStructMessage[T]())
+	switch t := reflect.TypeOf(x); t.Kind() {
+	case reflect.Struct:
+		return
+	case reflect.Pointer:
+		if t.Elem().Kind() == reflect.Struct {
+			return
+		}
 	}
+	panic(notStructMessage[T]())
 }
 
-// notStructMessage returns the message with which [mustBeStruct] might panic.
-// It exists to avoid change-detector tests should the message contents change.
+// notStructMessage returns the message with which [mustBeStructOrPointerToOne]
+// might panic. It exists to avoid change-detector tests should the message
+// contents change.
 func notStructMessage[T any]() string {
 	var x T
-	return fmt.Sprintf("%T is not a struct", x)
+	return fmt.Sprintf("%T is not a struct nor a pointer to a struct", x)
 }
 
 // An ExtraPayloadGettter provides strongly typed access to the extra payloads
@@ -139,33 +148,37 @@ type ExtraPayloadGetter[C ChainConfigHooks, R RulesHooks] struct {
 }
 
 // FromChainConfig returns the ChainConfig's extra payload.
-func (ExtraPayloadGetter[C, R]) FromChainConfig(c *ChainConfig) *C {
-	return pseudo.MustNewValue[*C](c.extraPayload()).Get()
+func (ExtraPayloadGetter[C, R]) FromChainConfig(c *ChainConfig) C {
+	return pseudo.MustNewValue[C](c.extraPayload()).Get()
+}
+
+// PointerFromChainConfig returns a pointer to the ChainConfig's extra payload.
+// This is guaranteed to be non-nil.
+func (ExtraPayloadGetter[C, R]) PointerFromChainConfig(c *ChainConfig) *C {
+	return pseudo.MustPointerTo[C](c.extraPayload()).Value.Get()
 }
 
 // hooksFromChainConfig is equivalent to FromChainConfig(), but returns an
 // interface instead of the concrete type implementing it; this allows it to be
-// used in non-generic code. If the concrete-type value is nil (typically
-// because no [Extras] were registered) a [noopHooks] is returned so it can be
-// used without nil checks.
+// used in non-generic code.
 func (e ExtraPayloadGetter[C, R]) hooksFromChainConfig(c *ChainConfig) ChainConfigHooks {
-	if h := e.FromChainConfig(c); h != nil {
-		return *h
-	}
-	return NOOPHooks{}
+	return e.FromChainConfig(c)
 }
 
 // FromRules returns the Rules' extra payload.
-func (ExtraPayloadGetter[C, R]) FromRules(r *Rules) *R {
-	return pseudo.MustNewValue[*R](r.extraPayload()).Get()
+func (ExtraPayloadGetter[C, R]) FromRules(r *Rules) R {
+	return pseudo.MustNewValue[R](r.extraPayload()).Get()
+}
+
+// PointerFromRules returns a pointer to the Rules's extra payload. This is
+// guaranteed to be non-nil.
+func (ExtraPayloadGetter[C, R]) PointerFromRules(r *Rules) *R {
+	return pseudo.MustPointerTo[R](r.extraPayload()).Value.Get()
 }
 
 // hooksFromRules is the [RulesHooks] equivalent of hooksFromChainConfig().
 func (e ExtraPayloadGetter[C, R]) hooksFromRules(r *Rules) RulesHooks {
-	if h := e.FromRules(r); h != nil {
-		return *h
-	}
-	return NOOPHooks{}
+	return e.FromRules(r)
 }
 
 // addRulesExtra is called at the end of [ChainConfig.Rules]; it exists to
@@ -189,7 +202,7 @@ func (c *ChainConfig) extraPayload() *pseudo.Type {
 		panic(fmt.Sprintf("%T.ExtraPayload() called before RegisterExtras()", c))
 	}
 	if c.extra == nil {
-		c.extra = registeredExtras.chainConfig.NilPointer()
+		c.extra = registeredExtras.newChainConfig()
 	}
 	return c.extra
 }
@@ -201,7 +214,7 @@ func (r *Rules) extraPayload() *pseudo.Type {
 		panic(fmt.Sprintf("%T.ExtraPayload() called before RegisterExtras()", r))
 	}
 	if r.extra == nil {
-		r.extra = registeredExtras.rules.NilPointer()
+		r.extra = registeredExtras.newRules()
 	}
 	return r.extra
 }

params/config.libevm_test.go

@@ -50,26 +50,36 @@ func TestRegisterExtras(t *testing.T) {
 			name: "Rules payload copied from ChainConfig payload",
 			register: func() {
 				RegisterExtras(Extras[ccExtraA, rulesExtraA]{
-					NewRules: func(cc *ChainConfig, r *Rules, ex *ccExtraA, _ *big.Int, _ bool, _ uint64) *rulesExtraA {
-						return &rulesExtraA{
+					NewRules: func(cc *ChainConfig, r *Rules, ex ccExtraA, _ *big.Int, _ bool, _ uint64) rulesExtraA {
+						return rulesExtraA{
 							A: ex.A,
 						}
 					},
 				})
 			},
-			ccExtra: pseudo.From(&ccExtraA{
+			ccExtra: pseudo.From(ccExtraA{
 				A: "hello",
 			}).Type,
-			wantRulesExtra: &rulesExtraA{
+			wantRulesExtra: rulesExtraA{
 				A: "hello",
 			},
 		},
 		{
-			name: "no NewForRules() function results in typed but nil pointer",
+			name: "no NewForRules() function results in zero value",
 			register: func() {
 				RegisterExtras(Extras[ccExtraB, rulesExtraB]{})
 			},
-			ccExtra: pseudo.From(&ccExtraB{
+			ccExtra: pseudo.From(ccExtraB{
+				B: "world",
+			}).Type,
+			wantRulesExtra: rulesExtraB{},
+		},
+		{
+			name: "no NewForRules() function results in nil pointer",
+			register: func() {
+				RegisterExtras(Extras[ccExtraB, *rulesExtraB]{})
+			},
+			ccExtra: pseudo.From(ccExtraB{
 				B: "world",
 			}).Type,
 			wantRulesExtra: (*rulesExtraB)(nil),
@@ -79,10 +89,10 @@ func TestRegisterExtras(t *testing.T) {
 			register: func() {
 				RegisterExtras(Extras[rawJSON, struct{ RulesHooks }]{})
 			},
-			ccExtra: pseudo.From(&rawJSON{
+			ccExtra: pseudo.From(rawJSON{
 				RawMessage: []byte(`"hello, world"`),
 			}).Type,
-			wantRulesExtra: (*struct{ RulesHooks })(nil),
+			wantRulesExtra: struct{ RulesHooks }{},
 		},
 	}
 
@@ -111,6 +121,75 @@ func TestRegisterExtras(t *testing.T) {
 	}
 }
 
+func TestModificationOfZeroExtras(t *testing.T) {
+	type (
+		ccExtra struct {
+			X int
+			NOOPHooks
+		}
+		rulesExtra struct {
+			X int
+			NOOPHooks
+		}
+	)
+
+	TestOnlyClearRegisteredExtras()
+	t.Cleanup(TestOnlyClearRegisteredExtras)
+	getter := RegisterExtras(Extras[ccExtra, rulesExtra]{})
+
+	config := new(ChainConfig)
+	rules := new(Rules)
+	// These assertion helpers are defined before any modifications so that the
+	// closure is demonstrably over the original zero values.
+	assertChainConfigExtra := func(t *testing.T, want ccExtra, msg string) {
+		t.Helper()
+		assert.Equalf(t, want, getter.FromChainConfig(config), "%T: "+msg, &config)
+	}
+	assertRulesExtra := func(t *testing.T, want rulesExtra, msg string) {
+		t.Helper()
+		assert.Equalf(t, want, getter.FromRules(rules), "%T: "+msg, &rules)
+	}
+
+	assertChainConfigExtra(t, ccExtra{}, "zero value")
+	assertRulesExtra(t, rulesExtra{}, "zero value")
+
+	const answer = 42
+	getter.PointerFromChainConfig(config).X = answer
+	assertChainConfigExtra(t, ccExtra{X: answer}, "after setting via pointer field")
+
+	const pi = 314159
+	getter.PointerFromRules(rules).X = pi
+	assertRulesExtra(t, rulesExtra{X: pi}, "after setting via pointer field")
+
+	ccReplace := ccExtra{X: 142857}
+	*getter.PointerFromChainConfig(config) = ccReplace
+	assertChainConfigExtra(t, ccReplace, "after replacement of entire extra via `*pointer = x`")
+
+	rulesReplace := rulesExtra{X: 18101986}
+	*getter.PointerFromRules(rules) = rulesReplace
+	assertRulesExtra(t, rulesReplace, "after replacement of entire extra via `*pointer = x`")
+
+	if t.Failed() {
+		// The test of shallow copying is now guaranteed to fail.
+		return
+	}
+	t.Run("shallow copy", func(t *testing.T) {
+		ccCopy := *config
+		rCopy := *rules
+
+		assert.Equal(t, getter.FromChainConfig(&ccCopy), ccReplace, "ChainConfig extras copied")
+		assert.Equal(t, getter.FromRules(&rCopy), rulesReplace, "Rules extras copied")
+
+		const seqUp = 123456789
+		getter.PointerFromChainConfig(&ccCopy).X = seqUp
+		assertChainConfigExtra(t, ccExtra{X: seqUp}, "original changed because copy only shallow")
+
+		const seqDown = 987654321
+		getter.PointerFromRules(&rCopy).X = seqDown
+		assertRulesExtra(t, rulesExtra{X: seqDown}, "original changed because copy only shallow")
+	})
+}
+
 func TestExtrasPanic(t *testing.T) {
 	TestOnlyClearRegisteredExtras()
 	defer TestOnlyClearRegisteredExtras()
@@ -131,7 +210,7 @@ func TestExtrasPanic(t *testing.T) {
 
 	assertPanics(
 		t, func() {
-			mustBeStruct[int]()
+			mustBeStructOrPointerToOne[int]()
 		},
 		notStructMessage[int](),
 	)