feat: Add support for named types with underlying basic types (#236)

## Why this should be merged

The current rlpgen partially supports named types with basic underlying,
and it generates the rlp without correct conversion i.e:
`w.WriteUint64(obj.Uint64NewT)`

This PR adds the full support it

## How this works

Adds check for `isNamedWithBasicUnderlying` and then returns a `op` with
`makeNamedBasicOp` which is basically a `basicOp` but with the `typ`
refers to the named type so that it can correctly encode decode with the
named type.

## How this was tested

Added UT tests

---------

Signed-off-by: Ceyhun Onur <ceyhunonur54@gmail.com>

Author: ceyonur

rlp/rlpgen/gen.go

@@ -687,6 +687,12 @@ func (bctx *buildContext) makeOp(name *types.Named, typ types.Type, tags rlpstru
 		if bctx.isDecoder(typ) {
 			return nil, fmt.Errorf("type %v implements rlp.Decoder with non-pointer receiver", typ)
 		}
+		if hasBasicUnderlying(typ) {
+			// libevm: named types are reduced to their underlying basic type in this loop.
+			// We're handling named types here by passing the named type as the main type.
+			// See [named.libevm.go] for more details.
+			return bctx.makeNamedBasicOp(typ)
+		}
 		// TODO: same check for encoder?
 		return bctx.makeOp(typ, typ.Underlying(), tags)
 	case *types.Pointer:

rlp/rlpgen/gen_test.go

@@ -47,7 +47,7 @@ func init() {
 	}
 }
 
-var tests = []string{"uints", "nil", "rawvalue", "optional", "bigint", "uint256", "alias"}
+var tests = []string{"uints", "nil", "rawvalue", "optional", "bigint", "uint256", "alias", "named.libevm"}
 
 func TestOutput(t *testing.T) {
 	for _, test := range tests {

rlp/rlpgen/named.libevm.go

@@ -0,0 +1,55 @@
+// Copyright 2025 the libevm authors.
+//
+// The libevm additions to go-ethereum are free software: you can redistribute
+// them and/or modify them under the terms of the GNU Lesser General Public License
+// as published by the Free Software Foundation, either version 3 of the License,
+// or (at your option) any later version.
+//
+// The libevm additions are distributed in the hope that they will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser
+// General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see
+// <http://www.gnu.org/licenses/>.
+
+package main
+
+import (
+	"fmt"
+	"go/types"
+)
+
+// makeNamedBasicOp is a convenience wrapper for basicOp.
+// It returns a basicOp with the named type as the main type instead of the underlying basic type.
+func (bctx *buildContext) makeNamedBasicOp(named *types.Named) (op, error) {
+	underlying := named.Underlying()
+	basic, ok := underlying.(*types.Basic)
+	if !ok {
+		return nil, fmt.Errorf("expected basic type, got %T", underlying)
+	}
+
+	// We use basic op because it actually supports necessary conversions (through writeNeedsConversion and decodeNeedsConversion)
+	// for named types.
+	// The only problem with that is it does not support the named type as the main type.
+	// So we use the named type as the main type instead of the underlying basic type.
+	baseOp, err := bctx.makeBasicOp(basic)
+	if err != nil {
+		return nil, err
+	}
+
+	op, ok := baseOp.(basicOp)
+	if !ok {
+		return nil, fmt.Errorf("expected basicOp, got %T", baseOp)
+	}
+	op.typ = named
+
+	return op, nil
+}
+
+// hasBasicUnderlying checks whether `named` has an underlying basic type.
+func hasBasicUnderlying(named *types.Named) bool {
+	_, ok := named.Underlying().(*types.Basic)
+	return ok
+}

rlp/rlpgen/testdata/named.libevm.in.txt

@@ -0,0 +1,15 @@
+// -*- mode: go -*-
+
+package test
+
+type (
+    BoolT   bool
+    Uint64T uint64
+    StringT string
+)
+
+type Test struct {
+    BoolNewT BoolT
+    Uint64NewT Uint64T
+    StringNewT StringT
+}

rlp/rlpgen/testdata/named.libevm.out.txt

@@ -0,0 +1,49 @@
+package test
+
+import "github.com/ava-labs/libevm/rlp"
+import "io"
+
+func (obj *Test) EncodeRLP(_w io.Writer) error {
+	w := rlp.NewEncoderBuffer(_w)
+	_tmp0 := w.List()
+	w.WriteBool(bool(obj.BoolNewT))
+	w.WriteUint64(uint64(obj.Uint64NewT))
+	w.WriteString(string(obj.StringNewT))
+	w.ListEnd(_tmp0)
+	return w.Flush()
+}
+
+func (obj *Test) DecodeRLP(dec *rlp.Stream) error {
+	var _tmp0 Test
+	{
+		if _, err := dec.List(); err != nil {
+			return err
+		}
+		// BoolNewT:
+		_tmp1, err := dec.Bool()
+		if err != nil {
+			return err
+		}
+		_tmp2 := BoolT(_tmp1)
+		_tmp0.BoolNewT = _tmp2
+		// Uint64NewT:
+		_tmp3, err := dec.Uint64()
+		if err != nil {
+			return err
+		}
+		_tmp4 := Uint64T(_tmp3)
+		_tmp0.Uint64NewT = _tmp4
+		// StringNewT:
+		_tmp5, err := dec.String()
+		if err != nil {
+			return err
+		}
+		_tmp6 := StringT(_tmp5)
+		_tmp0.StringNewT = _tmp6
+		if err := dec.ListEnd(); err != nil {
+			return err
+		}
+	}
+	*obj = _tmp0
+	return nil
+}