vm: render the top-level self as "main", and give remove_method's NameError its receiver

internal/vm/exception.go

@@ -42,6 +42,16 @@ func (vm *VM) undefinedMethodReceiver(self object.Value) string {
 		return kind + " " + vm.classDisplayName(c)
 	}
 	if vm.objSingleton(self) != nil {
+		// MRI's one exception inside this branch: the top-level self renders as
+		// "main", not as its identity string. name_err_mesg_to_str (error.c
+		// ruby_4_0:2679-2689) takes the singleton arm, sets the "an instance of"
+		// prefix to empty, and then chooses between rb_any_to_s(obj) and the
+		// literal "main" on `obj == rb_vm_top_self()`. rbgo had the arm and not
+		// the exception, so `self.nope` at the top level said "for
+		// #<Object:0x…>" where MRI says "for main".
+		if self == vm.main {
+			return "main"
+		}
 		return vm.objectIdentityRepr(self)
 	}
 	return "an instance of " + vm.classDisplayName(vm.classOf(self))

internal/vm/modulefunc.go

@@ -1,6 +1,10 @@
 package vm
 
-import "github.com/go-embedded-ruby/ruby/internal/object"
+import (
+	"fmt"
+
+	"github.com/go-embedded-ruby/ruby/internal/object"
+)
 
 // registerModuleExtras installs the module/class authoring directives that real
 // Ruby code (notably Puppet) leans on: module_function, the visibility setters
@@ -317,7 +321,12 @@ func (vm *VM) registerModuleExtras() {
 				vm.raiseFrozen(mod)
 			}
 			if _, ok := mod.methods[name]; !ok {
-				raise("NameError", "method '%s' not defined in %s", name, mod.ToS())
+				// MRI raises this through rb_name_err_raise (vm_method.c
+				// rb_mod_remove_method -> remove_method), whose FIRST argument is
+				// the exception's receiver -- so #receiver answers the module the
+				// removal was attempted on. rbgo built the same message through
+				// the bare raise() form and left it unrecorded.
+				vm.raiseNameError(fmt.Sprintf("method '%s' not defined in %s", name, mod.ToS()), name, mod)
 			}
 			delete(mod.methods, name)
 			bumpMethodSerial()

internal/vm/wave53_main_receiver_test.go

@@ -0,0 +1,63 @@
+// Copyright (c) the go-embedded-ruby/ruby authors
+//
+// SPDX-License-Identifier: BSD-3-Clause
+
+package vm
+
+import "testing"
+
+// TestTopLevelSelfRendersAsMain covers MRI's one exception inside the
+// singleton-receiver arm of name_err_mesg_to_str (error.c ruby_4_0:2679-2689):
+// an object carrying a singleton class renders as its identity string, EXCEPT
+// the top-level self, which renders as "main".
+//
+// Every want below is the byte-for-byte output of the local MRI 4.0.5 on that
+// exact source.
+func TestTopLevelSelfRendersAsMain(t *testing.T) {
+	t.Run("top-level self", func(t *testing.T) {
+		src := "begin; self.nope_xyz; rescue NoMethodError => e; puts e.message; end\n"
+		const want = "undefined method 'nope_xyz' for main\n"
+		if got := eval(t, src); got != want {
+			t.Errorf("got %q want %q", got, want)
+		}
+	})
+
+	// The rest of the arm must not move. An ordinary object that carries a
+	// singleton still renders as its identity, and — this is the part worth
+	// pinning — MRI does NOT dispatch #inspect or #to_s to build it. Overriding
+	// both changes nothing on ruby 4.0.5, which is why this message is
+	// constructed here rather than by sending to the receiver: an error path
+	// that runs user code can recurse or raise again.
+	t.Run("a singleton-carrying object still renders as its identity", func(t *testing.T) {
+		src := `o = Object.new
+def o.inspect; "CUSTOM-INSPECT"; end
+def o.to_s; "CUSTOM-TO-S"; end
+begin; o.nope; rescue NoMethodError => e; puts e.message.sub(/0x\h+/, "0xX"); end
+`
+		const want = "undefined method 'nope' for #<Object:0xX>\n"
+		if got := eval(t, src); got != want {
+			t.Errorf("got %q want %q — the override must NOT reach the message", got, want)
+		}
+	})
+
+	// And an object with no singleton takes the other arm entirely.
+	t.Run("no singleton", func(t *testing.T) {
+		src := "begin; Object.new.nope; rescue NoMethodError => e; puts e.message; end\n"
+		const want = "undefined method 'nope' for an instance of Object\n"
+		if got := eval(t, src); got != want {
+			t.Errorf("got %q want %q", got, want)
+		}
+	})
+}
+
+// TestRemoveMethodNameErrorCarriesItsReceiver: Module#remove_method on a name
+// that is not there raises through rb_name_err_raise (vm_method.c
+// rb_mod_remove_method), whose first argument IS the exception's receiver. rbgo
+// built the same message through the bare raise() form and recorded none.
+func TestRemoveMethodNameErrorCarriesItsReceiver(t *testing.T) {
+	src := "class K; end\nbegin; K.send(:remove_method, :nope); rescue NameError => e; p [e.message, e.name, e.receiver]; end\n"
+	const want = "[\"method 'nope' not defined in K\", :nope, K]\n"
+	if got := eval(t, src); got != want {
+		t.Errorf("got %q want %q", got, want)
+	}
+}