From dd859eae55cf4e69346851fe285b9af104c8ffb7 Mon Sep 17 00:00:00 2001
From: Felipe Pena <felipensp@gmail.com>
Date: Tue, 6 May 2025 06:48:55 -0300
Subject: [PATCH] cgen: fix interface `unsafe {nil}` comparison and
 initialization (fix #24374) (#24411)

---
 vlib/v/gen/c/auto_eq_methods.v                      |  6 ++++++
 vlib/v/gen/c/cgen.v                                 |  3 +++
 .../fn_returning_voidptr_casted_as_interface_test.v |  2 +-
 vlib/v/tests/interfaces/interface_nil_cmp_test.v    | 13 +++++++++++++
 ...uct_init_with_interface_pointer_and_embed_test.v |  2 +-
 5 files changed, 24 insertions(+), 2 deletions(-)
 create mode 100644 vlib/v/tests/interfaces/interface_nil_cmp_test.v

diff --git a/vlib/v/gen/c/auto_eq_methods.v b/vlib/v/gen/c/auto_eq_methods.v
index 84b8308c5f..42189c0252 100644
--- a/vlib/v/gen/c/auto_eq_methods.v
+++ b/vlib/v/gen/c/auto_eq_methods.v
@@ -258,7 +258,13 @@ fn (mut g Gen) gen_struct_equality_fn(left_type ast.Type) string {
 				&& (!field.typ.has_flag(.option) || !field.typ.is_ptr()) {
 				ptr := if field.typ.is_ptr() { '*'.repeat(field.typ.nr_muls()) } else { '' }
 				eq_fn := g.gen_interface_equality_fn(field.typ)
+				if ptr != '' {
+					fn_builder.write_string('((${left_arg} == (void*)0 && ${right_arg} == (void*)0) || (${left_arg} != (void*)0 && ${right_arg} != (void*)0 && ')
+				}
 				fn_builder.write_string('${eq_fn}_interface_eq(${ptr}${left_arg}, ${ptr}${right_arg})')
+				if ptr != '' {
+					fn_builder.write_string('))')
+				}
 			} else if field.typ.has_flag(.option) {
 				fn_builder.write_string('!memcmp(&${left_arg}.data, &${right_arg}.data, sizeof(${g.base_type(field.typ)}))')
 			} else {
diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index e560659401..5c9d79b053 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -2786,6 +2786,9 @@ fn (mut g Gen) call_cfn_for_casting_expr(fname string, expr ast.Expr, exp ast.Ty
 			g.write('&(${exp_styp.trim_right('*')}){._${got_styp.trim_right('*')}=')
 			rparen_n = 0
 			mutable_idx = got.idx()
+		} else if expr is ast.UnsafeExpr && expr.expr is ast.Nil {
+			g.write('(void*)0')
+			return
 		} else {
 			g.write('HEAP(${exp_styp}, ${fname}(')
 			rparen_n++
diff --git a/vlib/v/tests/interfaces/interface_edge_cases/fn_returning_voidptr_casted_as_interface_test.v b/vlib/v/tests/interfaces/interface_edge_cases/fn_returning_voidptr_casted_as_interface_test.v
index f4c7792e8e..380ac356a9 100644
--- a/vlib/v/tests/interfaces/interface_edge_cases/fn_returning_voidptr_casted_as_interface_test.v
+++ b/vlib/v/tests/interfaces/interface_edge_cases/fn_returning_voidptr_casted_as_interface_test.v
@@ -23,5 +23,5 @@ fn test_fn_returning_voidptr_casted_as_interface_works() {
 	// TODO: understand the root reason why msvc and
 	// `-cc clang-11 -cflags -fsanitize=memory` produce
 	// something like `&IAbc(e42aff650)` here
-	assert f(pi).contains('&IAbc(')
+	assert f(pi) == '&nil'
 }
diff --git a/vlib/v/tests/interfaces/interface_nil_cmp_test.v b/vlib/v/tests/interfaces/interface_nil_cmp_test.v
new file mode 100644
index 0000000000..3932320f79
--- /dev/null
+++ b/vlib/v/tests/interfaces/interface_nil_cmp_test.v
@@ -0,0 +1,13 @@
+interface Cfg {
+	id string
+}
+
+struct XX {
+	cfg &Cfg = unsafe { nil }
+}
+
+fn test_main() {
+	xx := XX{}
+	assert xx.cfg == unsafe { nil }
+	assert unsafe { nil } == xx.cfg
+}
diff --git a/vlib/v/tests/structs/struct_init_with_interface_pointer_and_embed_test.v b/vlib/v/tests/structs/struct_init_with_interface_pointer_and_embed_test.v
index 5416510977..52fa639d0b 100644
--- a/vlib/v/tests/structs/struct_init_with_interface_pointer_and_embed_test.v
+++ b/vlib/v/tests/structs/struct_init_with_interface_pointer_and_embed_test.v
@@ -24,6 +24,6 @@ fn test_struct_with_both_an_embed_and_a_pointer_to_interface_value_fields__initi
 	assert si.contains('Embed: Embed{')
 	assert si.contains('foo: 0')
 	assert si.contains('id: 0')
-	assert si.contains('parent: &Node(0x0)')
+	assert si.contains('parent: &nil')
 	assert si.contains('}')
 }