From 005e49db7134829bdcd2f29b16273fee70f81a84 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Sat, 27 Jun 2026 15:52:45 +0300
Subject: [PATCH] markused: mark mut-receiver interface implementors (fix
 skip-unused dropping anon fn) (#27380)

---
 vlib/v/markused/walker.v                      |  5 +++
 .../interface_mut_receiver_anon_fn.run.out    |  1 +
 .../interface_mut_receiver_anon_fn.vv         | 34 +++++++++++++++++++
 3 files changed, 40 insertions(+)
 create mode 100644 vlib/v/tests/skip_unused/interface_mut_receiver_anon_fn.run.out
 create mode 100644 vlib/v/tests/skip_unused/interface_mut_receiver_anon_fn.vv

diff --git a/vlib/v/markused/walker.v b/vlib/v/markused/walker.v
index 76db5e7fc..20a5b9a19 100644
--- a/vlib/v/markused/walker.v
+++ b/vlib/v/markused/walker.v
@@ -3177,6 +3177,11 @@ pub fn (mut w Walker) mark_by_sym(isym ast.TypeSymbol) {
 			}
 		}
 		ast.Interface {
+			// Include implementors that require a mutable receiver (stored in
+			// `mut_types`, not `types`); otherwise interface methods reached only
+			// via dispatch on a `mut` receiver are never walked, and symbols they
+			// reference (e.g. an anon fn passed to a C callback) get pruned by
+			// -skip-unused. See https://github.com/vlang/v/issues/27354
 			for typ in isym.info.implementor_types(true) {
 				if typ == ast.map_type {
 					w.features.used_maps++
diff --git a/vlib/v/tests/skip_unused/interface_mut_receiver_anon_fn.run.out b/vlib/v/tests/skip_unused/interface_mut_receiver_anon_fn.run.out
new file mode 100644
index 000000000..19f86f493
--- /dev/null
+++ b/vlib/v/tests/skip_unused/interface_mut_receiver_anon_fn.run.out
@@ -0,0 +1 @@
+done
diff --git a/vlib/v/tests/skip_unused/interface_mut_receiver_anon_fn.vv b/vlib/v/tests/skip_unused/interface_mut_receiver_anon_fn.vv
new file mode 100644
index 000000000..c5dee2cca
--- /dev/null
+++ b/vlib/v/tests/skip_unused/interface_mut_receiver_anon_fn.vv
@@ -0,0 +1,34 @@
+module main
+
+// Regression test for https://github.com/vlang/v/issues/27354
+// An anon fn passed to a C callback inside a method that is reached only via
+// interface dispatch on a `mut` receiver must be marked used by markused,
+// otherwise -skip-unused (the default) drops its definition and the generated
+// C fails with `use of undeclared identifier 'anon_fn_...'`.
+
+fn C.atexit(cb voidptr) int
+
+interface Handler {
+	setup()
+}
+
+struct Impl {
+mut:
+	n int
+}
+
+fn (mut i Impl) setup() {
+	C.atexit(fn () {})
+}
+
+fn run(mut h Handler) {
+	h.setup()
+}
+
+fn main() {
+	mut impl := Impl{
+		n: 99
+	}
+	run(mut impl)
+	println('done')
+}
-- 
2.39.5