cgen: fix auto str for interface struct member which implements str method

vlib/v/gen/c/auto_str_methods.v

@@ -1068,6 +1068,11 @@ fn struct_auto_str_func(sym &ast.TypeSymbol, lang ast.Language, _field_type ast.
 	} else if _field_type.has_flag(.option) || should_use_indent_func(sym.kind) {
 		obj := '${deref}it.${final_field_name}${sufix}'
 		if has_custom_str {
+			if sym.kind == .interface_ && (sym.info as ast.Interface).defines_method('str') {
+				iface_obj := 'it.${final_field_name}${sufix}'
+				dot := if field_type.is_ptr() { '->' } else { '.' }
+				return '${fn_name.trim_string_right('_str')}_name_table[${iface_obj}${dot}_typ]._method_str(${iface_obj}${dot}_object)', true
+			}
 			return '${fn_name}(${obj})', true
 		}
 		return 'indent_${fn_name}(${obj}, indent_count + 1)', true

vlib/v/gen/c/str.v

@@ -108,6 +108,7 @@ fn (mut g Gen) gen_expr_to_string(expr ast.Expr, etype ast.Type) {
 		unwrap_option := expr is ast.Ident && expr.or_expr.kind == .propagate_option
 		exp_typ := if unwrap_option { typ.clear_flag(.option) } else { typ }
 		is_ptr := exp_typ.is_ptr()
+		is_dump_expr := expr is ast.DumpExpr
 		is_var_mut := expr.is_auto_deref_var()
 		str_fn_name := g.get_str_fn(exp_typ)
 		if is_ptr && !is_var_mut {
@@ -130,7 +131,14 @@ fn (mut g Gen) gen_expr_to_string(expr ast.Expr, etype ast.Type) {
 		}
 		g.write('${str_fn_name}(')
 		if str_method_expects_ptr && !is_ptr {
-			g.write('&')
+			if is_dump_expr {
+				g.write('ADDR(${g.typ(typ)}, ')
+				defer {
+					g.write(')')
+				}
+			} else {
+				g.write('&')
+			}
 		} else if is_ptr && typ.has_flag(.option) {
 			g.write('*(${g.typ(typ)}*)&')
 		} else if !str_method_expects_ptr && !is_shared && (is_ptr || is_var_mut) {

vlib/v/tests/interface_auto_str_test.v

@@ -0,0 +1,31 @@
+interface Context {
+	str() string
+}
+
+struct OtherContext {
+	a string
+}
+
+pub fn (o &OtherContext) str() string {
+	return 'OtherContext'
+}
+
+struct WithContext {
+pub mut:
+	ctx Context
+}
+
+fn with_function(ctx Context) {
+	println(ctx)
+}
+
+fn test_main() {
+	mut s := WithContext{
+		ctx: OtherContext{}
+	}
+	assert dump(s) == WithContext{
+		ctx: OtherContext{}
+	}
+
+	assert dump(OtherContext{}) == OtherContext{}
+}