diff --git a/vlib/v/gen/c/assert.v b/vlib/v/gen/c/assert.v
index 46b48f7f00..5779177090 100644
--- a/vlib/v/gen/c/assert.v
+++ b/vlib/v/gen/c/assert.v
@@ -12,19 +12,11 @@ fn (mut g Gen) gen_assert_stmt(original_assert_statement ast.AssertStmt) {
 	mut node := original_assert_statement
 	g.writeln('// assert')
 	if mut node.expr is ast.InfixExpr {
-		if mut node.expr.left is ast.CallExpr {
-			node.expr.left = g.new_ctemp_var_then_gen(node.expr.left, node.expr.left_type)
-		} else if mut node.expr.left is ast.ParExpr {
-			if node.expr.left.expr is ast.CallExpr {
-				node.expr.left = g.new_ctemp_var_then_gen(node.expr.left.expr, node.expr.left_type)
-			}
+		if subst_expr := g.assert_subexpression_to_ctemp(node.expr.left, node.expr.left_type) {
+			node.expr.left = subst_expr
 		}
-		if mut node.expr.right is ast.CallExpr {
-			node.expr.right = g.new_ctemp_var_then_gen(node.expr.right, node.expr.right_type)
-		} else if mut node.expr.right is ast.ParExpr {
-			if node.expr.right.expr is ast.CallExpr {
-				node.expr.right = g.new_ctemp_var_then_gen(node.expr.right.expr, node.expr.right_type)
-			}
+		if subst_expr := g.assert_subexpression_to_ctemp(node.expr.right, node.expr.right_type) {
+			node.expr.right = subst_expr
 		}
 	}
 	g.inside_ternary++
@@ -60,6 +52,39 @@ fn (mut g Gen) gen_assert_stmt(original_assert_statement ast.AssertStmt) {
 	}
 }
 
+struct UnsupportedAssertCtempTransform {
+	msg  string
+	code int
+}
+
+const unsupported_ctemp_assert_transform = IError(UnsupportedAssertCtempTransform{})
+
+fn (mut g Gen) assert_subexpression_to_ctemp(expr ast.Expr, expr_type ast.Type) ?ast.Expr {
+	match expr {
+		ast.CallExpr {
+			return g.new_ctemp_var_then_gen(expr, expr_type)
+		}
+		ast.ParExpr {
+			if expr.expr is ast.CallExpr {
+				return g.new_ctemp_var_then_gen(expr.expr, expr_type)
+			}
+		}
+		ast.SelectorExpr {
+			if expr.expr is ast.CallExpr {
+				sym := g.table.get_final_type_symbol(g.unwrap_generic(expr.expr.return_type))
+				if sym.kind == .struct_ {
+					if (sym.info as ast.Struct).is_union {
+						return c.unsupported_ctemp_assert_transform
+					}
+				}
+				return g.new_ctemp_var_then_gen(expr, expr_type)
+			}
+		}
+		else {}
+	}
+	return c.unsupported_ctemp_assert_transform
+}
+
 fn (mut g Gen) gen_assert_postfailure_mode(node ast.AssertStmt) {
 	g.write_v_source_line_info(node.pos)
 	match g.pref.assert_failure_mode {
diff --git a/vlib/v/gen/c/testdata/assert_fncalls.c.must_have b/vlib/v/gen/c/testdata/assert_fncalls.c.must_have
new file mode 100644
index 0000000000..ec6c6e46e6
--- /dev/null
+++ b/vlib/v/gen/c/testdata/assert_fncalls.c.must_have
@@ -0,0 +1,9 @@
+int _t1 = main__fn_returning_array().len;
+if (!(_t1 == 1)) {
+v_assert_meta_info__t2.src = _SLIT("fn_returning_array().len == 1");
+v_assert_meta_info__t2.llabel = _SLIT("fn_returning_array().len");
+
+int _t3 = main__fn_returning_struct().x;
+if (!(_t3 == 123)) {
+v_assert_meta_info__t4.src = _SLIT("fn_returning_struct().x == 123");
+v_assert_meta_info__t4.llabel = _SLIT("fn_returning_struct().x");
diff --git a/vlib/v/gen/c/testdata/assert_fncalls.out b/vlib/v/gen/c/testdata/assert_fncalls.out
new file mode 100644
index 0000000000..04bbe63a4d
--- /dev/null
+++ b/vlib/v/gen/c/testdata/assert_fncalls.out
@@ -0,0 +1,4 @@
+[1]
+Abc{
+    x: 123
+}
diff --git a/vlib/v/gen/c/testdata/assert_fncalls.vv b/vlib/v/gen/c/testdata/assert_fncalls.vv
new file mode 100644
index 0000000000..ae554f522d
--- /dev/null
+++ b/vlib/v/gen/c/testdata/assert_fncalls.vv
@@ -0,0 +1,18 @@
+struct Abc {
+	x int
+}
+
+fn fn_returning_array() []int {
+	return [1]
+}
+
+fn fn_returning_struct() Abc {
+	return Abc{123}
+}
+
+fn main() {
+	assert fn_returning_array().len == 1
+	assert fn_returning_struct().x == 123
+	println(fn_returning_array())
+	println(fn_returning_struct())
+}
diff --git a/vlib/v/gen/js/sourcemap/source_map.v b/vlib/v/gen/js/sourcemap/source_map.v
index 44f79d1e86..bf864e13ae 100644
--- a/vlib/v/gen/js/sourcemap/source_map.v
+++ b/vlib/v/gen/js/sourcemap/source_map.v
@@ -39,7 +39,9 @@ pub fn new_sourcemap(file string, source_root string, sources_content_inline boo
 
 // Add a single mapping from original source line and column to the generated source's line and column for this source map being created.
 pub fn (mut sm SourceMap) add_mapping(source_name string, source_position SourcePositionType, gen_line u32, gen_column u32, name string) {
-	assert source_name.len != 0
+	if source_name.len == 0 {
+		panic('add_mapping, source_name should not be ""')
+	}
 
 	sources_ind := sm.sources.add(source_name)
 
@@ -53,7 +55,9 @@ pub fn (mut sm SourceMap) add_mapping(source_name string, source_position Source
 
 // Add multiple mappings from the same source
 pub fn (mut sm SourceMap) add_mapping_list(source_name string, mapping_list []MappingInput) ? {
-	assert source_name.len != 0
+	if source_name.len == 0 {
+		panic('add_mapping_list, source_name should not be ""')
+	}
 
 	sources_ind := sm.sources.add(source_name)
 
diff --git a/vlib/v/tests/assert_should_evaluate_args_just_once_test.v b/vlib/v/tests/assert_should_evaluate_args_just_once_test.v
new file mode 100644
index 0000000000..5a2d8a1887
--- /dev/null
+++ b/vlib/v/tests/assert_should_evaluate_args_just_once_test.v
@@ -0,0 +1,39 @@
+struct Abc {
+	ncalls int
+}
+
+[unsafe]
+fn fn_that_should_be_called_just_once_array() []int {
+	mut static ncalls := 0
+	ncalls++
+	println('${@FN} calls: $ncalls')
+	if ncalls > 1 {
+		assert false
+	}
+	return []int{len: ncalls, init: ncalls}
+}
+
+[unsafe]
+fn fn_that_should_be_called_just_once_struct() Abc {
+	mut static ncalls := 0
+	ncalls++
+	println('${@FN} calls: $ncalls')
+	if ncalls > 1 {
+		assert false
+	}
+	return Abc{
+		ncalls: ncalls
+	}
+}
+
+fn test_assert_calls_a_function_returning_an_array_just_once() {
+	unsafe {
+		assert fn_that_should_be_called_just_once_array().len == 1
+	}
+}
+
+fn test_assert_calls_a_function_returning_a_struct_just_once() {
+	unsafe {
+		assert fn_that_should_be_called_just_once_struct().ncalls == 1
+	}
+}