checker: do not allow dereferencing function calls on the left side of an =

vlib/v/checker/checker.v

@@ -3266,8 +3266,15 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 	is_decl := node.op == .decl_assign
 	for i, left in node.left {
 		if left is ast.CallExpr {
+			// ban `foo() = 10`
 			c.error('cannot call function `${left.name}()` on the left side of an assignment',
 				left.pos)
+		} else if left is ast.PrefixExpr {
+			// ban `*foo() = 10`
+			if left.right is ast.CallExpr && left.op == .mul {
+				c.error('cannot dereference a function call on the left side of an assignment, use a temporary variable',
+					left.pos)
+			}
 		} else if left is ast.IndexExpr {
 			if left.index is ast.RangeExpr {
 				c.error('cannot reassign using range expression on the left side of an assignment',

vlib/v/checker/tests/assign_deref_fn_call_on_left_side_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/assign_deref_fn_call_on_left_side_err.vv:8:2: error: cannot dereference a function call on the left side of an assignment, use a temporary variable
+    6 |
+    7 | fn main() {
+    8 |     *foo('s') = 1
+      |     ^
+    9 | }

vlib/v/checker/tests/assign_deref_fn_call_on_left_side_err.vv

@@ -0,0 +1,9 @@
+struct Foo{}
+
+fn foo(s string) &Foo {
+	return &Foo{}
+}
+
+fn main() {
+	*foo('s') = 1
+}

vlib/v/gen/c/cheaders.v

@@ -50,6 +50,7 @@ static inline void __sort_ptr(uintptr_t a[], bool b[], int l) {
 '
 
 const c_common_macros = '
+typedef unsigned char u8;
 #define EMPTY_VARG_INITIALIZATION 0
 #define EMPTY_STRUCT_DECLARATION
 #define EMPTY_STRUCT_INITIALIZATION
@@ -194,7 +195,7 @@ const c_helper_macros = '//============================== HELPER C MACROS ======
 //#define tos4(s, slen) ((string){.str=(s), .len=(slen)})
 // _SLIT0 is used as NULL string for literal arguments
 // `"" s` is used to enforce a string literal argument
-#define _SLIT0 (string){.len=0} 
+#define _SLIT0 (string){.len=0}
 #define _SLIT(s) ((string){.str=(byteptr)("" s), .len=(sizeof(s)-1), .is_lit=1})
 //#define _SLIT(s) ((string){.str=(byteptr)("" s), .len=(sizeof(s)-1), .is_lit=1})
 // take the address of an rvalue