cgen: fix assigning fn address (fix #24537) (#25158)

vlib/v/gen/c/assign.v

@@ -289,6 +289,7 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 
 	for i, mut left in node.left {
 		mut is_auto_heap := false
+		mut is_fn_var := false
 		mut var_type := node.left_types[i]
 		mut val_type := node.right_types[i]
 		val := node.right[i]
@@ -432,6 +433,9 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 					}
 				}
 				is_auto_heap = left.obj.is_auto_heap
+				if left.obj.typ != 0 && val is ast.PrefixExpr {
+					is_fn_var = g.table.final_sym(left.obj.typ).kind == .function
+				}
 			}
 		} else if mut left is ast.ComptimeSelector {
 			if left.typ_key != '' {
@@ -964,14 +968,14 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 					} else {
 						is_option_unwrapped := val is ast.Ident && val.or_expr.kind != .absent
 						is_option_auto_heap := is_auto_heap && is_option_unwrapped
-						if is_auto_heap {
+						if is_auto_heap && !is_fn_var {
 							if aligned != 0 {
 								g.write('HEAP_align(${styp}, (')
 							} else {
 								g.write('HEAP(${styp}, (')
 							}
 						}
-						if val.is_auto_deref_var() && !is_option_unwrapped {
+						if !is_fn_var && val.is_auto_deref_var() && !is_option_unwrapped {
 							g.write('*')
 						}
 						if (var_type.has_flag(.option) && val !in [ast.Ident, ast.SelectorExpr])
@@ -997,9 +1001,13 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 							g.fixed_array_var_init(tmp_var, false, unaliased_right_sym.info.elem_type,
 								unaliased_right_sym.info.size)
 						} else {
+							old_inside_assign_fn_var := g.inside_assign_fn_var
+							g.inside_assign_fn_var = val is ast.PrefixExpr && val.op == .amp
+								&& is_fn_var
 							g.expr(val)
+							g.inside_assign_fn_var = old_inside_assign_fn_var
 						}
-						if is_auto_heap && !is_option_auto_heap {
+						if !is_fn_var && is_auto_heap && !is_option_auto_heap {
 							if aligned != 0 {
 								g.write('), ${aligned})')
 							} else {

vlib/v/gen/c/cgen.v

@@ -163,6 +163,7 @@ mut:
 	inside_cinit              bool
 	inside_global_decl        bool
 	inside_interface_deref    bool
+	inside_assign_fn_var      bool
 	last_tmp_call_var         []string
 	last_if_option_type       ast.Type // stores the expected if type on nested if expr
 	loop_depth                int
@@ -5360,7 +5361,7 @@ fn (mut g Gen) ident(node ast.Ident) {
 		if node.obj is ast.Var {
 			is_auto_heap = node.obj.is_auto_heap
 				&& (!g.is_assign_lhs || g.assign_op != .decl_assign)
-			if is_auto_heap {
+			if is_auto_heap && !g.inside_assign_fn_var {
 				g.write('(*(')
 			}
 			is_option = is_option || node.obj.orig_type.has_flag(.option)
@@ -5478,7 +5479,7 @@ fn (mut g Gen) ident(node ast.Ident) {
 						}
 						g.write(')')
 					}
-					if is_auto_heap {
+					if is_auto_heap && !g.inside_assign_fn_var {
 						g.write('))')
 					}
 					return
@@ -5502,7 +5503,7 @@ fn (mut g Gen) ident(node ast.Ident) {
 		}
 	}
 	g.write(g.get_ternary_name(name))
-	if is_auto_heap {
+	if is_auto_heap && !g.inside_assign_fn_var {
 		g.write('))')
 		if is_option && node.or_expr.kind != .absent {
 			g.write('.data')

vlib/v/gen/c/testdata/assign_fn_addr.c.must_have

@@ -0,0 +1,2 @@
+string (*cb2) (void) = &cb1;
+string (*cb3) (void) = &cb2;

vlib/v/gen/c/testdata/assign_fn_addr.out

@@ -0,0 +1,3 @@
+true
+true
+true

vlib/v/gen/c/testdata/assign_fn_addr.vv

@@ -0,0 +1,12 @@
+const hello = 'hello world!'
+
+fn main() {
+	cb1 := fn () string {
+		return hello
+	}
+	cb2 := &cb1
+	cb3 := &cb2
+	println(voidptr(cb1) != voidptr(cb2))
+	println(voidptr(cb2) != voidptr(cb3))
+	println(voidptr(cb1) != voidptr(cb3))
+}