checker: do not allow modifying consts via mutable refs

vlib/os/os_test.v

@@ -586,3 +586,7 @@ fn test_truncate() {
 	assert newlen == os.file_size(filename)
 	os.rm(filename) or { panic(err) }
 }
+
+fn test_hostname() {
+	assert os.hostname().len > 2
+}

vlib/v/checker/checker.v

@@ -3222,7 +3222,8 @@ pub fn (mut c Checker) assign_stmt(mut assign_stmt ast.AssignStmt) {
 			c.type_implements(right_type, left_type, right.position())
 		}
 	}
-	// this needs to run after the assign stmt left exprs have been run through checker so that ident.obj is set
+	// this needs to run after the assign stmt left exprs have been run through checker
+	// so that ident.obj is set
 	// Check `x := &y` and `mut x := <-ch`
 	if right_first is ast.PrefixExpr {
 		node := right_first
@@ -3237,15 +3238,20 @@ pub fn (mut c Checker) assign_stmt(mut assign_stmt ast.AssignStmt) {
 			c.inside_ref_lit = (c.inside_ref_lit || node.op == .amp || is_shared)
 			c.expr(node.right)
 			c.inside_ref_lit = old_inside_ref_lit
+			if node.op == .amp {
 				if node.right is ast.Ident {
 					if node.right.obj is ast.Var {
 						v := node.right.obj
 						right_type0 = v.typ
-					if node.op == .amp {
 						if !v.is_mut && assigned_var.is_mut && !c.inside_unsafe {
 							c.error('`$node.right.name` is immutable, cannot have a mutable reference to it',
 								node.pos)
 						}
+					} else if node.right.obj is ast.ConstField {
+						if assigned_var.is_mut && !c.inside_unsafe {
+							c.error('`$node.right.name` is immutable, cannot have a mutable reference to it',
+								node.pos)
+						}
 					}
 				}
 			}

vlib/v/checker/tests/modify_const_with_ref.out

@@ -0,0 +1,14 @@
+g.v:9:6: warning: unused variable: `unused_var`
+    7 |
+    8 | fn main() {
+    9 |     mut unused_var := Foo{}
+      |         ~~~~~~~~~~
+   10 |     unused_var = Foo{}
+   11 |     mut c := &constant
+g.v:11:14: error: `constant` is immutable, cannot have a mutable reference to it
+    9 |     mut unused_var := Foo{}
+   10 |     unused_var = Foo{}
+   11 |     mut c := &constant
+      |              ^
+   12 |     c.value = 200
+   13 | }

vlib/v/checker/tests/modify_const_with_ref.vv

@@ -0,0 +1,13 @@
+struct Foo {
+mut:
+	value int
+}
+
+const constant = Foo{ 100 }
+
+fn main() {
+	mut unused_var := Foo{}
+	unused_var = Foo{}
+	mut c := &constant
+	c.value = 200
+}