checker: disallow `mut_ptr = &immutable_var`

Previously only initialization was detected.
Also add fn Ident.is_mut().

vlib/v/ast/ast.v

@@ -794,7 +794,21 @@ pub mut:
 	name   string
 	kind   IdentKind
 	info   IdentInfo
-	is_mut bool
+	is_mut bool // if mut *token* is before name. Use `is_mut()` to lookup mut variable
+}
+
+pub fn (i &Ident) is_mut() bool {
+	match i.obj {
+		Var {
+			return i.obj.is_mut
+		}
+		ConstField {
+			return false
+		}
+		AsmRegister, GlobalField {
+			return true
+		}
+	}
 }
 
 pub fn (i &Ident) var_info() IdentVar {

vlib/v/checker/assign.v

@@ -559,15 +559,10 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 					if right_node.right.obj is ast.Var {
 						v := right_node.right.obj
 						right_type0 = v.typ
-						if !v.is_mut && assigned_var.is_mut && !c.inside_unsafe {
-							c.error('`$right_node.right.name` is immutable, cannot have a mutable reference to it',
-								right_node.pos)
-						}
-					} else if right_node.right.obj is ast.ConstField {
-						if assigned_var.is_mut && !c.inside_unsafe {
-							c.error('`$right_node.right.name` is immutable, cannot have a mutable reference to it',
-								right_node.pos)
-						}
+					}
+					if !c.inside_unsafe && assigned_var.is_mut() && !right_node.right.is_mut() {
+						c.error('`$right_node.right.name` is immutable, cannot have a mutable reference to it',
+							right_node.pos)
 					}
 				}
 			}

vlib/v/checker/tests/modify_const_with_ref.out

@@ -1,14 +0,0 @@
-vlib/v/checker/tests/modify_const_with_ref.vv:11:11: 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:9:6: error: unused variable: `unused_var`
-    7 |
-    8 | fn main() {
-    9 |     mut unused_var := Foo{}
-      |         ~~~~~~~~~~
-   10 |     unused_var = Foo{}
-   11 |     mut c := &constant

vlib/v/checker/tests/modify_const_with_ref.vv

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

vlib/v/checker/tests/mut_assign_ref.out

@@ -0,0 +1,28 @@
+vlib/v/checker/tests/mut_assign_ref.vv:10:12: error: `f` is immutable, cannot have a mutable reference to it
+    8 | fn main() {
+    9 |     f := Foo{}
+   10 |     mut pf := &f
+      |               ^
+   11 |     pf = &f
+   12 |     p := &f
+vlib/v/checker/tests/mut_assign_ref.vv:11:7: error: `f` is immutable, cannot have a mutable reference to it
+    9 |     f := Foo{}
+   10 |     mut pf := &f
+   11 |     pf = &f
+      |          ^
+   12 |     p := &f
+   13 |     _ = p
+vlib/v/checker/tests/mut_assign_ref.vv:15:11: error: `constant` is immutable, cannot have a mutable reference to it
+   13 |     _ = p
+   14 | 
+   15 |     mut c := &constant
+      |              ^
+   16 |     c.value = 200
+   17 |     c = &constant
+vlib/v/checker/tests/mut_assign_ref.vv:17:6: error: `constant` is immutable, cannot have a mutable reference to it
+   15 |     mut c := &constant
+   16 |     c.value = 200
+   17 |     c = &constant
+      |         ^
+   18 |     ic := &constant // ok
+   19 |     _=ic

vlib/v/checker/tests/mut_assign_ref.vv

@@ -0,0 +1,24 @@
+struct Foo {
+mut:
+	value int
+}
+
+const constant = Foo{ 100 }
+
+fn main() {
+	f := Foo{}
+	mut pf := &f
+	pf = &f
+	p := &f
+	_ = p
+
+	mut c := &constant
+	c.value = 200
+	c = &constant
+	ic := &constant // ok
+	_=ic
+	
+	mut mf := f
+	pf = &mf // ok
+	_ = pf
+}