Fix detecting mutable slice including for `const`

vlib/v/ast/ast.v

@@ -792,7 +792,18 @@ 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 }
+		GlobalField { return true }
+		else {
+			panic('TODO')
+		}
+	}
 }
 
 pub fn (i &Ident) var_info() IdentVar {

vlib/v/checker/assign.v

@@ -338,7 +338,7 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 		if left_sym.kind == .array && mut left is ast.Ident && mut right is ast.IndexExpr {
 			sym := c.table.sym(right.left_type)
 			if sym.kind == .array && mut right.left is ast.Ident {
-				if left.is_mut && !right.left.is_mut {
+				if left.is_mut && !right.left.is_mut() {
 					if right.index is ast.RangeExpr {
 						c.error('cannot mutate immutable slice', right.left.pos)
 					} else {

vlib/v/checker/tests/index_expr_mutation.vv

@@ -1,7 +1,11 @@
+const imm_arr = [10, 20]
+
 fn test_immutable_slice() {
 	arr := [1, 2, 3, 4, 5]
 	mut s := arr[1..2]
 	s[0] = 0
+	mut s2 := imm_arr[..]
+	s2[0] = 0
 }
 
 fn test_immutable_element() {