checker: add a check for str[i] = `a` (#6107)

vlib/v/checker/checker.v

@@ -3076,6 +3076,9 @@ pub fn (mut c Checker) index_expr(mut node ast.IndexExpr) table.Type {
 		typ_sym.name.ends_with('ptr')) && !typ.has_flag(.variadic) { // byteptr, charptr etc
 		c.error('type `$typ_sym.name` does not support indexing', node.pos)
 	}
+	if typ_sym.kind == .string && !typ.is_ptr() && node.is_setter {
+		c.error('cannot assign to s[i] (strings are immutable)', node.pos)
+	}
 	if !c.inside_unsafe && (typ.is_ptr() || typ.is_pointer()) {
 		mut is_ok := false
 		if node.left is ast.Ident {

vlib/v/checker/tests/string_index_assign_error.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/string_index_assign_error.v:3:6: error: cannot assign to s[i] (strings are immutable)
+    1 | fn main() {
+    2 |     mut a := 'V is great!!'
+    3 |     a[0] = `R`
+      |      ~~~
+    4 | }

vlib/v/checker/tests/string_index_assign_error.vv

@@ -0,0 +1,4 @@
+fn main() {
+    mut a := 'V is great!!'
+    a[0] = `R`
+}