make the unchanged mutable variable error a warning in non-prod builds

vlib/compiler/compile_errors.v

@@ -23,6 +23,14 @@ fn (p mut Parser) error(s string) {
 	p.error_with_token_index(s, p.token_idx - 1)
 }
 
+fn (p mut Parser) warn_or_error(s string) {
+	if p.pref.is_prod {
+		p.error(s)
+	} else {
+		p.warn(s)
+	}
+}
+
 fn (p mut Parser) warn(s string) {
 	p.warn_with_token_index(s, p.token_idx - 1)
 }

vlib/compiler/fn.v

@@ -645,7 +645,7 @@ fn (p mut Parser) check_unused_and_mut_vars() {
 			p.production_error_with_token_index('`$var.name` declared and not used', var.token_idx)
 		}
 		if !var.is_changed && var.is_mut && !p.pref.is_repl && !p.pref.translated && var.typ != 'T*' && p.mod != 'ui' && var.typ != 'App*' {
-			p.error_with_token_index('`$var.name` is declared as mutable, but it was never changed', var.token_idx)
+			p.warn_or_error('`$var.name` is declared as mutable, but it was never changed')
 		}
 	}
 }