expr: fail fast if there are no operands

src/uu/expr/src/expr.rs

@@ -5,7 +5,7 @@
 
 use clap::{crate_version, Arg, ArgAction, Command};
 use uucore::{
-    error::{UResult, USimpleError},
+    error::{UResult, USimpleError, UUsageError},
     format_usage, help_about, help_section, help_usage,
 };
 
@@ -58,6 +58,10 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         .map(|v| v.into_iter().map(|s| s.as_ref()).collect::<Vec<_>>())
         .unwrap_or_default();
 
+    if token_strings.is_empty() {
+        return Err(UUsageError::new(2, "missing operand"));
+    }
+
     match process_expr(&token_strings[..]) {
         Ok(expr_result) => print_expr_ok(&expr_result),
         Err(expr_error) => Err(USimpleError::new(2, &expr_error)),

tests/by-util/test_expr.rs

@@ -11,7 +11,7 @@ fn test_no_arguments() {
     new_ucmd!()
         .fails()
         .code_is(2)
-        .stderr_only("expr: missing operand\n");
+        .usage_error("missing operand");
 }
 
 #[test]