From d2ede927367d87a79901cf45e4fee2dfa1c8f7dc Mon Sep 17 00:00:00 2001
From: ALXD <github@alxd.org>
Date: Mon, 20 Nov 2023 14:19:20 +0100
Subject: [PATCH 1/2] expr: make error messages align with GNU expr

---
 src/uu/expr/src/syntax_tree.rs | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/src/uu/expr/src/syntax_tree.rs b/src/uu/expr/src/syntax_tree.rs
index c55fb0bdc..b19c13c0a 100644
--- a/src/uu/expr/src/syntax_tree.rs
+++ b/src/uu/expr/src/syntax_tree.rs
@@ -13,6 +13,7 @@
 use num_bigint::BigInt;
 use num_traits::Zero;
 use onig::{Regex, RegexOptions, Syntax};
+use uucore::display::Quotable;
 
 use crate::tokens::Token;
 
@@ -214,7 +215,7 @@ pub fn tokens_to_ast(
         assert!(op_stack.is_empty());
 
         maybe_dump_rpn(&out_stack);
-        let result = ast_from_rpn(&mut out_stack);
+        let result = ast_from_rpn(&mut out_stack, None);
         if out_stack.is_empty() {
             maybe_dump_ast(&result);
             result
@@ -253,9 +254,12 @@ fn maybe_dump_rpn(rpn: &TokenStack) {
     }
 }
 
-fn ast_from_rpn(rpn: &mut TokenStack) -> Result<Box<AstNode>, String> {
+fn ast_from_rpn(rpn: &mut TokenStack, op_type: Option<&str>) -> Result<Box<AstNode>, String> {
     match rpn.pop() {
-        None => Err("syntax error (premature end of expression)".to_owned()),
+        None => Err(match op_type {
+            Some(value) => format!("syntax error: unexpected argument {}", value.quote()),
+            None => "missing operand".to_owned(),
+        }),
 
         Some((token_idx, Token::Value { value })) => Ok(AstNode::new_leaf(token_idx, &value)),
 
@@ -281,7 +285,7 @@ fn maybe_ast_node(
 ) -> Result<Box<AstNode>, String> {
     let mut operands = Vec::with_capacity(arity);
     for _ in 0..arity {
-        let operand = ast_from_rpn(rpn)?;
+        let operand = ast_from_rpn(rpn, Some(op_type))?;
         operands.push(operand);
     }
     operands.reverse();

From 8b650a7a9b5c6356425c37fa0ef49c28c49aea76 Mon Sep 17 00:00:00 2001
From: ALXD <github@alxd.org>
Date: Mon, 20 Nov 2023 14:29:06 +0100
Subject: [PATCH 2/2] expr: add tests for precise error messages

---
 tests/by-util/test_expr.rs | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/tests/by-util/test_expr.rs b/tests/by-util/test_expr.rs
index 28cfcf0ec..18125fe99 100644
--- a/tests/by-util/test_expr.rs
+++ b/tests/by-util/test_expr.rs
@@ -6,6 +6,14 @@
 
 use crate::common::util::TestScenario;
 
+#[test]
+fn test_no_arguments() {
+    new_ucmd!()
+        .fails()
+        .code_is(2)
+        .stderr_only("expr: missing operand\n");
+}
+
 #[test]
 fn test_simple_values() {
     // null or 0 => EXIT_VALUE == 1
@@ -275,6 +283,12 @@ fn test_substr() {
 
 #[test]
 fn test_invalid_substr() {
+    new_ucmd!()
+        .args(&["56", "substr"])
+        .fails()
+        .code_is(2)
+        .stderr_only("expr: syntax error: unexpected argument 'substr'\n");
+
     new_ucmd!()
         .args(&["substr", "abc", "0", "1"])
         .fails()