find: reject a signed -maxdepth/-mindepth value to match GNU

src/find/matchers/mod.rs

@@ -307,8 +307,10 @@ fn convert_arg_to_number(
     option_name: &str,
     value_as_string: &str,
 ) -> Result<usize, Box<dyn Error>> {
+    // Only accept plain decimal digits. Rust's `usize::from_str` also accepts a
+    // leading '+', but GNU find rejects a signed value like "+1" here.
     match value_as_string.parse::<usize>() {
-        Ok(val) => Ok(val),
+        Ok(val) if value_as_string.bytes().all(|b| b.is_ascii_digit()) => Ok(val),
         _ => Err(From::from(format!(
             "Expected a positive decimal integer argument to {option_name}, but got \
              `{value_as_string}'"

tests/test_find.rs

@@ -157,6 +157,18 @@ fn size_rejects_non_numeric_prefix() {
     }
 }
 
+#[test]
+fn depth_rejects_signed_value() {
+    // A signed depth like "+1" is rejected, matching GNU find.
+    for opt in ["-maxdepth", "-mindepth"] {
+        ucmd()
+            .args(&["./test_data", opt, "+1"])
+            .fails()
+            .stderr_contains("positive decimal integer argument");
+        ucmd().args(&["./test_data", opt, "1"]).succeeds();
+    }
+}
+
 #[test]
 fn multiple_matcher_failure() {
     ucmd()