tr: fix unescaped trailing backslash warning

This warning is being emitted whenever the input string ends with a
backslash, even if the backslash is escaped.

src/uu/tr/src/tr.rs

@@ -98,11 +98,24 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
     }
 
     if let Some(first) = sets.first() {
-        if let Some(b'\\') = os_str_as_bytes(first)?.last() {
-            show!(USimpleError::new(
-                0,
-                "warning: an unescaped backslash at end of string is not portable"
-            ));
+        let slice = os_str_as_bytes(first)?;
+
+        let mut iter = slice.iter();
+
+        if let Some(b'\\') = iter.next_back() {
+            match iter.next_back() {
+                Some(b'\\') => {
+                    // The trailing backslash has a backslash preceding it, so it is properly escaped
+                }
+                _ => {
+                    // The trailing backslash has a non-backslash character before it OR is the only character in the
+                    // string, so the warning applies
+                    show!(USimpleError::new(
+                        0,
+                        "warning: an unescaped backslash at end of string is not portable"
+                    ));
+                }
+            }
         }
     }
 

tests/by-util/test_tr.rs

@@ -1445,3 +1445,26 @@ fn test_truncate_non_utf8_set() {
         .succeeds()
         .stdout_is_bytes(b"\x010mp12");
 }
+
+#[test]
+#[cfg(unix)]
+fn test_unescaped_backslash_warning_false_positive() {
+    // Was erroneously printing this warning (even though the backslash was escaped):
+    // "tr: warning: an unescaped backslash at end of string is not portable"
+    new_ucmd!()
+        .args(&["-d", r"\\"])
+        .pipe_in(r"a\b\c\")
+        .succeeds()
+        .stdout_only("abc");
+}
+
+#[test]
+#[cfg(unix)]
+fn test_trailing_backslash_is_only_input_character() {
+    new_ucmd!()
+        .args(&["-d", r"\"])
+        .pipe_in(r"a\b\c\")
+        .succeeds()
+        .stderr_is("tr: warning: an unescaped backslash at end of string is not portable\n")
+        .stdout_is("abc");
+}