ln: report same file for any backup mode

src/uu/ln/src/ln.rs

@@ -406,8 +406,9 @@ pub fn link(src: &Path, dst: &Path, settings: &Settings) -> LnResult<()> {
 
     if dst.is_symlink() || dst.exists() {
         backup_path = backup_control::get_backup_path(settings.backup, dst, &settings.suffix);
-        if settings.backup == BackupMode::Existing && !settings.symbolic {
-            // when ln --backup f f, it should detect that it is the same file
+        if settings.backup != BackupMode::None && !settings.symbolic {
+            // `ln --backup f f`: the backup would move `f` away before the
+            // link is made, so detect the same file first, for any backup mode
             if paths_refer_to_same_file(src, dst, true) && is_same_entry(src, dst) {
                 return Err(LnError::SameFile(src.to_owned(), dst.to_owned()));
             }

tests/by-util/test_ln.rs

@@ -1016,6 +1016,42 @@ fn test_backup_same_file() {
         .stderr_contains("n: 'file1' and './file1' are the same file");
 }
 
+#[test]
+fn test_backup_simple_numbered_same_file() {
+    for backup in ["--backup=simple", "--backup=numbered"] {
+        for args in [
+            vec![backup, "a", "a"],
+            vec!["-f", backup, "a", "a"],
+            vec!["-f", backup, "./a", "a"],
+        ] {
+            let (at, mut ucmd) = at_and_ucmd!();
+            at.write("a", "content");
+
+            ucmd.args(&args)
+                .fails_with_code(1)
+                .stderr_contains("are the same file");
+
+            assert_eq!(at.read("a"), "content");
+            assert!(!at.file_exists("a~"));
+            assert!(!at.file_exists("a.~1~"));
+        }
+    }
+}
+
+#[test]
+fn test_backup_simple_same_file_into_directory() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.mkdir("d");
+    at.write("d/a", "content");
+
+    ucmd.args(&["-f", "--backup=simple", "d/a", "d"])
+        .fails_with_code(1)
+        .stderr_contains("are the same file");
+
+    assert_eq!(at.read("d/a"), "content");
+    assert!(!at.file_exists("d/a~"));
+}
+
 #[test]
 #[cfg(not(target_os = "android"))]
 fn test_backup_existing_hard_linked_under_different_name() {