[Priyanshu-choudhary]

Fixes #15527

Problem

With --backup=simple or --backup=numbered, linking a file onto itself gives a misleading error. ln renames the destination to its backup first, so the source is gone by the time the hard link is created:

$ echo x > a
$ ln -f --backup=simple a a
ln: failed to create hard link 'a' => 'a': No such file or directory

GNU coreutils 9.12 reports ln: 'a' and 'a' are the same file. Both exit 1 and leave a unchanged.

The same thing happens without -f, with --backup=numbered, with ./a a, and with a directory target (ln -f --backup=simple d/a d).

Fix

link() already detects the same file before backing up, but only for --backup=existing. This PR runs that check for every backup mode except none, since every other mode renames the destination before linking. Symbolic links are still excluded: ln -sf --backup=simple a a stays valid and behaves like GNU.

Testing

Compared against GNU coreutils 9.12 (LC_ALL=C, Linux):

Command	Before	After / GNU 9.12
ln -f --backup=simple a a	No such file or directory	'a' and 'a' are the same file
ln -f --backup=numbered a a	No such file or directory	'a' and 'a' are the same file
ln --backup=simple a a (no -f)	No such file or directory	'a' and 'a' are the same file
ln -f --backup=numbered ./a a	No such file or directory	'./a' and 'a' are the same file
ln -f --backup=simple d/a d	No such file or directory	'd/a' and 'd/a' are the same file
ln -f --backup=simple sl sl (symlink)	No such file or directory	'sl' and 'sl' are the same file

Unchanged and still matching GNU: --backup=existing, --backup=none, -f -b a a, ln -sf --backup=simple a a (succeeds, keeps a~), and ln -f --backup=simple a b where b is a hard link to a (succeeds, creates b~).

• Added test_backup_simple_numbered_same_file and test_backup_simple_same_file_into_directory. Both failed before the fix and pass after it.
• cargo test --features ln --no-default-features test_ln: 64 passed.
• cargo fmt --check and cargo clippy -p uu_ln --all-targets -- -D warnings are clean.

[cakebaker]

Thanks for your PR!