ln: Fix how it selects the form

ln had a bunch of problems:

1. `ln -s target` didn't work (2nd form in help).
2. `ln -t tmp` wasn't an error. We should check if files are
   empty first.
3. `ln -s file dir` didn't create dir/file.
4. `ln -s -T file dir` was removing `dir`.
5. Test cases for 4 say this is for compatibility with GNU
   coreutils but I couldn't find this feature.

src/ln/ln.rs

@@ -143,39 +143,45 @@ pub fn uumain(args: Vec<String>) -> i32 {
 }
 
 fn exec(files: &[PathBuf], settings: &Settings) -> i32 {
-    match settings.target_dir {
-        Some(ref name) => return link_files_in_dir(files, &PathBuf::from(name), &settings),
-        None => {}
+    if files.len() == 0 {
+        show_error!("missing file operand\nTry '{} --help' for more information.", NAME);
+        return 1;
     }
-    match files.len() {
-        0 => {
-            show_error!("missing file operand\nTry '{} --help' for more information.", NAME);
+
+    // Handle cases where we create links in a directory first.
+    if let Some(ref name) = settings.target_dir {
+        // 4th form: a directory is specified by -t.
+        return link_files_in_dir(files, &PathBuf::from(name), &settings);
+    }
+    if !settings.no_target_dir {
+        if files.len() == 1 {
+            // 2nd form: the target directory is the current directory.
+            return link_files_in_dir(files, &PathBuf::from("."), &settings);
+        }
+        let last_file = &PathBuf::from(files.last().unwrap());
+        if files.len() > 2 || last_file.is_dir() {
+            // 3rd form: create links in the last argument.
+            return link_files_in_dir(&files[0..files.len()-1], last_file, &settings);
+        }
+    }
+
+    // 1st form. Now there should be only two operands, but if -T is
+    // specified we may have a wrong number of operands.
+    if files.len() == 1 {
+        show_error!("missing destination file operand after '{}'", files[0].to_string_lossy());
+        return 1;
+    }
+    if files.len() > 2 {
+        show_error!("extra operand '{}'\nTry '{} --help' for more information.", files[2].display(), NAME);
+        return 1;
+    }
+    assert!(files.len() != 0);
+
+    match link(&files[0], &files[1], settings) {
+        Ok(_) => 0,
+        Err(e) => {
+            show_error!("{}", e);
             1
-        },
-        1 => match link(&files[0], &files[0], settings) {
-            Ok(_) => 0,
-            Err(e) => {
-                show_error!("{}", e);
-                1
-            }
-        },
-        2 => match link(&files[0], &files[1], settings) {
-            Ok(_) => 0,
-            Err(e) => {
-                show_error!("{}", e);
-                1
-            }
-        },
-        _ => {
-            if settings.no_target_dir {
-                show_error!("extra operand '{}'\nTry '{} --help' for more information.", files[2].display(), NAME);
-                return 1;
-            }
-            let (targets, dir) = match settings.target_dir {
-                Some(ref dir) => (files, PathBuf::from(dir.clone())),
-                None => (&files[0..files.len()-1], files[files.len()-1].clone())
-            };
-            link_files_in_dir(targets, &dir, settings)
         }
     }
 }
@@ -219,10 +225,6 @@ fn link_files_in_dir(files: &[PathBuf], target_dir: &PathBuf, settings: &Setting
 fn link(src: &PathBuf, dst: &PathBuf, settings: &Settings) -> Result<()> {
     let mut backup_path = None;
 
-    if dst.is_dir() && settings.no_target_dir {
-        try!(fs::remove_dir(dst));
-    }
-
     if is_symlink(dst) || dst.exists() {
         match settings.overwrite {
             OverwriteMode::NoClobber => {},

tests/test_ln.rs

@@ -274,7 +274,7 @@ fn test_symlink_target_dir_from_dir() {
 }
 
 #[test]
-fn test_symlink_overwrite_dir() {
+fn test_symlink_overwrite_dir_fail() {
     let (at, mut ucmd) = at_and_ucmd!();
     let path_a = "test_symlink_overwrite_dir_a";
     let path_b = "test_symlink_overwrite_dir_b";
@@ -282,35 +282,7 @@ fn test_symlink_overwrite_dir() {
     at.touch(path_a);
     at.mkdir(path_b);
 
-    ucmd.args(&["-s", "-T", path_a, path_b]).succeeds().no_stderr();
-
-    assert!(at.file_exists(path_a));
-    assert!(at.is_symlink(path_b));
-    assert_eq!(at.resolve_link(path_b), path_a);
-}
-
-#[test]
-fn test_symlink_overwrite_nonempty_dir() {
-    let (at, mut ucmd) = at_and_ucmd!();
-    let path_a = "test_symlink_overwrite_nonempty_dir_a";
-    let path_b = "test_symlink_overwrite_nonempty_dir_b";
-    let dummy = "test_symlink_overwrite_nonempty_dir_b/file";
-
-    at.touch(path_a);
-    at.mkdir(path_b);
-    at.touch(dummy);
-
-    // Not same error as GNU; the error message is a Rust builtin
-    // TODO: test (and implement) correct error message (or at least decide whether to do so)
-    // Current: "ln: error: Directory not empty (os error 66)"
-    // GNU:     "ln: cannot link 'a' to 'b': Directory not empty"
-    
-    // Verbose output for the link should not be shown on failure
-    assert!(ucmd.args(&["-v", "-T", "-s", path_a, path_b])
-            .fails().no_stdout().stderr.len() > 0);
-
-    assert!(at.file_exists(path_a));
-    assert!(at.dir_exists(path_b));
+    assert!(ucmd.args(&["-s", "-T", path_a, path_b]).fails().stderr.len() > 0);
 }
 
 #[test]
@@ -348,3 +320,58 @@ fn test_symlink_verbose() {
     scene.ucmd().args(&["-v", "-b", file_a, file_b])
         .succeeds().stdout_only(format!("'{}' -> '{}' (backup: '{}~')\n", file_b, file_a, file_b));
 }
+
+#[test]
+fn test_symlink_target_only() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    let dir = "test_symlink_target_only";
+
+    at.mkdir(dir);
+
+    assert!(ucmd.args(&["-s", "-t", dir]).fails().stderr.len() > 0);
+}
+
+#[test]
+fn test_symlink_implicit_target_dir() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    let dir = "test_symlink_implicit_target_dir";
+    let file = "test_symlink_implicit_target_dir/file";
+
+    at.mkdir(dir);
+    at.touch(file);
+
+    ucmd.args(&["-s", file]).succeeds().no_stderr();
+
+    assert!(at.file_exists("file"));
+    assert!(at.is_symlink("file"));
+    assert_eq!(at.resolve_link("file"), file);
+}
+
+#[test]
+fn test_symlink_to_dir_2args() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    let filename = "test_symlink_to_dir_2args_file";
+    let from_file = &format!("{}/{}", at.as_string(), filename);
+    let to_dir = "test_symlink_to_dir_2args_to_dir";
+    let to_file = &format!("{}/{}", to_dir, filename);
+
+    at.mkdir(to_dir);
+    at.touch(from_file);
+
+    ucmd.args(&["-s", from_file, to_dir]).succeeds().no_stderr();
+
+    assert!(at.file_exists(to_file));
+    assert!(at.is_symlink(to_file));
+    assert_eq!(at.resolve_link(to_file), filename);
+}
+
+#[test]
+fn test_symlink_missing_destination() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    let file = "test_symlink_missing_destination";
+
+    at.touch(file);
+
+    ucmd.args(&["-s", "-T", file]).fails()
+        .stderr_is(format!("ln: error: missing destination file operand after '{}'", file));
+}