install: refuse a -t target that is not a directory with one source

src/uu/install/locales/en-US.ftl

@@ -39,6 +39,7 @@ install-error-invalid-user = invalid user: { $user }
 install-error-invalid-group = invalid group: { $group }
 install-error-omitting-directory = omitting directory { $path }
 install-error-not-a-directory = failed to access { $path }: Not a directory
+install-error-target-dir-access = failed to access { $path }: { $error }
 install-error-existing-file-not-directory = cannot create directory { $path }: File exists
 install-error-override-directory-failed = cannot overwrite directory { $dir } with non-directory { $file }
 install-error-same-file = { $file1 } and { $file2 } are the same file

src/uu/install/locales/fr-FR.ftl

@@ -39,6 +39,7 @@ install-error-invalid-user = utilisateur invalide : { $user }
 install-error-invalid-group = groupe invalide : { $group }
 install-error-omitting-directory = omission du répertoire { $path }
 install-error-not-a-directory = échec de l'accès à { $path } : N'est pas un répertoire
+install-error-target-dir-access = échec de l'accès à { $path } : { $error }
 install-error-existing-file-not-directory = impossible de créer le répertoire { $path } : Le fichier existe
 install-error-override-directory-failed = impossible d'écraser le répertoire { $dir } avec un non-répertoire { $file }
 install-error-same-file = { $file1 } et { $file2 } sont le même fichier

src/uu/install/src/install.rs

@@ -117,6 +117,9 @@ enum InstallError {
     #[error("{}", translate!("install-error-not-a-directory", "path" => .0.quote()))]
     NotADirectory(PathBuf),
 
+    #[error("{}", translate!("install-error-target-dir-access", "path" => .0.quote(), "error" => strip_errno(.1)))]
+    TargetDirAccess(PathBuf, #[source] std::io::Error),
+
     #[error("{}", translate!("install-error-existing-file-not-directory", "path" => .0.quote()))]
     ExistingFileNotADirectory(PathBuf),
 
@@ -759,7 +762,9 @@ fn standard(mut paths: Vec<OsString>, b: &Behavior) -> UResult<()> {
         }
     }
 
-    if sources.len() > 1 {
+    // A `-t` target is always the directory to install into, never the file
+    // to install as, even with a single source.
+    if sources.len() > 1 || b.target_dir.is_some() {
         copy_files_into_dir(sources, &target, b)
     } else {
         let source = sources.first().unwrap();
@@ -835,6 +840,13 @@ fn metadata_for_source(path: &Path) -> UResult<fs::Metadata> {
 ///
 fn copy_files_into_dir(files: &[PathBuf], target_dir: &Path, b: &Behavior) -> UResult<()> {
     if !target_dir.is_dir() {
+        if b.target_dir.is_some() {
+            // GNU names why the `-t` directory cannot be used.
+            let reason = metadata(target_dir)
+                .err()
+                .unwrap_or_else(|| rustix::io::Errno::NOTDIR.into());
+            return Err(InstallError::TargetDirAccess(target_dir.to_path_buf(), reason).into());
+        }
         return Err(InstallError::TargetDirIsntDir(target_dir.to_path_buf()).into());
     }
     let mut installed_destinations: HashSet<PathBuf> = HashSet::with_capacity(files.len());

tests/by-util/test_install.rs

@@ -2203,6 +2203,32 @@ fn test_target_file_ends_with_slash() {
         .stderr_contains("failed to access 'dir/target_file/': Not a directory");
 }
 
+/// Without `-D`, a `-t` target that is not a directory is refused, even with a
+/// single source, rather than installed over or created as a file.
+#[test]
+fn test_install_target_dir_not_a_directory() {
+    let scene = TestScenario::new(util_name!());
+    let at = &scene.fixtures;
+    at.write("source", "new");
+    at.write("regular", "old");
+    at.symlink_file("nowhere", "dangling");
+
+    for (target, reason) in [
+        ("regular", "Not a directory"),
+        ("dangling", "No such file or directory"),
+        ("missing", "No such file or directory"),
+    ] {
+        scene
+            .ucmd()
+            .args(&["-t", target, "source"])
+            .fails()
+            .stderr_only(format!("install: failed to access '{target}': {reason}\n"));
+    }
+    assert_eq!(at.read("regular"), "old");
+    assert!(at.is_symlink("dangling"));
+    assert!(!at.file_exists("missing"));
+}
+
 #[test]
 fn test_install_root_combined() {
     let ts = TestScenario::new(util_name!());