head: don't add trailing newline to end of file

Prevent `head` from adding a trailing newline to the end of a file
that did not originally have one when using `head --lines=-0`.

src/uu/head/src/head.rs

@@ -38,6 +38,7 @@ mod options {
 mod lines;
 mod parse;
 mod take;
+use lines::lines;
 use lines::zlines;
 use take::take_all_but;
 use take::take_lines;
@@ -285,8 +286,8 @@ fn read_but_last_n_lines(
             stdout.write_all(&bytes?)?;
         }
     } else {
-        for line in take_all_but(input.lines(), n) {
-            println!("{}", line?);
+        for line in take_all_but(lines(input), n) {
+            print!("{}", line?);
         }
     }
     Ok(())

src/uu/head/src/lines.rs

@@ -1,11 +1,75 @@
+//  * This file is part of the uutils coreutils package.
+//  *
+//  * For the full copyright and license information, please view the LICENSE
+//  * file that was distributed with this source code.
 // spell-checker:ignore (vars) zline zlines
-
-//! Iterate over zero-terminated lines.
+//! Iterate over lines, including the line ending character(s).
+//!
+//! This module provides the [`lines`] and [`zlines`] functions,
+//! similar to the [`BufRead::lines`] method. While the
+//! [`BufRead::lines`] method yields [`String`] instances that do not
+//! include the line ending characters (`"\n"` or `"\r\n"`), our
+//! functions yield [`String`] instances that include the line ending
+//! characters. This is useful if the input data does not end with a
+//! newline character and you want to preserve the exact form of the
+//! input data.
 use std::io::BufRead;
 
 /// The zero byte, representing the null character.
 const ZERO: u8 = 0;
 
+/// Returns an iterator over the lines, including line ending characters.
+///
+/// This function is just like [`BufRead::lines`], but it includes the
+/// line ending characters in each yielded [`String`] if the input
+/// data has them.
+///
+/// # Examples
+///
+/// If the input data does not end with a newline character (`'\n'`),
+/// then the last [`String`] yielded by this iterator also does not
+/// end with a newline:
+///
+/// ```rust,ignore
+/// use std::io::BufRead;
+/// use std::io::Cursor;
+///
+/// let cursor = Cursor::new(b"x\ny\nz");
+/// let mut it = cursor.lines();
+///
+/// assert_eq!(it.next(), Some(String::from("x\n")));
+/// assert_eq!(it.next(), Some(String::from("y\n")));
+/// assert_eq!(it.next(), Some(String::from("z")));
+/// assert_eq!(it.next(), None);
+/// ```
+pub(crate) fn lines<B>(reader: B) -> Lines<B>
+where
+    B: BufRead,
+{
+    Lines { buf: reader }
+}
+
+/// An iterator over the lines of an instance of `BufRead`.
+///
+/// This struct is generally created by calling [`lines`] on a `BufRead`.
+/// Please see the documentation of [`lines`] for more details.
+pub(crate) struct Lines<B> {
+    buf: B,
+}
+
+impl<B: BufRead> Iterator for Lines<B> {
+    type Item = std::io::Result<String>;
+
+    fn next(&mut self) -> Option<std::io::Result<String>> {
+        let mut buf = String::new();
+        match self.buf.read_line(&mut buf) {
+            Ok(0) => None,
+            Ok(_n) => Some(Ok(buf)),
+            Err(e) => Some(Err(e)),
+        }
+    }
+}
+
 /// Returns an iterator over the lines of the given reader.
 ///
 /// The iterator returned from this function will yield instances of
@@ -50,6 +114,7 @@ impl<B: BufRead> Iterator for ZLines<B> {
 #[cfg(test)]
 mod tests {
 
+    use crate::lines::lines;
     use crate::lines::zlines;
     use std::io::Cursor;
 
@@ -72,4 +137,15 @@ mod tests {
         assert_eq!(iter.next(), Some(b"z".to_vec()));
         assert_eq!(iter.next(), None);
     }
+
+    #[test]
+    fn test_lines() {
+        let cursor = Cursor::new(b"x\ny\nz");
+        let mut it = lines(cursor).map(|l| l.unwrap());
+
+        assert_eq!(it.next(), Some(String::from("x\n")));
+        assert_eq!(it.next(), Some(String::from("y\n")));
+        assert_eq!(it.next(), Some(String::from("z")));
+        assert_eq!(it.next(), None);
+    }
 }

tests/by-util/test_head.rs

@@ -157,11 +157,17 @@ fn test_negative_byte_syntax() {
 #[test]
 fn test_negative_zero_lines() {
     new_ucmd!()
-        .args(&["--lines=-0"])
+        .arg("--lines=-0")
         .pipe_in("a\nb\n")
         .succeeds()
         .stdout_is("a\nb\n");
+    new_ucmd!()
+        .arg("--lines=-0")
+        .pipe_in("a\nb")
+        .succeeds()
+        .stdout_is("a\nb");
 }
+
 #[test]
 fn test_negative_zero_bytes() {
     new_ucmd!()