Use ANSI escape code to clear line

This eliminates the trailing spaces that Cargo used to use for clearing
a line of output. Trailing spaces are problematic because they take up
space on the next line when reducing the width of a terminal.

src/cargo/core/shell.rs

@@ -120,6 +120,13 @@ impl Shell {
         self.err.as_write()
     }
 
+    /// Erase from cursor to end of line.
+    pub fn err_erase_line(&mut self) {
+        if let ShellOut::Stream { tty: true, .. } = self.err {
+            imp::err_erase_line(self);
+        }
+    }
+
     /// Shortcut to right-align and color green a status message.
     pub fn status<T, U>(&mut self, status: T, message: U) -> CargoResult<()>
     where
@@ -332,6 +339,8 @@ mod imp {
 
     use libc;
 
+    use super::Shell;
+
     pub fn stderr_width() -> Option<usize> {
         unsafe {
             let mut winsize: libc::winsize = mem::zeroed();
@@ -345,10 +354,19 @@ mod imp {
             }
         }
     }
+
+    pub fn err_erase_line(shell: &mut Shell) {
+        // This is the "EL - Erase in Line" sequence. It clears from the cursor
+        // to the end of line.
+        // https://en.wikipedia.org/wiki/ANSI_escape_code#CSI_sequences
+        let _ = shell.err().write_all(b"\x1B[K");
+    }
 }
 
 #[cfg(all(unix, not(any(target_os = "linux", target_os = "macos"))))]
 mod imp {
+    pub(super) use super::default_err_erase_line as err_erase_line;
+
     pub fn stderr_width() -> Option<usize> {
         None
     }
@@ -366,6 +384,8 @@ mod imp {
     use self::winapi::um::wincon::*;
     use self::winapi::um::winnt::*;
 
+    pub(super) use super::default_err_erase_line as err_erase_line;
+
     pub fn stderr_width() -> Option<usize> {
         unsafe {
             let stdout = GetStdHandle(STD_ERROR_HANDLE);
@@ -408,3 +428,14 @@ mod imp {
         }
     }
 }
+
+#[cfg(any(
+    all(unix, not(any(target_os = "linux", target_os = "macos"))),
+    windows,
+))]
+fn default_err_erase_line(shell: &mut Shell) {
+    if let Some(max_width) = imp::stderr_width() {
+        let blank = " ".repeat(max_width);
+        drop(write!(shell.err(), "{}\r", blank));
+    }
+}

src/cargo/util/progress.rs

@@ -196,9 +196,7 @@ impl<'cfg> State<'cfg> {
     }
 
     fn clear(&mut self) {
-        self.try_update_max_width();
-        let blank = " ".repeat(self.format.max_width);
-        drop(write!(self.config.shell().err(), "{}\r", blank));
+        self.config.shell().err_erase_line();
     }
 
     fn try_update_max_width(&mut self) {