This patch combines the two upstream commits below, the patched code has been
tested to build succesfully on both rustc 1.34 (in buster armel at the time of
writing) and rustc 1.41 (in buster on other architectures at the time of 
writing)

commit e383956f3fc9f313d8cf979f1a9772bea9eb1eb8
Author: gwenn <gtreguier@gmail.com>
Date:   Fri May 17 19:20:14 2019 +0200

    Try to fix nightly build
    
    See #217

commit 6e5099f644d81e8546bf4a1a2c21e002597f5bc6
Author: gwenn <gtreguier@gmail.com>
Date:   Fri May 17 20:52:06 2019 +0200

    Another try

Index: rust-rustyline-3.0.0/examples/example.rs
===================================================================
--- rust-rustyline-3.0.0.orig/examples/example.rs
+++ rust-rustyline-3.0.0/examples/example.rs
@@ -76,7 +76,7 @@ fn main() {
         let readline = rl.readline(PROMPT);
         match readline {
             Ok(line) => {
-                rl.add_history_entry(line.as_ref());
+                rl.add_history_entry(line.as_str());
                 println!("Line: {}", line);
             }
             Err(ReadlineError::Interrupted) => {
Index: rust-rustyline-3.0.0/src/history.rs
===================================================================
--- rust-rustyline-3.0.0.orig/src/history.rs
+++ rust-rustyline-3.0.0/src/history.rs
@@ -148,7 +148,7 @@ impl History {
         let file = File::open(&path)?;
         let rdr = BufReader::new(file);
         for line in rdr.lines() {
-            self.add(line?.as_ref()); // TODO truncate to MAX_LINE
+            self.add(line?); // TODO truncate to MAX_LINE
         }
         Ok(())
     }
Index: rust-rustyline-3.0.0/src/lib.rs
===================================================================
--- rust-rustyline-3.0.0.orig/src/lib.rs
+++ rust-rustyline-3.0.0/src/lib.rs
@@ -652,7 +652,7 @@ fn readline_raw<H: Helper>(
     let user_input = readline_edit(prompt, initial, editor, &original_mode);
     if editor.config.auto_add_history() {
         if let Ok(ref line) = user_input {
-            editor.add_history_entry(line.as_ref());
+            editor.add_history_entry(line.as_str());
         }
     }
     drop(guard); // disable_raw_mode(original_mode)?;