ripgrep: move minimum version to Rust stable

This also updates some code to make use of our more liberal versioning
requirement, including the use of crossbeam-channel instead of the MsQueue
from the older an unmaintained crossbeam 0.3. This does regrettably add
a sizable number of dependencies, however, compile times seem mostly
unaffected.

Closes #1019

ignore/examples/walk.rs

@@ -1,14 +1,12 @@
-extern crate crossbeam;
+extern crate crossbeam_channel as channel;
 extern crate ignore;
 extern crate walkdir;
 
 use std::env;
 use std::io::{self, Write};
 use std::path::Path;
-use std::sync::Arc;
 use std::thread;
 
-use crossbeam::sync::MsQueue;
 use ignore::WalkBuilder;
 use walkdir::WalkDir;
 
@@ -16,7 +14,7 @@ fn main() {
     let mut path = env::args().nth(1).unwrap();
     let mut parallel = false;
     let mut simple = false;
-    let queue: Arc<MsQueue<Option<DirEntry>>> = Arc::new(MsQueue::new());
+    let (tx, rx) = channel::bounded::<DirEntry>(100);
     if path == "parallel" {
         path = env::args().nth(2).unwrap();
         parallel = true;
@@ -25,10 +23,9 @@ fn main() {
         simple = true;
     }
 
-    let stdout_queue = queue.clone();
     let stdout_thread = thread::spawn(move || {
         let mut stdout = io::BufWriter::new(io::stdout());
-        while let Some(dent) = stdout_queue.pop() {
+        for dent in rx {
             write_path(&mut stdout, dent.path());
         }
     });
@@ -36,26 +33,26 @@ fn main() {
     if parallel {
         let walker = WalkBuilder::new(path).threads(6).build_parallel();
         walker.run(|| {
-            let queue = queue.clone();
+            let tx = tx.clone();
             Box::new(move |result| {
                 use ignore::WalkState::*;
 
-                queue.push(Some(DirEntry::Y(result.unwrap())));
+                tx.send(DirEntry::Y(result.unwrap()));
                 Continue
             })
         });
     } else if simple {
         let walker = WalkDir::new(path);
         for result in walker {
-            queue.push(Some(DirEntry::X(result.unwrap())));
+            tx.send(DirEntry::X(result.unwrap()));
         }
     } else {
         let walker = WalkBuilder::new(path).build();
         for result in walker {
-            queue.push(Some(DirEntry::Y(result.unwrap())));
+            tx.send(DirEntry::Y(result.unwrap()));
         }
     }
-    queue.push(None);
+    drop(tx);
     stdout_thread.join().unwrap();
 }