Store maximum queue length

Previously, the queue length was constantly decreasing as we built crates, which
meant that we were incorrectly displaying the progress bar. In debug builds,
this even led to panics (due to underflow on subtraction).

src/cargo/core/compiler/job_queue.rs

@@ -95,6 +95,9 @@ pub struct JobQueue<'a, 'cfg> {
 /// It is created from JobQueue when we have fully assembled the crate graph
 /// (i.e., all package dependencies are known).
 struct DrainState<'a, 'cfg> {
+    // This is the length of the DependencyQueue when starting out
+    total_units: usize,
+
     queue: DependencyQueue<Unit<'a>, Artifact, Job>,
     tx: Sender<Message>,
     rx: Receiver<Message>,
@@ -341,6 +344,7 @@ impl<'a, 'cfg> JobQueue<'a, 'cfg> {
         let (tx, rx) = channel();
         let progress = Progress::with_style("Building", ProgressStyle::Ratio, cx.bcx.config);
         let state = DrainState {
+            total_units: self.queue.len(),
             queue: self.queue,
             tx,
             rx,
@@ -713,7 +717,7 @@ impl<'a, 'cfg> DrainState<'a, 'cfg> {
             .collect::<Vec<_>>();
         drop(self.progress.tick_now(
             self.finished,
-            self.queue.len(),
+            self.total_units,
             &format!(": {}", active_names.join(", ")),
         ));
     }

src/cargo/util/progress.rs

@@ -224,6 +224,7 @@ impl<'cfg> State<'cfg> {
 
 impl Format {
     fn progress(&self, cur: usize, max: usize) -> Option<String> {
+        assert!(cur <= max);
         // Render the percentage at the far right and then figure how long the
         // progress bar is
         let pct = (cur as f64) / (max as f64);