[Windows] Work around non-monotonic clocks in the self-profiler

On Windows, the high-resolution timestamp api doesn't seem to always be
monotonic. This can cause panics when the self-profiler uses the
`Instant` api to find elapsed time.

Work around this by detecting the case where now is less than the start
time and just use 0 elapsed ticks as the measurement.

Fixes rust-lang#51648

src/librustc/util/profiling.rs

@@ -12,7 +12,7 @@ use session::config::Options;
 
 use std::fs;
 use std::io::{self, StdoutLock, Write};
-use std::time::Instant;
+use std::time::{Duration, Instant};
 
 macro_rules! define_categories {
     ($($name:ident,)*) => {
@@ -197,7 +197,20 @@ impl SelfProfiler {
     }
 
     fn stop_timer(&mut self) -> u64 {
-        let elapsed = self.current_timer.elapsed();
+        let elapsed = if cfg!(windows) {
+            // On Windows, timers don't always appear to be monotonic (see #51648)
+            // which can lead to panics when calculating elapsed time.
+            // Work around this by testing to see if the current time is less than
+            // our recorded time, and if it is, just returning 0.
+            let now = Instant::now();
+            if self.current_timer >= now {
+                Duration::new(0, 0)
+            } else {
+                self.current_timer.elapsed()
+            }
+        } else {
+            self.current_timer.elapsed()
+        };
 
         self.current_timer = Instant::now();