detect 1 in a million changes

src/cargo/core/compiler/fingerprint/mod.rs

@@ -1887,6 +1887,9 @@ where
         None
     };
 
+    let mut truncated_precision_count : u64 = 0;
+    let mut total_checked: u64 = 0;
+    let mut first : Option<StaleItem> = None;
     for path in paths {
         let path = path.as_ref();
 
@@ -1931,10 +1934,20 @@ where
         // Additionally, the build can span different volumes, some which support high-precision
         // file timestamps and some that don't (e.g. mounted Docker volumes). This can make
         // one of the timestamps lose the nanosecond part and appear up to a second in the past.
+        total_checked += 1;
         let truncated_precision =
             path_mtime.nanoseconds() == 0 || reference_mtime.nanoseconds() == 0;
 
         let fresh = if truncated_precision {
+            truncated_precision_count += 1;
+            if first.is_none() {
+               first = Some(StaleItem::ChangedFile {
+                   reference: reference.to_path_buf(),
+                   reference_mtime,
+                   stale: path.to_path_buf(),
+                   stale_mtime: path_mtime,
+               }); 
+            }
             path_mtime.unix_seconds() <= reference_mtime.unix_seconds()
         } else {
             path_mtime <= reference_mtime