update comment based on further research

Author: RalfJung

src/cargo/core/compiler/fingerprint.rs

@@ -744,10 +744,23 @@ where
                 return true;
             }
         };
-        // Equal mtimes could mean that the input was changed in that same second,
-        // but *after* the output was generated.  So this means they are stale.
-        // In theory, cargo is using nanosecond precision throughout so this
-        // should not make a difference -- but it was necessary to fix #5918.
+
+        // Note that equal mtimes are considered "stale". For filesystems with
+        // not much timestamp precision like 1s this is a conservative approximation
+        // to handle the case where a file is modified within the same second after
+        // a build finishes. We want to make sure that incremental rebuilds pick that up!
+        //
+        // For filesystems with nanosecond precision it's been seen in the wild that
+        // its "nanosecond precision" isn't really nanosecond-accurate. It turns out that
+        // kernels may cache the current time so files created at different times actually
+        // list the same nanosecond precision. Some digging on #5919 picked up that the
+        // kernel caches the current time between timer ticks, which could mean that if
+        // a file is updated at most 10ms after a build finishes then Cargo may not
+        // pick up the build changes.
+        //
+        // All in all, the equality check here is a conservative assumption that,
+        // if equal, files were changed just after a previous build finished.
+        // It's hoped this doesn't cause too many issues in practice!
         if mtime2 >= mtime {
             info!("stale: {} -- {} vs {}", path.display(), mtime2, mtime);
             true