fix cargo not doing anything when the input and output mtimes are equal

src/cargo/core/compiler/fingerprint.rs

@@ -744,7 +744,24 @@ where
                 return true;
             }
         };
-        if mtime2 > mtime {
+
+        // 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
         } else {

tests/testsuite/build_script.rs

@@ -2545,6 +2545,7 @@ fn rebuild_only_on_explicit_paths() {
     sleep_ms(1000);
     File::create(p.root().join("foo")).unwrap();
     File::create(p.root().join("bar")).unwrap();
+    sleep_ms(1000); // make sure the to-be-created outfile has a timestamp distinct from the infiles
 
     // now the exist, so run once, catch the mtime, then shouldn't run again
     println!("run with");