back out 5919

Author: Eh2406

src/cargo/core/compiler/fingerprint.rs

@@ -761,8 +761,9 @@ where
             }
         };
 
-        // Note that equal mtimes are considered "stale". For filesystems with
-        // not much timestamp precision like 1s this is a conservative approximation
+        // TODO: fix #5918
+        // Note that equal mtimes should be considered "stale". For filesystems with
+        // not much timestamp precision like 1s this is would be a conservative approximation
         // to handle the case where a file is modified within the same second after
         // a build starts. We want to make sure that incremental rebuilds pick that up!
         //
@@ -774,10 +775,11 @@ where
         // a file is updated at most 10ms after a build starts then Cargo may not
         // pick up the build changes.
         //
-        // All in all, the equality check here is a conservative assumption that,
+        // All in all, an equality check here would be 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 {
+        // Unfortunately this became problematic when (in #6484) cargo switch to more accurately
+        // measuring the start time of builds.
+        if mtime2 > mtime {
             info!("stale: {} -- {} vs {}", path.display(), mtime2, mtime);
             true
         } else {

tests/testsuite/freshness.rs

@@ -1283,6 +1283,9 @@ fn bust_patched_dep() {
         .build();
 
     p.cargo("build").run();
+    if is_coarse_mtime() {
+        sleep_ms(1000);
+    }
 
     File::create(&p.root().join("reg1new/src/lib.rs")).unwrap();
     if is_coarse_mtime() {
@@ -1385,6 +1388,7 @@ fn rebuild_on_mid_build_file_modification() {
 
     let t = thread::spawn(move || {
         let socket = server.accept().unwrap().0;
+        sleep_ms(1000);
         let mut file = OpenOptions::new()
             .write(true)
             .append(true)

tests/testsuite/path.rs

@@ -356,6 +356,7 @@ fn deep_dependencies_trigger_rebuild() {
     //
     // We base recompilation off mtime, so sleep for at least a second to ensure
     // that this write will change the mtime.
+    sleep_ms(1000);
     File::create(&p.root().join("baz/src/baz.rs"))
         .unwrap()
         .write_all(br#"pub fn baz() { println!("hello!"); }"#)
@@ -372,6 +373,7 @@ fn deep_dependencies_trigger_rebuild() {
         .run();
 
     // Make sure an update to bar doesn't trigger baz
+    sleep_ms(1000);
     File::create(&p.root().join("bar/src/bar.rs"))
         .unwrap()
         .write_all(