Back touch the output instead of having a new important file.

This still round trips through the file system just in case it is on a different clock.

src/cargo/core/compiler/custom_build.rs

@@ -332,10 +332,14 @@ fn build_work<'a, 'cfg>(cx: &mut Context<'a, 'cfg>, unit: &Unit<'a>) -> CargoRes
             state.build_plan(invocation_name, cmd.clone(), Arc::new(Vec::new()));
         } else {
             state.running(&cmd);
+            // `invoked.timestamp` is used to get `FileTime::from_system_time(SystemTime::now());`
+            // using the exact clock that this file system is using.
+            let timestamp = output_file.with_file_name("invoked.timestamp");
             paths::write(
-                &output_file.with_file_name("invoked.timestamp"),
+                &timestamp,
                 b"This file has an mtime of when this build-script was started.",
             )?;
+            let timestamp = paths::mtime(&timestamp)?;
             let output = if extra_verbose {
                 let prefix = format!("[{} {}] ", id.name(), id.version());
                 state.capture_output(&cmd, Some(prefix), true)
@@ -358,6 +362,7 @@ fn build_work<'a, 'cfg>(cx: &mut Context<'a, 'cfg>, unit: &Unit<'a>) -> CargoRes
             // state informing what variables were discovered via our script as
             // well.
             paths::write(&output_file, &output.stdout)?;
+            filetime::set_file_times(output_file, timestamp, timestamp)?;
             paths::write(&err_file, &output.stderr)?;
             paths::write(&root_output_file, util::path2bytes(&script_out_dir)?)?;
             let parsed_output =

src/cargo/core/compiler/fingerprint.rs

@@ -239,7 +239,7 @@ impl Fingerprint {
         for local in self.local.iter() {
             match *local {
                 LocalFingerprint::MtimeBased(ref slot, ref path) => {
-                    let path = root.join(&path.with_file_name("invoked.timestamp"));
+                    let path = root.join(path);
                     let mtime = paths::mtime(&path)?;
                     *slot.0.lock().unwrap() = Some(mtime);
                 }
@@ -746,7 +746,7 @@ where
     I: IntoIterator,
     I::Item: AsRef<Path>,
 {
-    let mtime = match paths::mtime(&output.with_file_name("invoked.timestamp")) {
+    let mtime = match paths::mtime(output) {
         Ok(mtime) => mtime,
         Err(..) => return None,
     };

src/cargo/core/compiler/mod.rs

@@ -292,10 +292,14 @@ fn rustc<'a, 'cfg>(
         }
 
         state.running(&rustc);
+        // `invoked.timestamp` is used to get `FileTime::from_system_time(SystemTime::now());`
+        // using the exact clock that this file system is using.
+        let timestamp = dep_info_loc.with_file_name("invoked.timestamp");
         paths::write(
-            &dep_info_loc.with_file_name("invoked.timestamp"),
+            &timestamp,
             b"This file has an mtime of when rustc was started.",
         )?;
+        let timestamp = paths::mtime(&timestamp)?;
         if json_messages {
             exec.exec_json(
                 rustc,
@@ -338,6 +342,7 @@ fn rustc<'a, 'cfg>(
                         rustc_dep_info_loc.display()
                     ))
                 })?;
+            filetime::set_file_times(dep_info_loc, timestamp, timestamp)?;
         }
 
         Ok(())