Fix unused_must_use, error in the futrue (#6999)

* Error on unused must_use

* Fix unused must_use

src/rust/engine/async_semaphore/src/lib.rs

@@ -1,3 +1,4 @@
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,

src/rust/engine/boxfuture/src/lib.rs

@@ -2,6 +2,7 @@
 // https://github.com/alexcrichton/futures-rs/issues/228 has background for its removal.
 // This avoids needing to call Box::new() around every future that we produce.
 
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,

src/rust/engine/build_utils/src/lib.rs

@@ -1,3 +1,4 @@
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,

src/rust/engine/fs/brfs/src/main.rs

@@ -1,3 +1,4 @@
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,

src/rust/engine/fs/fs_util/src/main.rs

@@ -1,3 +1,4 @@
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,

src/rust/engine/fs/src/lib.rs

@@ -1,6 +1,7 @@
 // Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 // Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,

src/rust/engine/graph/src/lib.rs

@@ -1,6 +1,7 @@
 // Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 // Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,

src/rust/engine/hashing/src/lib.rs

@@ -1,6 +1,7 @@
 // Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 // Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,

src/rust/engine/process_execution/src/lib.rs

@@ -1,3 +1,4 @@
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,

src/rust/engine/process_executor/src/main.rs

@@ -1,3 +1,4 @@
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,

src/rust/engine/resettable/src/lib.rs

@@ -1,3 +1,4 @@
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,

src/rust/engine/serverset/src/lib.rs

@@ -1,6 +1,7 @@
 // Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 // Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,
@@ -289,8 +290,7 @@ impl<T: Clone + Send + Sync + 'static> Serverset<T> {
 
 impl<T: std::fmt::Debug> std::fmt::Debug for Serverset<T> {
   fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
-    write!(f, "Serverset {{ {:?} }}", self.inner.servers);
-    Ok(())
+    write!(f, "Serverset {{ {:?} }}", self.inner.servers)
   }
 }
 

src/rust/engine/src/lib.rs

@@ -1,6 +1,7 @@
 // Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 // Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,

src/rust/engine/testutil/local_cas/src/main.rs

@@ -1,3 +1,4 @@
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,

src/rust/engine/testutil/src/lib.rs

@@ -1,3 +1,4 @@
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,

src/rust/engine/ui/src/display.rs

@@ -1,3 +1,4 @@
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,

src/rust/engine/ui/src/main.rs

@@ -1,3 +1,4 @@
+#![deny(unused_must_use)]
 // Enable all clippy lints except for many of the pedantic ones. It's a shame this needs to be copied and pasted across crates, but there doesn't appear to be a way to include inner attributes from a common source.
 #![deny(
   clippy::all,