metrics: use mocked AtomicU64 in IO metrics driver (#4649)

.github/workflows/ci.yml

@@ -228,6 +228,7 @@ jobs:
           - powerpc64-unknown-linux-gnu
           - mips-unknown-linux-gnu
           - arm-linux-androideabi
+          - mipsel-unknown-linux-musl
     steps:
       - uses: actions/checkout@v2
       - name: Install Rust ${{ env.rust_stable }}
@@ -241,7 +242,14 @@ jobs:
         with:
           use-cross: true
           command: check
-          args: --workspace --target ${{ matrix.target }}
+          args: --workspace --all-features --target ${{ matrix.target }}
+      - uses: actions-rs/cargo@v1
+        with:
+          use-cross: true
+          command: check
+          args: --workspace --all-features --target ${{ matrix.target }}
+        env:
+          RUSTFLAGS: --cfg tokio_unstable -Dwarnings
 
   features:
     name: features

Cross.toml

@@ -0,0 +1,4 @@
+[build.env]
+passthrough = [
+    "RUSTFLAGS",
+]

tokio/src/loom/std/atomic_u64.rs

@@ -75,4 +75,12 @@ cfg_not_has_atomic_u64! {
             self.compare_exchange(current, new, success, failure)
         }
     }
+
+    impl Default for AtomicU64 {
+        fn default() -> AtomicU64 {
+            Self {
+                inner: Mutex::new(0),
+            }
+        }
+    }
 }

tokio/src/runtime/metrics/io.rs

@@ -1,6 +1,6 @@
 #![cfg_attr(not(feature = "net"), allow(dead_code))]
 
-use std::sync::atomic::{AtomicU64, Ordering::Relaxed};
+use crate::loom::sync::atomic::{AtomicU64, Ordering::Relaxed};
 
 #[derive(Default)]
 pub(crate) struct IoDriverMetrics {