Avoid warnings under miri

src/map.rs

@@ -7,10 +7,7 @@ use std::error::Error;
 use std::fmt::{self, Debug, Display, Formatter};
 use std::hash::{BuildHasher, Hash, Hasher};
 use std::iter::FromIterator;
-use std::sync::{
-    atomic::{AtomicIsize, AtomicUsize, Ordering},
-    Once,
-};
+use std::sync::atomic::{AtomicIsize, Ordering};
 
 const ISIZE_BITS: usize = core::mem::size_of::<isize>() * 8;
 
@@ -62,8 +59,10 @@ const MAX_RESIZERS: isize = (1 << (ISIZE_BITS - RESIZE_STAMP_BITS)) - 1;
 /// The bit shift for recording size stamp in `size_ctl`.
 const RESIZE_STAMP_SHIFT: usize = ISIZE_BITS - RESIZE_STAMP_BITS;
 
-static NCPU_INITIALIZER: Once = Once::new();
-static NCPU: AtomicUsize = AtomicUsize::new(0);
+#[cfg(not(miri))]
+static NCPU_INITIALIZER: std::sync::Once = std::sync::Once::new();
+#[cfg(not(miri))]
+static NCPU: std::sync::atomic::AtomicUsize = std::sync::atomic::AtomicUsize::new(0);
 
 macro_rules! load_factor {
     ($n: expr) => {