rustc_metadata: use configurable AtomicBool for privateness flag

This switches to using a `Cell` for single-threaded rustc.

compiler/rustc_data_structures/src/sync.rs

@@ -143,6 +143,20 @@ pub fn fetch_or(&self, val: bool, _: Ordering) -> bool {
                 self.0.set(val);
                 result
             }
+            pub fn fetch_update(
+                &self,
+                _order_set: Ordering,
+                _order_get: Ordering,
+                mut f: impl FnMut(bool) -> Option<bool>,
+            ) -> Result<bool, bool> {
+                let prev = self.0.get();
+                if let Some(next) = f(prev) {
+                    self.0.set(next);
+                    Ok(prev)
+                } else {
+                    Err(prev)
+                }
+            }
         }
 
         impl<T: Copy + PartialEq> Atomic<T> {

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -9,7 +9,7 @@
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::owned_slice::OwnedSlice;
 use rustc_data_structures::svh::Svh;
-use rustc_data_structures::sync::{AppendOnlyVec, Lock, Lrc, OnceCell};
+use rustc_data_structures::sync::{AppendOnlyVec, AtomicBool, Lock, Lrc, OnceCell};
 use rustc_data_structures::unhash::UnhashMap;
 use rustc_expand::base::{SyntaxExtension, SyntaxExtensionKind};
 use rustc_expand::proc_macro::{AttrProcMacro, BangProcMacro, DeriveProcMacro};
@@ -40,7 +40,7 @@
 use std::iter::TrustedLen;
 use std::num::NonZeroUsize;
 use std::path::Path;
-use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::atomic::Ordering;
 use std::{io, iter, mem};
 
 pub(super) use cstore_impl::provide;