Revert "Change prefetch to avoid deadlock"

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -677,7 +677,8 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         let debugger_visualizers =
             stat!("debugger-visualizers", || self.encode_debugger_visualizers());
 
-        // Encode exported symbols info. This is prefetched in `encode_metadata`.
+        // Encode exported symbols info. This is prefetched in `encode_metadata` so we encode
+        // this as late as possible to give the prefetching as much time as possible to complete.
         let exported_symbols = stat!("exported-symbols", || {
             self.encode_exported_symbols(tcx.exported_symbols(LOCAL_CRATE))
         });
@@ -2217,13 +2218,21 @@ pub fn encode_metadata(tcx: TyCtxt<'_>, path: &Path) {
     // there's no need to do dep-graph tracking for any of it.
     tcx.dep_graph.assert_ignored();
 
-    if tcx.sess.threads() != 1 {
-        // Prefetch some queries used by metadata encoding.
-        // This is not necessary for correctness, but is only done for performance reasons.
-        // It can be removed if it turns out to cause trouble or be detrimental to performance.
-        join(|| prefetch_mir(tcx), || tcx.exported_symbols(LOCAL_CRATE));
-    }
+    join(
+        || encode_metadata_impl(tcx, path),
+        || {
+            if tcx.sess.threads() == 1 {
+                return;
+            }
+            // Prefetch some queries used by metadata encoding.
+            // This is not necessary for correctness, but is only done for performance reasons.
+            // It can be removed if it turns out to cause trouble or be detrimental to performance.
+            join(|| prefetch_mir(tcx), || tcx.exported_symbols(LOCAL_CRATE));
+        },
+    );
+}
 
+fn encode_metadata_impl(tcx: TyCtxt<'_>, path: &Path) {
     let mut encoder = opaque::FileEncoder::new(path)
         .unwrap_or_else(|err| tcx.dcx().emit_fatal(FailCreateFileEncoder { err }));
     encoder.emit_raw_bytes(METADATA_HEADER);