rustc_span: Replace a HashMap<_, ()> with HashSet

Now that `HashSet::entry()` exists, we don't need to fake it with a map.

compiler/rustc_span/src/hygiene.rs

@@ -26,6 +26,7 @@
 
 use std::cell::RefCell;
 use std::collections::hash_map::Entry;
+use std::collections::hash_set::Entry as SetEntry;
 use std::fmt;
 use std::hash::Hash;
 
@@ -1270,7 +1271,7 @@ pub struct HygieneDecodeContext {
     inner: Lock<HygieneDecodeContextInner>,
 
     /// A set of serialized `SyntaxContext` ids that are currently being decoded on each thread.
-    local_in_progress: WorkerLocal<RefCell<FxHashMap<u32, ()>>>,
+    local_in_progress: WorkerLocal<RefCell<FxHashSet<u32>>>,
 }
 
 /// Register an expansion which has been decoded from the on-disk-cache for the local crate.
@@ -1364,14 +1365,14 @@ pub fn decode_syntax_context<D: Decoder, F: FnOnce(&mut D, u32) -> SyntaxContext
         match inner.decoding.entry(raw_id) {
             Entry::Occupied(ctxt_entry) => {
                 match context.local_in_progress.borrow_mut().entry(raw_id) {
-                    Entry::Occupied(..) => {
+                    SetEntry::Occupied(..) => {
                         // We're decoding this already on the current thread. Return here
                         // and let the function higher up the stack finish decoding to handle
                         // recursive cases.
                         return *ctxt_entry.get();
                     }
-                    Entry::Vacant(entry) => {
-                        entry.insert(());
+                    SetEntry::Vacant(entry) => {
+                        entry.insert();
 
                         // Some other thread is current decoding this. Race with it.
                         *ctxt_entry.get()
@@ -1380,7 +1381,7 @@ pub fn decode_syntax_context<D: Decoder, F: FnOnce(&mut D, u32) -> SyntaxContext
             }
             Entry::Vacant(entry) => {
                 // We are the first thread to start decoding. Mark the current thread as being progress.
-                context.local_in_progress.borrow_mut().insert(raw_id, ());
+                context.local_in_progress.borrow_mut().insert(raw_id);
 
                 // Allocate and store SyntaxContext id *before* calling the decoder function,
                 // as the SyntaxContextData may reference itself.

compiler/rustc_span/src/lib.rs

@@ -22,6 +22,7 @@
 #![feature(array_windows)]
 #![feature(cfg_match)]
 #![feature(core_io_borrowed_buf)]
+#![feature(hash_set_entry)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]
 #![feature(min_specialization)]