Auto merge of #132282 - Noratrieb:it-is-the-end-of-serial, r=cjgillot

Delete the `cfg(not(parallel))` serial compiler

Since it's inception a long time ago, the parallel compiler and its cfgs have been a maintenance burden. This was a necessary evil the allow iteration while not degrading performance because of synchronization overhead.

But this time is over. Thanks to the amazing work by the parallel working group (and the dyn sync crimes), the parallel compiler has now been fast enough to be shipped by default in nightly for quite a while now.
Stable and beta have still been on the serial compiler, because they can't use `-Zthreads` anyways.
But this is quite suboptimal:
- the maintenance burden still sucks
- we're not testing the serial compiler in nightly

Because of these reasons, it's time to end it. The serial compiler has served us well in the years since it was split from the parallel one, but it's over now.

Let the knight slay one head of the two-headed dragon!

#113349

Note that the default is still 1 thread, as more than 1 thread is still fairly broken.

cc `@onur-ozkan` to see if i did the bootstrap field removal correctly, `@SparrowLii` on the sync parts

2 files changed, 1 insertions, 21 deletions

diff --git a/compiler/rustc_span/src/hygiene.rs b/compiler/rustc_span/src/hygiene.rs
index c57d516fcae..a9ca5dbe509 100644
--- a/compiler/rustc_span/src/hygiene.rs
+++ b/compiler/rustc_span/src/hygiene.rs
@@ -1300,7 +1300,6 @@ pub fn register_expn_id(
     let expn_id = ExpnId { krate, local_id };
     HygieneData::with(|hygiene_data| {
         let _old_data = hygiene_data.foreign_expn_data.insert(expn_id, data);
-        debug_assert!(_old_data.is_none() || cfg!(parallel_compiler));
         let _old_hash = hygiene_data.foreign_expn_hashes.insert(expn_id, hash);
         debug_assert!(_old_hash.is_none() || _old_hash == Some(hash));
         let _old_id = hygiene_data.expn_hash_to_expn_id.insert(hash, expn_id);
@@ -1423,18 +1422,7 @@ pub fn decode_syntax_context<D: Decoder, F: FnOnce(&mut D, u32) -> SyntaxContext
             ctxt_data = old.clone();
         }
 
-        let dummy = std::mem::replace(
-            &mut hygiene_data.syntax_context_data[ctxt.as_u32() as usize],
-            ctxt_data,
-        );
-        if cfg!(not(parallel_compiler)) {
-            // Make sure nothing weird happened while `decode_data` was running.
-            // We used `kw::Empty` for the dummy value and we expect nothing to be
-            // modifying the dummy entry.
-            // This does not hold for the parallel compiler as another thread may
-            // have inserted the fully decoded data.
-            assert_eq!(dummy.dollar_crate_name, kw::Empty);
-        }
+        hygiene_data.syntax_context_data[ctxt.as_u32() as usize] = ctxt_data;
     });
 
     // Mark the context as completed
diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index ef775068515..baa1c2c91c7 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -521,14 +521,6 @@ impl SpanData {
     }
 }
 
-// The interner is pointed to by a thread local value which is only set on the main thread
-// with parallelization is disabled. So we don't allow `Span` to transfer between threads
-// to avoid panics and other errors, even though it would be memory safe to do so.
-#[cfg(not(parallel_compiler))]
-impl !Send for Span {}
-#[cfg(not(parallel_compiler))]
-impl !Sync for Span {}
-
 impl PartialOrd for Span {
     fn partial_cmp(&self, rhs: &Self) -> Option<Ordering> {
         PartialOrd::partial_cmp(&self.data(), &rhs.data())