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!

Author: Noratrieb

Diff for: compiler/rustc/Cargo.toml

@@ -31,5 +31,4 @@ jemalloc = ['dep:jemalloc-sys']
 llvm = ['rustc_driver_impl/llvm']
 max_level_info = ['rustc_driver_impl/max_level_info']
 rustc_randomized_layouts = ['rustc_driver_impl/rustc_randomized_layouts']
-rustc_use_parallel_compiler = ['rustc_driver_impl/rustc_use_parallel_compiler']
 # tidy-alphabetical-end

Diff for: compiler/rustc_ast/src/tokenstream.rs

@@ -38,7 +38,6 @@ pub enum TokenTree {
 }
 
 // Ensure all fields of `TokenTree` are `DynSend` and `DynSync`.
-#[cfg(parallel_compiler)]
 fn _dummy()
 where
     Token: sync::DynSend + sync::DynSync,

Diff for: compiler/rustc_baked_icu_data/Cargo.toml

@@ -8,11 +8,6 @@ edition = "2021"
 icu_list = "1.2"
 icu_locid = "1.2"
 icu_locid_transform = "1.3.2"
-icu_provider = "1.2"
+icu_provider = { version = "1.2", features = ["sync"] }
 zerovec = "0.10.0"
 # tidy-alphabetical-end
-
-[features]
-# tidy-alphabetical-start
-rustc_use_parallel_compiler = ['icu_provider/sync']
-# tidy-alphabetical-end

Diff for: compiler/rustc_data_structures/Cargo.toml

@@ -10,11 +10,11 @@ bitflags = "2.4.1"
 either = "1.0"
 elsa = "=1.7.1"
 ena = "0.14.3"
-indexmap = { version = "2.4.0" }
+indexmap = { version = "2.4.0", features = ["rustc-rayon"] }
 jobserver_crate = { version = "0.1.28", package = "jobserver" }
 measureme = "11"
 rustc-hash = "2.0.0"
-rustc-rayon = { version = "0.5.0", optional = true }
+rustc-rayon = "0.5.0"
 rustc-stable-hash = { version = "0.1.0", features = ["nightly"] }
 rustc_arena = { path = "../rustc_arena" }
 rustc_graphviz = { path = "../rustc_graphviz" }
@@ -53,8 +53,3 @@ memmap2 = "0.2.1"
 
 [target.'cfg(not(target_has_atomic = "64"))'.dependencies]
 portable-atomic = "1.5.1"
-
-[features]
-# tidy-alphabetical-start
-rustc_use_parallel_compiler = ["indexmap/rustc-rayon", "dep:rustc-rayon"]
-# tidy-alphabetical-end

Diff for: compiler/rustc_data_structures/src/lib.rs

@@ -10,7 +10,6 @@
 #![allow(internal_features)]
 #![allow(rustc::default_hash_types)]
 #![allow(rustc::potential_query_instability)]
-#![cfg_attr(not(parallel_compiler), feature(cell_leak))]
 #![deny(unsafe_op_in_unsafe_fn)]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]