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!

compiler/rustc_error_messages/Cargo.toml

@@ -19,8 +19,3 @@ rustc_span = { path = "../rustc_span" }
 tracing = "0.1"
 unic-langid = { version = "0.9.0", features = ["macros"] }
 # tidy-alphabetical-end
-
-[features]
-# tidy-alphabetical-start
-rustc_use_parallel_compiler = ['rustc_baked_icu_data/rustc_use_parallel_compiler']
-# tidy-alphabetical-end

compiler/rustc_error_messages/src/lib.rs

@@ -8,12 +8,9 @@
 // tidy-alphabetical-end
 
 use std::borrow::Cow;
-#[cfg(not(parallel_compiler))]
-use std::cell::LazyCell as Lazy;
 use std::error::Error;
 use std::path::{Path, PathBuf};
-#[cfg(parallel_compiler)]
-use std::sync::LazyLock as Lazy;
+use std::sync::LazyLock;
 use std::{fmt, fs, io};
 
 use fluent_bundle::FluentResource;
@@ -21,9 +18,6 @@ pub use fluent_bundle::types::FluentType;
 pub use fluent_bundle::{self, FluentArgs, FluentError, FluentValue};
 use fluent_syntax::parser::ParserError;
 use icu_provider_adapters::fallback::{LocaleFallbackProvider, LocaleFallbacker};
-#[cfg(not(parallel_compiler))]
-use intl_memoizer::IntlLangMemoizer;
-#[cfg(parallel_compiler)]
 use intl_memoizer::concurrent::IntlLangMemoizer;
 use rustc_data_structures::sync::{IntoDynSyncSend, Lrc};
 use rustc_macros::{Decodable, Encodable};
@@ -34,12 +28,6 @@ pub use unic_langid::{LanguageIdentifier, langid};
 pub type FluentBundle =
     IntoDynSyncSend<fluent_bundle::bundle::FluentBundle<FluentResource, IntlLangMemoizer>>;
 
-#[cfg(not(parallel_compiler))]
-fn new_bundle(locales: Vec<LanguageIdentifier>) -> FluentBundle {
-    IntoDynSyncSend(fluent_bundle::bundle::FluentBundle::new(locales))
-}
-
-#[cfg(parallel_compiler)]
 fn new_bundle(locales: Vec<LanguageIdentifier>) -> FluentBundle {
     IntoDynSyncSend(fluent_bundle::bundle::FluentBundle::new_concurrent(locales))
 }
@@ -217,7 +205,7 @@ fn register_functions(bundle: &mut FluentBundle) {
 
 /// Type alias for the result of `fallback_fluent_bundle` - a reference-counted pointer to a lazily
 /// evaluated fluent bundle.
-pub type LazyFallbackBundle = Lrc<Lazy<FluentBundle, impl FnOnce() -> FluentBundle>>;
+pub type LazyFallbackBundle = Lrc<LazyLock<FluentBundle, impl FnOnce() -> FluentBundle>>;
 
 /// Return the default `FluentBundle` with standard "en-US" diagnostic messages.
 #[instrument(level = "trace", skip(resources))]
@@ -225,7 +213,7 @@ pub fn fallback_fluent_bundle(
     resources: Vec<&'static str>,
     with_directionality_markers: bool,
 ) -> LazyFallbackBundle {
-    Lrc::new(Lazy::new(move || {
+    Lrc::new(LazyLock::new(move || {
         let mut fallback_bundle = new_bundle(vec![langid!("en-US")]);
 
         register_functions(&mut fallback_bundle);
@@ -548,15 +536,6 @@ pub fn fluent_value_from_str_list_sep_by_and(l: Vec<Cow<'_, str>>) -> FluentValu
             Cow::Owned(result)
         }
 
-        #[cfg(not(parallel_compiler))]
-        fn as_string_threadsafe(
-            &self,
-            _intls: &intl_memoizer::concurrent::IntlLangMemoizer,
-        ) -> Cow<'static, str> {
-            unreachable!("`as_string_threadsafe` is not used in non-parallel rustc")
-        }
-
-        #[cfg(parallel_compiler)]
         fn as_string_threadsafe(
             &self,
             intls: &intl_memoizer::concurrent::IntlLangMemoizer,