enable parallel rustc front end in nightly builds

Cargo.lock

@@ -2982,9 +2982,9 @@ dependencies = [
 
 [[package]]
 name = "portable-atomic"
-version = "1.4.2"
+version = "1.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f32154ba0af3a075eefa1eda8bb414ee928f62303a54ea85b8d6638ff1a6ee9e"
+checksum = "3bccab0e7fd7cc19f820a1c8c91720af652d0c88dc9664dd72aef2614f04af3b"
 
 [[package]]
 name = "ppv-lite86"
@@ -3712,6 +3712,7 @@ dependencies = [
  "measureme",
  "memmap2",
  "parking_lot 0.12.1",
+ "portable-atomic",
  "rustc-hash",
  "rustc-rayon",
  "rustc-rayon-core",

compiler/rustc_data_structures/Cargo.toml

@@ -47,6 +47,9 @@ features = [
 memmap2 = "0.2.1"
 # tidy-alphabetical-end
 
+[target.'cfg(any(target_arch = "powerpc", target_arch = "mips"))'.dependencies]
+portable-atomic = "1.5.1"
+
 [features]
 # tidy-alphabetical-start
 rustc_use_parallel_compiler = ["indexmap/rustc-rayon", "rustc-rayon", "rustc-rayon-core"]

compiler/rustc_data_structures/src/marker.rs

@@ -138,7 +138,6 @@ cfg_match! {
             [std::sync::atomic::AtomicUsize]
             [std::sync::atomic::AtomicU8]
             [std::sync::atomic::AtomicU32]
-            [std::sync::atomic::AtomicU64]
             [std::backtrace::Backtrace]
             [std::io::Error]
             [std::fs::File]
@@ -148,6 +147,18 @@ cfg_match! {
             [crate::owned_slice::OwnedSlice]
         );
 
+        // PowerPC and MIPS platforms with 32-bit pointers do not
+        // have AtomicU64 type.
+        #[cfg(not(any(target_arch = "powerpc", target_arch = "mips")))]
+        already_sync!(
+            [std::sync::atomic::AtomicU64]
+        );
+
+        #[cfg(any(target_arch = "powerpc", target_arch = "mips"))]
+        already_sync!(
+            [portable_atomic::AtomicU64]
+        );
+
         macro_rules! impl_dyn_sync {
             ($($($attr: meta)* [$ty: ty where $($generics2: tt)*])*) => {
                 $(unsafe impl<$($generics2)*> DynSync for $ty {})*

compiler/rustc_data_structures/src/sync.rs

@@ -265,7 +265,15 @@ cfg_match! {
 
         pub use std::sync::OnceLock;
 
-        pub use std::sync::atomic::{AtomicBool, AtomicUsize, AtomicU32, AtomicU64};
+        pub use std::sync::atomic::{AtomicBool, AtomicUsize, AtomicU32};
+
+        // PowerPC and MIPS platforms with 32-bit pointers do not
+        // have AtomicU64 type.
+        #[cfg(not(any(target_arch = "powerpc", target_arch = "mips")))]
+        pub use std::sync::atomic::AtomicU64;
+
+        #[cfg(any(target_arch = "powerpc", target_arch = "mips"))]
+        pub use portable_atomic::AtomicU64;
 
         pub use std::sync::Arc as Lrc;
         pub use std::sync::Weak as Weak;

config.example.toml

@@ -30,7 +30,7 @@
 #
 # If `change-id` does not match the version that is currently running,
 # `x.py` will prompt you to update it and check the related PR for more details.
-change-id = 116998
+change-id = 117435
 
 # =============================================================================
 # Tweaking how LLVM is compiled
@@ -553,10 +553,11 @@ change-id = 116998
 # Whether to always use incremental compilation when building rustc
 #incremental = false
 
-# Build a multi-threaded rustc
-# FIXME(#75760): Some UI tests fail when this option is enabled.
-# NOTE: This option is NOT SUPPORTED. See #48685.
-#parallel-compiler = false
+# Build a multi-threaded rustc. This allows users to use parallel rustc
+# via the unstable option `-Z threads=n`.
+# Since stable/beta channels only allow using stable features,
+# `parallel-compiler = false` should be set for these channels.
+#parallel-compiler = true
 
 # The default linker that will be hard-coded into the generated
 # compiler for targets that don't specify a default linker explicitly

src/bootstrap/src/core/config/config.rs

@@ -1072,6 +1072,7 @@ impl Config {
         config.bindir = "bin".into();
         config.dist_include_mingw_linker = true;
         config.dist_compression_profile = "fast".into();
+        config.rustc_parallel = true;
 
         config.stdout_is_tty = std::io::stdout().is_terminal();
         config.stderr_is_tty = std::io::stderr().is_terminal();
@@ -1429,7 +1430,9 @@ impl Config {
             set(&mut config.use_lld, rust.use_lld);
             set(&mut config.lld_enabled, rust.lld);
             set(&mut config.llvm_tools_enabled, rust.llvm_tools);
-            config.rustc_parallel = rust.parallel_compiler.unwrap_or(false);
+            config.rustc_parallel = rust
+                .parallel_compiler
+                .unwrap_or(config.channel == "dev" || config.channel == "nightly");
             config.rustc_default_linker = rust.default_linker;
             config.musl_root = rust.musl_root.map(PathBuf::from);
             config.save_toolstates = rust.save_toolstates.map(PathBuf::from);

src/bootstrap/src/lib.rs

@@ -77,7 +77,7 @@ const LLD_FILE_NAMES: &[&str] = &["ld.lld", "ld64.lld", "lld-link", "wasm-ld"];
 ///
 /// If you make any major changes (such as adding new values or changing default values), please
 /// ensure that the associated PR ID is added to the end of this list.
-pub const CONFIG_CHANGE_HISTORY: &[usize] = &[115898, 116998];
+pub const CONFIG_CHANGE_HISTORY: &[usize] = &[115898, 116998, 117435];
 
 /// Extra --check-cfg to add when building
 /// (Mode restriction, config name, config values (if any))

src/ci/run.sh

@@ -98,8 +98,8 @@ if [ "$DEPLOY$DEPLOY_ALT" = "1" ]; then
   if [ "$NO_LLVM_ASSERTIONS" = "1" ]; then
     RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --disable-llvm-assertions"
   elif [ "$DEPLOY_ALT" != "" ]; then
-    if [ "$NO_PARALLEL_COMPILER" = "" ]; then
-      RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.parallel-compiler"
+    if [ "$ALT_PARALLEL_COMPILER" = "" ]; then
+      RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.parallel-compiler=false"
     fi
     RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --enable-llvm-assertions"
     RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.verify-llvm-ir"

src/tools/tidy/src/deps.rs

@@ -211,6 +211,7 @@ const PERMITTED_RUSTC_DEPENDENCIES: &[&str] = &[
     "perf-event-open-sys",
     "pin-project-lite",
     "polonius-engine",
+    "portable-atomic", // dependency for platforms doesn't support `AtomicU64` in std
     "ppv-lite86",
     "proc-macro-hack",
     "proc-macro2",