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

6 files changed, 15 insertions, 24 deletions

diff --git a/src/bootstrap/src/core/builder/cargo.rs b/src/bootstrap/src/core/builder/cargo.rs
index 134da5db96d..0688a1d6892 100644
--- a/src/bootstrap/src/core/builder/cargo.rs
+++ b/src/bootstrap/src/core/builder/cargo.rs
@@ -1198,15 +1198,6 @@ impl Builder<'_> {
             rustflags.arg("-Zinline-mir-preserve-debug");
         }
 
-        if self.config.rustc_parallel
-            && matches!(mode, Mode::ToolRustc | Mode::Rustc | Mode::Codegen)
-        {
-            // keep in sync with `bootstrap/lib.rs:Build::rustc_features`
-            // `cfg` option for rustc, `features` option for cargo, for conditional compilation
-            rustflags.arg("--cfg=parallel_compiler");
-            rustdocflags.arg("--cfg=parallel_compiler");
-        }
-
         Cargo {
             command: cargo,
             compiler,
diff --git a/src/bootstrap/src/core/config/config.rs b/src/bootstrap/src/core/config/config.rs
index f977c285a74..0d49cf0aac4 100644
--- a/src/bootstrap/src/core/config/config.rs
+++ b/src/bootstrap/src/core/config/config.rs
@@ -278,7 +278,6 @@ pub struct Config {
     pub rust_strip: bool,
     pub rust_frame_pointers: bool,
     pub rust_stack_protector: Option<String>,
-    pub rustc_parallel: bool,
     pub rustc_default_linker: Option<String>,
     pub rust_optimize_tests: bool,
     pub rust_dist_src: bool,
@@ -1224,7 +1223,6 @@ impl Config {
             bindir: "bin".into(),
             dist_include_mingw_linker: true,
             dist_compression_profile: "fast".into(),
-            rustc_parallel: true,
 
             stdout_is_tty: std::io::stdout().is_terminal(),
             stderr_is_tty: std::io::stderr().is_terminal(),
@@ -1773,8 +1771,14 @@ impl Config {
 
             config.rust_randomize_layout = randomize_layout.unwrap_or_default();
             config.llvm_tools_enabled = llvm_tools.unwrap_or(true);
-            config.rustc_parallel =
-                parallel_compiler.unwrap_or(config.channel == "dev" || config.channel == "nightly");
+
+            // FIXME: Remove this option at the end of 2024.
+            if parallel_compiler.is_some() {
+                println!(
+                    "WARNING: The `rust.parallel-compiler` option is deprecated and does nothing. The parallel compiler (with one thread) is now the default"
+                );
+            }
+
             config.llvm_enzyme =
                 llvm_enzyme.unwrap_or(config.channel == "dev" || config.channel == "nightly");
             config.rustc_default_linker = default_linker;
diff --git a/src/bootstrap/src/lib.rs b/src/bootstrap/src/lib.rs
index ba74cabcd30..c384fd6bf43 100644
--- a/src/bootstrap/src/lib.rs
+++ b/src/bootstrap/src/lib.rs
@@ -80,11 +80,8 @@ const EXTRA_CHECK_CFGS: &[(Option<Mode>, &str, Option<&[&'static str]>)] = &[
     (Some(Mode::Rustc), "llvm_enzyme", None),
     (Some(Mode::Codegen), "llvm_enzyme", None),
     (Some(Mode::ToolRustc), "llvm_enzyme", None),
-    (Some(Mode::Rustc), "parallel_compiler", None),
-    (Some(Mode::ToolRustc), "parallel_compiler", None),
     (Some(Mode::ToolRustc), "rust_analyzer", None),
     (Some(Mode::ToolStd), "rust_analyzer", None),
-    (Some(Mode::Codegen), "parallel_compiler", None),
     // Any library specific cfgs like `target_os`, `target_arch` should be put in
     // priority the `[lints.rust.unexpected_cfgs.check-cfg]` table
     // in the appropriate `library/{std,alloc,core}/Cargo.toml`
@@ -695,9 +692,6 @@ impl Build {
             features.push("llvm");
         }
         // keep in sync with `bootstrap/compile.rs:rustc_cargo_env`
-        if self.config.rustc_parallel && check("rustc_use_parallel_compiler") {
-            features.push("rustc_use_parallel_compiler");
-        }
         if self.config.rust_randomize_layout {
             features.push("rustc_randomized_layouts");
         }
diff --git a/src/bootstrap/src/utils/change_tracker.rs b/src/bootstrap/src/utils/change_tracker.rs
index 0915ec15a2f..1d05f94e3be 100644
--- a/src/bootstrap/src/utils/change_tracker.rs
+++ b/src/bootstrap/src/utils/change_tracker.rs
@@ -291,6 +291,11 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         summary: "New option `llvm.offload` to control whether the llvm offload runtime for GPU support is built. Implicitly enables the openmp runtime as dependency.",
     },
     ChangeInfo {
+        change_id: 132282,
+        severity: ChangeSeverity::Warning,
+        summary: "Deprecated `rust.parallel_compiler` as the compiler now always defaults to being parallel (with 1 thread)",
+    },
+    ChangeInfo {
         change_id: 132494,
         severity: ChangeSeverity::Info,
         summary: "`download-rustc='if-unchanged'` is now a default option for library profile.",
diff --git a/src/ci/run.sh b/src/ci/run.sh
index 9f39ad9c55c..c771a03ad74 100755
--- a/src/ci/run.sh
+++ b/src/ci/run.sh
@@ -120,9 +120,6 @@ if [ "$DEPLOY$DEPLOY_ALT" = "1" ]; then
   if [ "$NO_LLVM_ASSERTIONS" = "1" ]; then
     RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --disable-llvm-assertions"
   elif [ "$DEPLOY_ALT" != "" ]; then
-    if [ "$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"
   fi
@@ -263,7 +260,7 @@ fi
 
 if [ "$RUN_CHECK_WITH_PARALLEL_QUERIES" != "" ]; then
   rm -f config.toml
-  $SRC/configure --set change-id=99999999 --set rust.parallel-compiler
+  $SRC/configure --set change-id=99999999
 
   # Save the build metrics before we wipe the directory
   if [ "$HAS_METRICS" = 1 ]; then
diff --git a/src/librustdoc/lib.rs b/src/librustdoc/lib.rs
index 78dc0b8e2f9..436d36d899e 100644
--- a/src/librustdoc/lib.rs
+++ b/src/librustdoc/lib.rs
@@ -206,7 +206,7 @@ fn init_logging(early_dcx: &EarlyDiagCtxt) {
         .with_verbose_exit(true)
         .with_verbose_entry(true)
         .with_indent_amount(2);
-    #[cfg(all(parallel_compiler, debug_assertions))]
+    #[cfg(debug_assertions)]
     let layer = layer.with_thread_ids(true).with_thread_names(true);
 
     use tracing_subscriber::layer::SubscriberExt;