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!

4 files changed, 13 insertions, 19 deletions

diff --git a/src/bootstrap/src/core/builder/cargo.rs b/src/bootstrap/src/core/builder/cargo.rs
index c3d0994f164..7b93a68dedc 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 8115aea033d..13ee664302c 100644
--- a/src/bootstrap/src/core/config/config.rs
+++ b/src/bootstrap/src/core/config/config.rs
@@ -276,7 +276,6 @@ 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,
@@ -1222,7 +1221,6 @@ 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(),
@@ -1771,8 +1769,14 @@ 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.",