Add parallel-frontend-threads to bootstrap.toml and enable multi-threaded parallel compilation

bootstrap.example.toml

@@ -859,6 +859,14 @@
 # Trigger a `DebugBreak` after an internal compiler error during bootstrap on Windows
 #rust.break-on-ice = true
 
+# Set the number of threads for the compiler frontend used during compilation of Rust code (passed to `-Zthreads`).
+# The valid options are:
+# 0 - Set the number of threads according to the detected number of threads of the host system
+# 1 - Use a single thread for compilation of Rust code (the default)
+# N - Number of threads used for compilation of Rust code
+#
+#rust.parallel-frontend-threads = 1
+
 # =============================================================================
 # Distribution options
 #

src/bootstrap/configure.py

@@ -340,6 +340,11 @@ o(
     "don't truncate options when printing them in this configure script",
 )
 v("set", None, "set arbitrary key/value pairs in TOML configuration")
+v(
+    "parallel-frontend-threads",
+    "rust.parallel-frontend-threads",
+    "number of parallel threads for rustc compilation",
+)
 
 
 def p(msg):

src/bootstrap/src/core/builder/cargo.rs

@@ -679,6 +679,12 @@ impl Builder<'_> {
         // cargo would implicitly add it, it was discover that sometimes bootstrap only use
         // `rustflags` without `cargo` making it required.
         rustflags.arg("-Zunstable-options");
+
+        // Add parallel frontend threads configuration
+        if let Some(threads) = self.config.rust_parallel_frontend_threads {
+            rustflags.arg(&format!("-Zthreads={threads}"));
+        }
+
         for (restricted_mode, name, values) in EXTRA_CHECK_CFGS {
             if restricted_mode.is_none() || *restricted_mode == Some(mode) {
                 rustflags.arg(&check_cfg_arg(name, *values));

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

@@ -191,7 +191,6 @@ pub struct Config {
     pub rust_optimize: RustOptimize,
     pub rust_codegen_units: Option<u32>,
     pub rust_codegen_units_std: Option<u32>,
-
     pub rustc_debug_assertions: bool,
     pub std_debug_assertions: bool,
     pub tools_debug_assertions: bool,
@@ -222,6 +221,8 @@ pub struct Config {
     pub rust_validate_mir_opts: Option<u32>,
     pub rust_std_features: BTreeSet<String>,
     pub rust_break_on_ice: bool,
+    pub rust_parallel_frontend_threads: Option<u32>,
+
     pub llvm_profile_use: Option<String>,
     pub llvm_profile_generate: bool,
     pub llvm_libunwind_default: Option<LlvmLibunwind>,
@@ -534,6 +535,7 @@ impl Config {
             backtrace_on_ice: rust_backtrace_on_ice,
             verify_llvm_ir: rust_verify_llvm_ir,
             thin_lto_import_instr_limit: rust_thin_lto_import_instr_limit,
+            parallel_frontend_threads: rust_parallel_frontend_threads,
             remap_debuginfo: rust_remap_debuginfo,
             jemalloc: rust_jemalloc,
             test_compare_mode: rust_test_compare_mode,
@@ -1298,6 +1300,7 @@ impl Config {
             rust_overflow_checks_std: rust_overflow_checks_std
                 .or(rust_overflow_checks)
                 .unwrap_or(rust_debug == Some(true)),
+            rust_parallel_frontend_threads: rust_parallel_frontend_threads.map(threads_from_config),
             rust_profile_generate: flags_rust_profile_generate.or(rust_profile_generate),
             rust_profile_use: flags_rust_profile_use.or(rust_profile_use),
             rust_randomize_layout: rust_randomize_layout.unwrap_or(false),

src/bootstrap/src/core/config/toml/rust.rs

@@ -66,6 +66,7 @@ define_config! {
         validate_mir_opts: Option<u32> = "validate-mir-opts",
         std_features: Option<BTreeSet<String>> = "std-features",
         break_on_ice: Option<bool> = "break-on-ice",
+        parallel_frontend_threads: Option<u32> = "parallel-frontend-threads",
     }
 }
 
@@ -357,6 +358,7 @@ pub fn check_incompatible_options_for_ci_rustc(
         validate_mir_opts: _,
         frame_pointers: _,
         break_on_ice: _,
+        parallel_frontend_threads: _,
     } = ci_rust_config;
 
     // There are two kinds of checks for CI rustc incompatible options:

src/bootstrap/src/utils/change_tracker.rs

@@ -546,4 +546,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: "The default value of the `gcc.download-ci-gcc` option has been changed to `true`.",
     },
+    ChangeInfo {
+        change_id: 146458,
+        severity: ChangeSeverity::Info,
+        summary: "There is now a bootstrap option called `rust.parallel-frontend-threads`, which can be used to set the number of threads for the compiler frontend used during compilation of Rust code.",
+    },
 ];