Auto merge of #117435 - SparrowLii:nightly_parallel, r=oli-obk

enable parallel rustc front end in nightly builds

Refers to the [MCP](rust-lang/compiler-team#681), this pr does:
1. Enable the parallel front end in nightly builds, and keep the default number of threads as 1. Then users can use the parallel rustc front end via -Z threads=n option.

2. Set it up to serial front end for beta/stable builds via bootstrap.

3. Switch over the alt builders from parallel rustc to serial, so we have artifacts without parallel to test against the artifacts with parallel.

r? `@oli-obk`

cc `@cjgillot` `@nnethercote` `@bjorn3` `@Kobzol`

Author: bors

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"