Propagate parallel_compiler feature through rustc crates. Turned off feature gives change of builded crates: 238 -> 224.

compiler/rustc/Cargo.toml

@@ -19,3 +19,4 @@ features = ['unprefixed_malloc_on_supported_platforms']
 jemalloc = ['tikv-jemalloc-sys']
 llvm = ['rustc_driver/llvm']
 max_level_info = ['rustc_driver/max_level_info']
+rustc_use_parallel_compiler = ['rustc_driver/rustc_use_parallel_compiler']

compiler/rustc_data_structures/Cargo.toml

@@ -9,7 +9,7 @@ doctest = false
 [dependencies]
 arrayvec = { version = "0.7", default-features = false }
 ena = "0.14"
-indexmap = { version = "1.8.0", features = ["rustc-rayon"] }
+indexmap = { version = "1.8.0" }
 tracing = "0.1"
 jobserver_crate = { version = "0.1.13", package = "jobserver" }
 rustc_serialize = { path = "../rustc_serialize" }
@@ -17,8 +17,8 @@ rustc_macros = { path = "../rustc_macros" }
 rustc_graphviz = { path = "../rustc_graphviz" }
 cfg-if = "0.1.2"
 stable_deref_trait = "1.0.0"
-rayon = { version = "0.3.2", package = "rustc-rayon" }
+rayon = { version = "0.3.2", package = "rustc-rayon", optional = true }
-rayon-core = { version = "0.3.2", package = "rustc-rayon-core" }
+rayon-core = { version = "0.3.2", package = "rustc-rayon-core", optional = true }
 rustc-hash = "1.1.0"
 smallvec = { version = "1.6.1", features = ["const_generics", "union", "may_dangle"] }
 rustc_index = { path = "../rustc_index", package = "rustc_index" }
@@ -36,3 +36,6 @@ winapi = { version = "0.3", features = ["fileapi", "psapi", "winerror"] }
 
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
 memmap2 = "0.2.1"
+
+[features]
+rustc_use_parallel_compiler = ["indexmap/rustc-rayon", "rayon", "rayon-core"]

compiler/rustc_driver/Cargo.toml

@@ -39,3 +39,5 @@ winapi = { version = "0.3", features = ["consoleapi", "debugapi", "processenv"]
 [features]
 llvm = ['rustc_interface/llvm']
 max_level_info = ['rustc_log/max_level_info']
+rustc_use_parallel_compiler = ['rustc_data_structures/rustc_use_parallel_compiler', 'rustc_interface/rustc_use_parallel_compiler',
+    'rustc_middle/rustc_use_parallel_compiler']

compiler/rustc_interface/Cargo.toml

@@ -10,8 +10,8 @@ doctest = false
 libc = "0.2"
 libloading = "0.7.1"
 tracing = "0.1"
-rustc-rayon-core = "0.3.2"
+rustc-rayon-core = { version = "0.3.2", optional = true }
-rayon = { version = "0.3.2", package = "rustc-rayon" }
+rayon = { version = "0.3.2", package = "rustc-rayon", optional = true }
 smallvec = { version = "1.6.1", features = ["union", "may_dangle"] }
 rustc_ast = { path = "../rustc_ast" }
 rustc_attr = { path = "../rustc_attr" }
@@ -57,3 +57,4 @@ rustc_target = { path = "../rustc_target" }
 
 [features]
 llvm = ['rustc_codegen_llvm']
+rustc_use_parallel_compiler = ['rayon', 'rustc-rayon-core', 'rustc_query_impl/rustc_use_parallel_compiler']

compiler/rustc_middle/Cargo.toml

@@ -12,8 +12,8 @@ bitflags = "1.2.1"
 either = "1.5.0"
 gsgdt = "0.1.2"
 tracing = "0.1"
-rustc-rayon = "0.3.2"
+rustc-rayon = { version = "0.3.2", optional = true }
-rustc-rayon-core = "0.3.2"
+rustc-rayon-core = { version = "0.3.2", optional = true }
 polonius-engine = "0.13.0"
 rustc_apfloat = { path = "../rustc_apfloat" }
 rustc_attr = { path = "../rustc_attr" }
@@ -35,3 +35,6 @@ rustc_session = { path = "../rustc_session" }
 rustc_type_ir = { path = "../rustc_type_ir" }
 rand = "0.8.4"
 rand_xoshiro = "0.6.0"
+
+[features]
+rustc_use_parallel_compiler = ["rustc-rayon", "rustc-rayon-core"]

compiler/rustc_query_impl/Cargo.toml

@@ -8,7 +8,7 @@ doctest = false
 
 [dependencies]
 measureme = "10.0.0"
-rustc-rayon-core = "0.3.2"
+rustc-rayon-core = { version = "0.3.2", optional = true }
 rustc_ast = { path = "../rustc_ast" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }
@@ -20,3 +20,6 @@ rustc_query_system = { path = "../rustc_query_system" }
 rustc_serialize = { path = "../rustc_serialize" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
+
+[features]
+rustc_use_parallel_compiler = ["rustc-rayon-core", "rustc_query_system/rustc_use_parallel_compiler"]

compiler/rustc_query_system/Cargo.toml

@@ -9,7 +9,7 @@ doctest = false
 [dependencies]
 rustc_arena = { path = "../rustc_arena" }
 tracing = "0.1"
-rustc-rayon-core = "0.3.2"
+rustc-rayon-core = { version = "0.3.2", optional = true }
 rustc_ast = { path = "../rustc_ast" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }
@@ -23,3 +23,6 @@ rustc_span = { path = "../rustc_span" }
 rustc_target = { path = "../rustc_target" }
 parking_lot = "0.11"
 smallvec = { version = "1.6.1", features = ["union", "may_dangle"] }
+
+[features]
+rustc_use_parallel_compiler = ["rustc-rayon-core"]

src/bootstrap/compile.rs

@@ -689,6 +689,8 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetS
     }
 
     if builder.config.rustc_parallel {
+        // keep in sync with `bootstrap/lib.rs:Build::rustc_features`
+        // `cfg` option for rustc, `features` option for cargo, for conditional compilation
         cargo.rustflag("--cfg=parallel_compiler");
         cargo.rustdocflag("--cfg=parallel_compiler");
     }

src/bootstrap/lib.rs

@@ -729,12 +729,16 @@ impl Build {
 
     /// Gets the space-separated set of activated features for the compiler.
     fn rustc_features(&self, kind: Kind) -> String {
-        let mut features = String::new();
+        let mut features = vec![];
         if self.config.jemalloc {
-            features.push_str("jemalloc");
+            features.push("jemalloc");
         }
         if self.config.llvm_enabled() || kind == Kind::Check {
-            features.push_str(" llvm");
+            features.push("llvm");
+        }
+        // keep in sync with `bootstrap/compile.rs:rustc_cargo_env`
+        if self.config.rustc_parallel {
+            features.push("rustc_use_parallel_compiler");
         }
 
         // If debug logging is on, then we want the default for tracing:
@@ -743,10 +747,10 @@ impl Build {
         // if its unset, if debug_assertions is on, then debug_logging will also be on
         // as well as tracing *ignoring* this feature when debug_assertions is on
         if !self.config.rust_debug_logging {
-            features.push_str(" max_level_info");
+            features.push("max_level_info");
         }
 
-        features
+        features.join(" ")
     }
 
     /// Component directory that Cargo will produce output into (e.g.