Avoid building C++ for rustc_llvm with --compile-time-deps

This saves about 30s.

compiler/rustc/Cargo.toml

@@ -27,6 +27,7 @@ features = ['unprefixed_malloc_on_supported_platforms']
 
 [features]
 # tidy-alphabetical-start
+check_only = ['rustc_driver_impl/check_only']
 jemalloc = ['dep:tikv-jemalloc-sys']
 llvm = ['rustc_driver_impl/llvm']
 max_level_info = ['rustc_driver_impl/max_level_info']

compiler/rustc_codegen_llvm/Cargo.toml

@@ -43,3 +43,6 @@ serde_json = "1"
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 tracing = "0.1"
 # tidy-alphabetical-end
+
+[features]
+check_only = ["rustc_llvm/check_only"]

compiler/rustc_driver_impl/Cargo.toml

@@ -72,6 +72,7 @@ ctrlc = "3.4.4"
 
 [features]
 # tidy-alphabetical-start
+check_only = ['rustc_interface/check_only']
 llvm = ['rustc_interface/llvm']
 max_level_info = ['rustc_log/max_level_info']
 rustc_randomized_layouts = [

compiler/rustc_interface/Cargo.toml

@@ -56,5 +56,6 @@ rustc_abi = { path = "../rustc_abi" }
 
 [features]
 # tidy-alphabetical-start
+check_only = ['rustc_codegen_llvm?/check_only']
 llvm = ['dep:rustc_codegen_llvm']
 # tidy-alphabetical-end

compiler/rustc_llvm/Cargo.toml

@@ -14,3 +14,7 @@ libc = "0.2.73"
 # pinned `cc` in `rustc_codegen_ssa` if you update `cc` here.
 cc = "=1.2.16"
 # tidy-alphabetical-end
+
+[features]
+# Used by ./x.py check --compile-time-deps to skip building C++ code
+check_only = []

compiler/rustc_llvm/build.rs

@@ -106,6 +106,10 @@ fn output(cmd: &mut Command) -> String {
 }
 
 fn main() {
+    if cfg!(feature = "check_only") {
+        return;
+    }
+
     for component in REQUIRED_COMPONENTS.iter().chain(OPTIONAL_COMPONENTS.iter()) {
         println!("cargo:rustc-check-cfg=cfg(llvm_component,values(\"{component}\"))");
     }

src/bootstrap/src/lib.rs

@@ -775,6 +775,9 @@ fn rustc_features(&self, kind: Kind, target: TargetSelection, crates: &[String])
         if self.config.rust_randomize_layout && check("rustc_randomized_layouts") {
             features.push("rustc_randomized_layouts");
         }
+        if self.config.compile_time_deps && kind == Kind::Check {
+            features.push("check_only");
+        }
 
         // If debug logging is on, then we want the default for tracing:
         // https://github.com/tokio-rs/tracing/blob/3dd5c03d907afdf2c39444a29931833335171554/tracing/src/level_filters.rs#L26