Do not rebuild LLVM for x.py check

src/bootstrap/builder.rs

@@ -552,6 +552,12 @@ impl<'a> Builder<'a> {
              .arg("--target")
              .arg(target);
 
+        // Set a flag for `check` so that certain build scripts can do less work
+        // (e.g. not building/requiring LLVM).
+        if cmd == "check" {
+            cargo.env("RUST_CHECK", "1");
+        }
+
         // If we were invoked from `make` then that's already got a jobserver
         // set up for us so no need to tell Cargo about jobs all over again.
         if env::var_os("MAKEFLAGS").is_none() && env::var_os("MFLAGS").is_none() {

src/bootstrap/check.rs

@@ -11,7 +11,6 @@
 //! Implementation of compiling the compiler and standard library, in "check" mode.
 
 use compile::{run_cargo, std_cargo, test_cargo, rustc_cargo, rustc_cargo_env, add_to_sysroot};
-use compile::build_codegen_backend;
 use builder::{RunConfig, Builder, ShouldRun, Step};
 use {Compiler, Mode};
 use cache::{INTERNER, Interned};
@@ -138,11 +137,11 @@ impl Step for CodegenBackend {
         let backend = self.backend;
 
         let mut cargo = builder.cargo(compiler, Mode::Librustc, target, "check");
-        let mut features = build.rustc_features().to_string();
+        let features = build.rustc_features().to_string();
         cargo.arg("--manifest-path").arg(build.src.join("src/librustc_trans/Cargo.toml"));
         rustc_cargo_env(build, &mut cargo);
 
-        features += &build_codegen_backend(&builder, &mut cargo, &compiler, target, backend);
+        // We won't build LLVM if it's not available, as it shouldn't affect `check`.
 
         let _folder = build.fold_output(|| format!("stage{}-rustc_trans", compiler.stage));
         run_cargo(build,

src/librustc_llvm/build.rs

@@ -28,6 +28,11 @@ fn detect_llvm_link() -> (&'static str, &'static str) {
 }
 
 fn main() {
+    if env::var_os("RUST_CHECK").is_some() {
+        // If we're just running `check`, there's no need for LLVM to be built.
+        return;
+    }
+
     let target = env::var("TARGET").expect("TARGET was not set");
     let llvm_config = env::var_os("LLVM_CONFIG")
         .map(PathBuf::from)