Don't trigger an LLVM build from check builds using the stage 0 compiler

Author: Zalathar

src/bootstrap/src/core/build_steps/compile.rs

@@ -1387,8 +1387,10 @@ pub fn rustc_cargo_env(
         let building_is_expensive =
             crate::core::build_steps::llvm::prebuilt_llvm_config(builder, target, false)
                 .should_build();
-        // `top_stage == stage` might be false for `check --stage 1`, if we are building the stage 1 compiler
-        let can_skip_build = builder.kind == Kind::Check && builder.top_stage == build_stage;
+
+        // If we are doing a check build using the pre-built stage 0 compiler,
+        // there should be no need to build LLVM.
+        let can_skip_build = builder.kind == Kind::Check && build_stage == 0;
         let should_skip_build = building_is_expensive && can_skip_build;
         if !should_skip_build {
             rustc_llvm_env(builder, cargo, target)

src/bootstrap/src/core/builder/tests.rs

@@ -630,6 +630,46 @@ fn any_debug() {
     assert_eq!(x.downcast_ref::<MyStruct>(), Some(&MyStruct { x: 7 }));
 }
 
+#[test]
+fn test_check_compiler_doesnt_build_llvm() {
+    // Checking the stage 1 compiler uses the bootstrap compiler, and therefore
+    // should not need to build LLVM.
+    {
+        let config = configure_with_args(
+            &["check", "compiler", "--stage=1"],
+            &[TEST_TRIPLE_1],
+            &[TEST_TRIPLE_1],
+        );
+        let mut cache = run_build(&config.paths.clone(), config);
+
+        assert!(!cache.contains::<llvm::Llvm>());
+    }
+
+    // Checking the stage 1 library uses the stage 1 compiler, so it will need LLVM.
+    {
+        let config = configure_with_args(
+            &["check", "library", "--stage=1"],
+            &[TEST_TRIPLE_1],
+            &[TEST_TRIPLE_1],
+        );
+        let mut cache = run_build(&config.paths.clone(), config);
+
+        assert!(cache.contains::<llvm::Llvm>());
+    }
+
+    // Checking the stage 2 compiler uses the stage 1 compiler, so it will need LLVM.
+    {
+        let config = configure_with_args(
+            &["check", "compiler", "--stage=2"],
+            &[TEST_TRIPLE_1],
+            &[TEST_TRIPLE_1],
+        );
+        let mut cache = run_build(&config.paths.clone(), config);
+
+        assert!(cache.contains::<llvm::Llvm>());
+    }
+}
+
 /// These tests use insta for snapshot testing.
 /// See bootstrap's README on how to bless the snapshots.
 mod snapshot {