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

Author: Zalathar

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

@@ -1323,7 +1323,7 @@ pub fn rustc_cargo_env(
     builder: &Builder<'_>,
     cargo: &mut Cargo,
     target: TargetSelection,
-    build_stage: u32,
+    _build_stage: u32,
 ) {
     // Set some configuration variables picked up by build scripts and
     // the compiler alike
@@ -1384,13 +1384,12 @@ pub fn rustc_cargo_env(
     // detected that LLVM is already built and good to go which helps prevent
     // busting caches (e.g. like #71152).
     if builder.config.llvm_enabled(target) {
-        let building_is_expensive =
+        let building_llvm_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;
-        let should_skip_build = building_is_expensive && can_skip_build;
-        if !should_skip_build {
+
+        let skip_llvm = (builder.kind == Kind::Check) && building_llvm_is_expensive;
+        if !skip_llvm {
             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 {
@@ -1294,7 +1334,6 @@ mod snapshot {
             ctx.config("check")
                 .path("compiler")
                 .render_steps(), @r"
-        [build] llvm <host>
         [check] rustc 0 <host> -> rustc 1 <host>
         [check] rustc 0 <host> -> cranelift 1 <host>
         [check] rustc 0 <host> -> gcc 1 <host>
@@ -1304,7 +1343,6 @@ mod snapshot {
             ctx.config("check")
                 .path("rustc")
                 .render_steps(), @r"
-        [build] llvm <host>
         [check] rustc 0 <host> -> rustc 1 <host>
         ");
     }
@@ -1324,7 +1362,6 @@ mod snapshot {
                 .path("compiler")
                 .stage(1)
                 .render_steps(), @r"
-        [build] llvm <host>
         [check] rustc 0 <host> -> rustc 1 <host>
         [check] rustc 0 <host> -> cranelift 1 <host>
         [check] rustc 0 <host> -> gcc 1 <host>
@@ -1456,7 +1493,6 @@ mod snapshot {
                 .paths(&["library", "compiler"])
                 .args(&args)
                 .render_steps(), @r"
-        [build] llvm <host>
         [check] rustc 0 <host> -> rustc 1 <host>
         [check] rustc 0 <host> -> cranelift 1 <host>
         [check] rustc 0 <host> -> gcc 1 <host>
@@ -1470,7 +1506,6 @@ mod snapshot {
             ctx.config("check")
                 .path("miri")
                 .render_steps(), @r"
-        [build] llvm <host>
         [check] rustc 0 <host> -> rustc 1 <host>
         [check] rustc 0 <host> -> Miri 1 <host>
         ");
@@ -1491,7 +1526,6 @@ mod snapshot {
                 .path("miri")
                 .stage(1)
                 .render_steps(), @r"
-        [build] llvm <host>
         [check] rustc 0 <host> -> rustc 1 <host>
         [check] rustc 0 <host> -> Miri 1 <host>
         ");
@@ -1544,7 +1578,6 @@ mod snapshot {
             ctx.config("check")
                 .path("rustc_codegen_cranelift")
                 .render_steps(), @r"
-        [build] llvm <host>
         [check] rustc 0 <host> -> rustc 1 <host>
         [check] rustc 0 <host> -> cranelift 1 <host>
         [check] rustc 0 <host> -> gcc 1 <host>
@@ -1558,7 +1591,6 @@ mod snapshot {
             ctx.config("check")
                 .path("rust-analyzer")
                 .render_steps(), @r"
-        [build] llvm <host>
         [check] rustc 0 <host> -> rustc 1 <host>
         [check] rustc 0 <host> -> rust-analyzer 1 <host>
         ");