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
@@ -1379,18 +1379,24 @@ pub fn rustc_cargo_env(
         cargo.rustflag("--cfg=llvm_enzyme");
     }
 
-    // Note that this is disabled if LLVM itself is disabled or we're in a check
-    // build. If we are in a check build we still go ahead here presuming we've
-    // detected that LLVM is already built and good to go which helps prevent
-    // busting caches (e.g. like #71152).
+    // These conditionals represent a tension between three forces:
+    // - For non-check builds, we need to define some LLVM-related environment
+    //   variables, requiring LLVM to have been built.
+    // - For check builds, we want to avoid building LLVM if possible.
+    // - Check builds and non-check builds should have the same environment if
+    //   possible, to avoid unnecessary rebuilds due to cache-busting.
+    //
+    // Therefore we try to avoid building LLVM for check builds, but only if
+    // building LLVM would be expensive. If "building" LLVM is cheap
+    // (i.e. it's already built or is downloadable), we prefer to maintain a
+    // consistent environment between check and non-check builds.
     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)
         }
     }
@@ -1407,6 +1413,9 @@ pub fn rustc_cargo_env(
 
 /// Pass down configuration from the LLVM build into the build of
 /// rustc_llvm and rustc_codegen_llvm.
+///
+/// Note that this has the side-effect of _building LLVM_, which is sometimes
+/// unwanted (e.g. for check builds).
 fn rustc_llvm_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetSelection) {
     if builder.config.is_rust_llvm(target) {
         cargo.env("LLVM_RUSTLLVM", "1");

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

@@ -1298,7 +1298,6 @@ 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>
@@ -1312,7 +1311,6 @@ mod snapshot {
             ctx.config("check")
                 .path("rustc")
                 .render_steps(), @r"
-        [build] llvm <host>
         [check] rustc 0 <host> -> rustc 1 <host>
         ");
     }
@@ -1332,7 +1330,6 @@ 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>
@@ -1464,7 +1461,6 @@ 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>
@@ -1478,7 +1474,6 @@ 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>
         ");
@@ -1499,7 +1494,6 @@ 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>
         ");
@@ -1552,7 +1546,6 @@ 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>
@@ -1566,7 +1559,6 @@ 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>
         ");