Don't skip building LLVM if already built

Author: Mark-Simulacrum

src/bootstrap/builder.rs

@@ -765,9 +765,17 @@ impl<'a> Builder<'a> {
         }
 
         // Set a flag for `check`/`clippy`/`fix`, so that certain build
-        // scripts can do less work (e.g. not building/requiring LLVM).
+        // scripts can do less work (i.e. not building/requiring LLVM).
         if cmd == "check" || cmd == "clippy" || cmd == "fix" {
-            cargo.env("RUST_CHECK", "1");
+            // If we've not yet built LLVM, or it's stale, then bust
+            // the librustc_llvm cache. That will always work, even though it
+            // may mean that on the next non-check build we'll need to rebuild
+            // librustc_llvm. But if LLVM is stale, that'll be a tiny amount
+            // of work comparitively, and we'd likely need to rebuild it anyway,
+            // so that's okay.
+            if crate::native::prebuilt_llvm_config(self, target).is_err() {
+                cargo.env("RUST_CHECK", "1");
+            }
         }
 
         let stage = if compiler.stage == 0 && self.local_rebuild {

src/bootstrap/compile.rs

@@ -517,9 +517,13 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo, target: Interne
     // librustc_llvm and librustc_codegen_llvm.
     //
     // Note that this is disabled if LLVM itself is disabled or we're in a check
-    // build, where if we're in a check build there's no need to build all of
-    // LLVM and such.
-    if builder.config.llvm_enabled() && builder.kind != Kind::Check {
+    // build. If we are in a check build we still go ahead here presuming we've
+    // detected that LLVM is alreay built and good to go which helps prevent
+    // busting caches (e.g. like #71152).
+    if builder.config.llvm_enabled()
+        && (builder.kind != Kind::Check
+            || crate::native::prebuilt_llvm_config(builder, target).is_ok())
+    {
         if builder.is_rust_llvm(target) {
             cargo.env("LLVM_RUSTLLVM", "1");
         }

src/librustc_llvm/build.rs

@@ -15,9 +15,9 @@ fn detect_llvm_link() -> (&'static str, &'static str) {
 }
 
 fn main() {
+    println!("cargo:rerun-if-env-changed=RUST_CHECK");
     if env::var_os("RUST_CHECK").is_some() {
         // If we're just running `check`, there's no need for LLVM to be built.
-        println!("cargo:rerun-if-env-changed=RUST_CHECK");
         return;
     }