simplify LLVM submodule handling

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

@@ -1205,7 +1205,8 @@ pub fn rustc_cargo_env(
     // busting caches (e.g. like #71152).
     if builder.config.llvm_enabled(target) {
         let building_is_expensive =
-            crate::core::build_steps::llvm::prebuilt_llvm_config(builder, target).should_build();
+            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 == stage;
         let should_skip_build = building_is_expensive && can_skip_build;

src/bootstrap/src/core/build_steps/dist.rs

@@ -2036,7 +2036,7 @@ fn maybe_install_llvm(
         }
         !builder.config.dry_run()
     } else if let llvm::LlvmBuildStatus::AlreadyBuilt(llvm::LlvmResult { llvm_config, .. }) =
-        llvm::prebuilt_llvm_config(builder, target)
+        llvm::prebuilt_llvm_config(builder, target, true)
     {
         let mut cmd = command(llvm_config);
         cmd.arg("--libfiles");

src/bootstrap/src/core/build_steps/llvm.rs

@@ -87,10 +87,14 @@ impl LdFlags {
 ///
 /// This will return the llvm-config if it can get it (but it will not build it
 /// if not).
-pub fn prebuilt_llvm_config(builder: &Builder<'_>, target: TargetSelection) -> LlvmBuildStatus {
-    // If we have llvm submodule initialized already, sync it.
-    builder.update_existing_submodule("src/llvm-project");
-
+pub fn prebuilt_llvm_config(
+    builder: &Builder<'_>,
+    target: TargetSelection,
+    // Certain commands (like `x test mir-opt --bless`) may call this function with different targets,
+    // which could bypass the CI LLVM early-return even if `builder.config.llvm_from_ci` is true.
+    // This flag should be `true` only if the caller needs the LLVM sources (e.g., if it will build LLVM).
+    handle_submodule_when_needed: bool,
+) -> LlvmBuildStatus {
     builder.config.maybe_download_ci_llvm();
 
     // If we're using a custom LLVM bail out here, but we can only use a
@@ -109,9 +113,10 @@ pub fn prebuilt_llvm_config(builder: &Builder<'_>, target: TargetSelection) -> L
         }
     }
 
-    // Initialize the llvm submodule if not initialized already.
-    // If submodules are disabled, this does nothing.
-    builder.config.update_submodule("src/llvm-project");
+    if handle_submodule_when_needed {
+        // If submodules are disabled, this does nothing.
+        builder.config.update_submodule("src/llvm-project");
+    }
 
     let root = "src/llvm-project/llvm";
     let out_dir = builder.llvm_out(target);
@@ -284,7 +289,7 @@ impl Step for Llvm {
         };
 
         // If LLVM has already been built or been downloaded through download-ci-llvm, we avoid building it again.
-        let Meta { stamp, res, out_dir, root } = match prebuilt_llvm_config(builder, target) {
+        let Meta { stamp, res, out_dir, root } = match prebuilt_llvm_config(builder, target, true) {
             LlvmBuildStatus::AlreadyBuilt(p) => return p,
             LlvmBuildStatus::ShouldBuild(m) => m,
         };

src/bootstrap/src/core/builder.rs

@@ -1537,7 +1537,9 @@ impl<'a> Builder<'a> {
             // rustc_llvm. But if LLVM is stale, that'll be a tiny amount
             // of work comparatively, and we'd likely need to rebuild it anyway,
             // so that's okay.
-            if crate::core::build_steps::llvm::prebuilt_llvm_config(self, target).should_build() {
+            if crate::core::build_steps::llvm::prebuilt_llvm_config(self, target, false)
+                .should_build()
+            {
                 cargo.env("RUST_CHECK", "1");
             }
         }