Auto merge of #101833 - jyn514:cross-compile-compiler-builtins, r=Mark-Simulacrum

Make the `c` feature for `compiler-builtins` an explicit opt-in

Its build script doesn't support cross-compilation. I tried fixing it, but the cc crate itself doesn't appear to support cross-compiling to windows either unless you use the -gnu toolchain:
```
  error occurred: Failed to find tool. Is `lib.exe` installed?
```

Fixes https://github.com/rust-lang/rust/issues/101172.

3 files changed, 29 insertions, 22 deletions

diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index c13e83f6c86..58cf3edc317 100644
--- a/src/bootstrap/compile.rs
+++ b/src/bootstrap/compile.rs
@@ -299,9 +299,7 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, stage: u32, car
 
     // Determine if we're going to compile in optimized C intrinsics to
     // the `compiler-builtins` crate. These intrinsics live in LLVM's
-    // `compiler-rt` repository, but our `src/llvm-project` submodule isn't
-    // always checked out, so we need to conditionally look for this. (e.g. if
-    // an external LLVM is used we skip the LLVM submodule checkout).
+    // `compiler-rt` repository.
     //
     // Note that this shouldn't affect the correctness of `compiler-builtins`,
     // but only its speed. Some intrinsics in C haven't been translated to Rust
@@ -312,8 +310,15 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, stage: u32, car
     // If `compiler-rt` is available ensure that the `c` feature of the
     // `compiler-builtins` crate is enabled and it's configured to learn where
     // `compiler-rt` is located.
-    let compiler_builtins_root = builder.src.join("src/llvm-project/compiler-rt");
-    let compiler_builtins_c_feature = if compiler_builtins_root.exists() {
+    let compiler_builtins_c_feature = if builder.config.optimized_compiler_builtins {
+        if !builder.is_rust_llvm(target) {
+            panic!(
+                "need a managed LLVM submodule for optimized intrinsics support; unset `llvm-config` or `optimized-compiler-builtins`"
+            );
+        }
+
+        builder.update_submodule(&Path::new("src").join("llvm-project"));
+        let compiler_builtins_root = builder.src.join("src/llvm-project/compiler-rt");
         // Note that `libprofiler_builtins/build.rs` also computes this so if
         // you're changing something here please also change that.
         cargo.env("RUST_COMPILER_RT_ROOT", &compiler_builtins_root);
diff --git a/src/bootstrap/config.rs b/src/bootstrap/config.rs
index 74530dec97b..f29b5170ea5 100644
--- a/src/bootstrap/config.rs
+++ b/src/bootstrap/config.rs
@@ -73,6 +73,8 @@ pub struct Config {
     pub color: Color,
     pub patch_binaries_for_nix: bool,
     pub stage0_metadata: Stage0Metadata,
+    /// Whether to use the `c` feature of the `compiler_builtins` crate.
+    pub optimized_compiler_builtins: bool,
 
     pub on_fail: Option<String>,
     pub stage: u32,
@@ -597,6 +599,7 @@ define_config! {
         bench_stage: Option<u32> = "bench-stage",
         patch_binaries_for_nix: Option<bool> = "patch-binaries-for-nix",
         metrics: Option<bool> = "metrics",
+        optimized_compiler_builtins: Option<bool> = "optimized-compiler-builtins",
     }
 }
 
@@ -966,6 +969,7 @@ impl Config {
         set(&mut config.print_step_timings, build.print_step_timings);
         set(&mut config.print_step_rusage, build.print_step_rusage);
         set(&mut config.patch_binaries_for_nix, build.patch_binaries_for_nix);
+        set(&mut config.optimized_compiler_builtins, build.optimized_compiler_builtins);
 
         config.verbose = cmp::max(config.verbose, flags.verbose);
 
diff --git a/src/bootstrap/dist.rs b/src/bootstrap/dist.rs
index f387496883b..42352285182 100644
--- a/src/bootstrap/dist.rs
+++ b/src/bootstrap/dist.rs
@@ -1844,23 +1844,21 @@ fn add_env(builder: &Builder<'_>, cmd: &mut Command, target: TargetSelection) {
 ///
 /// Returns whether the files were actually copied.
 fn maybe_install_llvm(builder: &Builder<'_>, target: TargetSelection, dst_libdir: &Path) -> bool {
-    if let Some(config) = builder.config.target_config.get(&target) {
-        if config.llvm_config.is_some() && !builder.config.llvm_from_ci {
-            // If the LLVM was externally provided, then we don't currently copy
-            // artifacts into the sysroot. This is not necessarily the right
-            // choice (in particular, it will require the LLVM dylib to be in
-            // the linker's load path at runtime), but the common use case for
-            // external LLVMs is distribution provided LLVMs, and in that case
-            // they're usually in the standard search path (e.g., /usr/lib) and
-            // copying them here is going to cause problems as we may end up
-            // with the wrong files and isn't what distributions want.
-            //
-            // This behavior may be revisited in the future though.
-            //
-            // If the LLVM is coming from ourselves (just from CI) though, we
-            // still want to install it, as it otherwise won't be available.
-            return false;
-        }
+    if !builder.is_rust_llvm(target) {
+        // If the LLVM was externally provided, then we don't currently copy
+        // artifacts into the sysroot. This is not necessarily the right
+        // choice (in particular, it will require the LLVM dylib to be in
+        // the linker's load path at runtime), but the common use case for
+        // external LLVMs is distribution provided LLVMs, and in that case
+        // they're usually in the standard search path (e.g., /usr/lib) and
+        // copying them here is going to cause problems as we may end up
+        // with the wrong files and isn't what distributions want.
+        //
+        // This behavior may be revisited in the future though.
+        //
+        // If the LLVM is coming from ourselves (just from CI) though, we
+        // still want to install it, as it otherwise won't be available.
+        return false;
     }
 
     // On macOS, rustc (and LLVM tools) link to an unversioned libLLVM.dylib