Make the c feature for compiler-builtins opt-in instead of inferred

The build script for `compiler_builtins` 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?
```

Rather than trying to fix it or special-case the platforms without bugs,
make it opt-in instead of automatic.

Author: jyn514

config.example.toml

@@ -328,6 +328,10 @@ changelog-seen = 2
 # on this runtime, such as `-C profile-generate` or `-C instrument-coverage`).
 #profiler = false
 
+# Use the optimized LLVM C intrinsics for `compiler_builtins`, rather than Rust intrinsics.
+# Requires the LLVM submodule to be managed by bootstrap (i.e. not external).
+#optimized-compiler-builtins = false
+
 # Indicates whether the native libraries linked into Cargo will be statically
 # linked or not.
 #cargo-native-static = false

src/bootstrap/compile.rs

@@ -338,9 +338,7 @@ 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
@@ -351,8 +349,15 @@ 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);

src/bootstrap/config.rs

@@ -138,6 +138,8 @@ 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 stdout_is_tty: bool,
     pub stderr_is_tty: bool,
@@ -801,6 +803,7 @@ define_config! {
         patch_binaries_for_nix: Option<bool> = "patch-binaries-for-nix",
         // NOTE: only parsed by bootstrap.py, `--feature build-metrics` enables metrics unconditionally
         metrics: Option<bool> = "metrics",
+        optimized_compiler_builtins: Option<bool> = "optimized-compiler-builtins",
     }
 }
 
@@ -1295,6 +1298,7 @@ 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 as usize);
 

src/bootstrap/dist.rs

@@ -1964,23 +1964,21 @@ fn install_llvm_file(builder: &Builder<'_>, source: &Path, destination: &Path) {
 ///
 /// 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

src/ci/docker/host-x86_64/disabled/dist-x86_64-haiku/Dockerfile

@@ -47,4 +47,6 @@ ENV RUST_CONFIGURE_ARGS --disable-jemalloc \
   --set=$TARGET.cc=x86_64-unknown-haiku-gcc \
   --set=$TARGET.cxx=x86_64-unknown-haiku-g++ \
   --set=$TARGET.llvm-config=/bin/llvm-config-haiku
+ENV EXTERNAL_LLVM 1
+
 ENV SCRIPT python3 ../x.py dist --host=$HOST --target=$HOST

src/ci/docker/host-x86_64/dist-various-2/Dockerfile

@@ -138,4 +138,6 @@ ENV RUST_CONFIGURE_ARGS --enable-extended --enable-lld --disable-docs \
   --set target.wasm32-wasi.wasi-root=/wasm32-wasi \
   --musl-root-armv7=/musl-armv7
 
+ENV EXTERNAL_LLVM 1
+
 ENV SCRIPT python3 ../x.py dist --host='' --target $TARGETS

src/ci/docker/host-x86_64/x86_64-gnu-llvm-14/Dockerfile

@@ -37,6 +37,7 @@ RUN sh /scripts/sccache.sh
 # We are disabling CI LLVM since this builder is intentionally using a host
 # LLVM, rather than the typical src/llvm-project LLVM.
 ENV NO_DOWNLOAD_CI_LLVM 1
+ENV EXTERNAL_LLVM 1
 
 # This is not the latest LLVM version, so some components required by tests may
 # be missing.

src/ci/run.sh

@@ -79,6 +79,14 @@ fi
 # space required for CI artifacts.
 RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --dist-compression-formats=xz"
 
+# Enable the `c` feature for compiler_builtins, but only when the `compiler-rt` source is available.
+if [ "$EXTERNAL_LLVM" = "" ]; then
+  RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set build.optimized-compiler-builtins"
+elif [ "$DEPLOY$DEPLOY_ALT" = "1" ]; then
+    echo "error: dist builds should always use in-tree LLVM so we use optimized compiler-rt!" >&2
+    exit 1
+fi
+
 if [ "$DIST_SRC" = "" ]; then
   RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --disable-dist-src"
 fi