Avoid infinite recursion when cranelift is the default codegen backend

Author: bjorn3

scripts/setup_rust_fork.sh

@@ -4,7 +4,9 @@ set -e
 # Compiletest expects all standard library paths to start with /rustc/FAKE_PREFIX.
 # CG_CLIF_STDLIB_REMAP_PATH_PREFIX will cause cg_clif's build system to pass
 # --remap-path-prefix to handle this.
-CG_CLIF_STDLIB_REMAP_PATH_PREFIX=/rustc/FAKE_PREFIX ./y.sh build
+# CG_CLIF_FORCE_GNU_AS will force usage of as instead of the LLVM backend of rustc as we
+# the LLVM backend isn't compiled in here.
+CG_CLIF_FORCE_GNU_AS=1 CG_CLIF_STDLIB_REMAP_PATH_PREFIX=/rustc/FAKE_PREFIX ./y.sh build
 
 echo "[SETUP] Rust fork"
 git clone https://github.com/rust-lang/rust.git || true

scripts/test_bootstrap.sh

@@ -11,5 +11,7 @@ rm -r compiler/rustc_codegen_cranelift/{Cargo.*,src}
 cp ../Cargo.* compiler/rustc_codegen_cranelift/
 cp -r ../src compiler/rustc_codegen_cranelift/src
 
-./x.py build --stage 1 library/std
+# CG_CLIF_FORCE_GNU_AS will force usage of as instead of the LLVM backend of rustc as we
+# the LLVM backend isn't compiled in here.
+CG_CLIF_FORCE_GNU_AS=1 ./x.py build --stage 1 library/std
 popd

src/global_asm.rs

@@ -140,7 +140,7 @@ pub(crate) fn compile_global_asm(
     );
 
     // Assemble `global_asm`
-    if false {
+    if option_env!("CG_CLIF_FORCE_GNU_AS").is_some() {
         let mut child = Command::new(&config.assembler)
             .arg("-o")
             .arg(&global_asm_object_file)
@@ -164,6 +164,7 @@ pub(crate) fn compile_global_asm(
             .arg(&global_asm_object_file)
             .arg("-")
             .arg("-Abad_asm_style")
+            .arg("-Zcodegen-backend=llvm")
             .stdin(Stdio::piped())
             .spawn()
             .expect("Failed to spawn `as`.");