Rollup merge of #117328 - lqd:cranelift-rocket, r=Mark-Simulacrum

pass `CODEGEN_BACKENDS` to docker

The backends to build are now defined in the `CODEGEN_BACKENDS` env var. It's correctly set in CI, but wasn't passed to docker, hence cg_clif wasn't actually built in #81746.

r? `@Kobzol:` I locally tried `CODEGEN_BACKENDS="cranelift" DEPLOY=1 src/ci/docker/run.sh dist-x86_64-linux` and this change was enough for `ci/run.sh` to read the env var.

So I'll try as-is and we'll see.

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

@@ -1305,6 +1305,10 @@ impl Step for CodegenBackend {
             return None;
         }
 
+        if !builder.config.rust_codegen_backends.contains(&self.backend) {
+            return None;
+        }
+
         if self.backend == "cranelift" {
             if !target_supports_cranelift_backend(self.compiler.host) {
                 builder.info("target not supported by rustc_codegen_cranelift. skipping");
@@ -1343,12 +1347,15 @@ impl Step for CodegenBackend {
         let backends_dst = PathBuf::from("lib").join(&backends_rel);
 
         let backend_name = format!("rustc_codegen_{}", backend);
+        let mut found_backend = false;
         for backend in fs::read_dir(&backends_src).unwrap() {
             let file_name = backend.unwrap().file_name();
             if file_name.to_str().unwrap().contains(&backend_name) {
                 tarball.add_file(backends_src.join(file_name), &backends_dst, 0o644);
+                found_backend = true;
             }
         }
+        assert!(found_backend);
 
         Some(tarball.generate())
     }

src/ci/docker/run.sh

@@ -288,6 +288,7 @@ docker \
   --env DIST_TRY_BUILD \
   --env PR_CI_JOB \
   --env OBJDIR_ON_HOST="$objdir" \
+  --env CODEGEN_BACKENDS \
   --init \
   --rm \
   rust-ci \