Stop the host Hermes compiler build from targeting visionOS

.changeset/prebuilt-hermes-visionos-env.md

@@ -0,0 +1,13 @@
+---
+"react-native-node-api": patch
+---
+
+Fix `prebuilt-hermes` failing to configure the host Hermes compiler with "Host
+compiler appears to require libatomic, but cannot find it". It exported all
+three deployment targets Hermes' `build-apple-framework.sh` can ask for to every
+command it ran, including the host compiler build. `XROS_DEPLOYMENT_TARGET` is
+also a clang driver variable, so clang targeted visionOS against the macOS
+sysroot, and every API marked unavailable there — `pthread_mutexattr_init`, the
+`fd_set` helpers reached through `unistd.h` — failed to compile. Each platform
+build now gets only the deployment target it needs, and the host compiler build
+gets none.

.github/workflows/hermes-prebuilt.yml

@@ -67,6 +67,37 @@ jobs:
         run: |
           path=$(pnpm exec react-native-node-api prebuilt-hermes --no-download)
           echo "path=$path" >> "$GITHUB_OUTPUT"
+      # CMake reports a failed feature check as a bare "not found", with the
+      # compiler's actual complaint only in its configure log. Surface that log
+      # here so a failure is diagnosable without another round trip.
+      - name: Dump CMake configure log
+        if: failure()
+        run: |
+          log=$(find "$PWD" -name CMakeConfigureLog.yaml -path '*build_host_hermesc*' | head -1)
+          if [ -z "$log" ]; then
+            echo "No CMakeConfigureLog.yaml found"
+            find "$PWD" -name 'CMakeError.log' -path '*build_host_hermesc*' -exec cat {} \;
+            exit 0
+          fi
+          echo "::group::Environment seen by CMake"
+          env | sort
+          xcode-select --print-path
+          xcrun --show-sdk-path || true
+          echo "::endgroup::"
+          echo "::group::unistd.h check"
+          grep -n -B 5 -A 45 'unistd\.h' "$log" | head -150
+          echo "::endgroup::"
+          echo "::group::atomics check"
+          grep -n -B 5 -A 60 'HAVE_CXX_ATOMICS_WITHOUT_LIB' "$log" | head -180
+          echo "::endgroup::"
+          cp "$log" "$RUNNER_TEMP/CMakeConfigureLog.yaml"
+      - name: Upload CMake configure log
+        if: failure()
+        uses: actions/upload-artifact@v7
+        with:
+          name: hermesc-cmake-configure-log
+          path: ${{ runner.temp }}/CMakeConfigureLog.yaml
+          if-no-files-found: ignore
       # --latest=false keeps these out of the "latest release" slot, which
       # belongs to the package releases changesets publishes.
       - name: Publish as a release asset

packages/host/src/node/cli/hermes-prebuilt.ts

@@ -27,13 +27,25 @@ const RELEASES_URL =
 
 export const DEFAULT_PLATFORMS = ["iphoneos", "iphonesimulator"];
 
-// Passed to Hermes' build-apple-framework.sh, which errors out rather than
-// assume one. These match React Native's own podspec declarations.
-const DEPLOYMENT_TARGETS = {
-  IOS_DEPLOYMENT_TARGET: "15.1",
-  MAC_DEPLOYMENT_TARGET: "10.15",
-  XROS_DEPLOYMENT_TARGET: "1.0",
-};
+/**
+ * The deployment target build-apple-framework.sh reads for a platform — it
+ * errors out rather than assume one. These match React Native's own podspec
+ * declarations.
+ *
+ * Only the variable that platform needs is passed. XROS_DEPLOYMENT_TARGET is
+ * also a clang driver variable, so leaking it into a build that isn't for
+ * visionOS makes clang target visionOS against whatever sysroot it was given —
+ * and every API marked unavailable there then fails to compile.
+ */
+function getDeploymentTarget(platform: string): Record<string, string> {
+  if (platform === "macosx") {
+    return { MAC_DEPLOYMENT_TARGET: "10.15" };
+  } else if (platform === "xros" || platform === "xrsimulator") {
+    return { XROS_DEPLOYMENT_TARGET: "1.0" };
+  } else {
+    return { IOS_DEPLOYMENT_TARGET: "15.1" };
+  }
+}
 
 export const BUILD_TYPES = ["debug", "release"] as const;
 export type BuildType = (typeof BUILD_TYPES)[number];
@@ -162,15 +174,19 @@ async function buildArchive({
   // vendored copy: the framework and the app linking it share jsi::Runtime.
   const jsiPath = path.join(reactNativePath, "ReactCommon", "jsi");
 
-  const run = (command: string, args: string[]) =>
+  const run = (
+    command: string,
+    args: string[],
+    extraEnv: Record<string, string> = {},
+  ) =>
     spawn(command, args, {
       cwd: hermesPath,
       outputMode: "inherit",
       // Keeps the build log off stdout, which callers parse for the final path.
       stdout: process.stderr,
       env: {
-        ...DEPLOYMENT_TARGETS,
         ...process.env,
+        ...extraEnv,
         JSI_PATH: jsiPath,
         BUILD_TYPE: buildType === "debug" ? "Debug" : "Release",
         HERMES_OVERRIDE_HERMESC_PATH: importHostCompilersPath,
@@ -202,7 +218,11 @@ async function buildArchive({
   }
 
   for (const platform of platforms) {
-    await run("./utils/build-apple-framework.sh", [platform]);
+    await run(
+      "./utils/build-apple-framework.sh",
+      [platform],
+      getDeploymentTarget(platform),
+    );
   }
 
   const frameworksPath = path.join(