Use rustc_codegen_llvm's get_dylib_metadata to unblock proc macros. (EmbarkStudios#268)

Author: eddyb

crates/rustc_codegen_spirv/src/lib.rs

@@ -230,8 +230,16 @@ impl MetadataLoader for SpirvMetadataLoader {
         link::read_metadata(path)
     }
 
-    fn get_dylib_metadata(&self, _: &Target, _: &Path) -> Result<MetadataRef, String> {
-        Err("TODO: implement get_dylib_metadata".to_string())
+    fn get_dylib_metadata(&self, target: &Target, path: &Path) -> Result<MetadataRef, String> {
+        // HACK(eddyb) this is needed to allow metadata loading for proc macros
+        // (compiled as host dylibs); perhaps it'd be better to use the `object`
+        // crate, like `rustc_codegen_cranelift` does.
+        // NOTE(eddyb) while both `::new()` and `.metadata_loader()` call `Box::new`,
+        // they only do so with ZST values, and so we don't pointlessly allocate.
+        extern crate rustc_codegen_llvm;
+        rustc_codegen_llvm::LlvmCodegenBackend::new()
+            .metadata_loader()
+            .get_dylib_metadata(target, path)
     }
 }