Improve backwards compatibility with old .rh files

crates/compiler/build/src/program.rs

@@ -807,7 +807,7 @@ fn build_loaded_file<'a>(
     };
 
     // the preprocessed host is stored beside the platform's main.roc
-    let preprocessed_host_path = if linking_strategy == LinkingStrategy::Legacy {
+    let mut preprocessed_host_path = if linking_strategy == LinkingStrategy::Legacy {
         if let roc_target::OperatingSystem::Wasi = operating_system {
             // when compiling a wasm application, we implicitly assume here that the host is in zig
             // and has a file called "host.zig"
@@ -835,9 +835,35 @@ fn build_loaded_file<'a>(
         None
     } else if is_platform_prebuilt {
         if !preprocessed_host_path.exists() {
-            invalid_prebuilt_platform(prebuilt_requested, preprocessed_host_path);
+            // Check for .rh and .o files using the old "x86_64" instead of the new "x64", for
+            // backwards compatibility with prebuilt platforms created before
+            // https://github.com/roc-lang/roc/pull/557 landed.
+            //
+            // Eventually (e.g. certainly after the end of 2023) this fixup can be removed.
+            if preprocessed_host_path.ends_with("-x64.rh")
+                || preprocessed_host_path.ends_with("-x64.o")
+            {
+                // Fixup the filename to the new format.
+                preprocessed_host_path = preprocessed_host_path.with_file_name(
+                    preprocessed_host_path
+                        .file_name()
+                        .unwrap()
+                        .to_str()
+                        .unwrap()
+                        .replace("-x64.", "-x86_64."),
+                );
+
+                // Try again now that we've fixed the filename.
+                if !preprocessed_host_path.exists() {
+                    invalid_prebuilt_platform(prebuilt_requested, preprocessed_host_path);
+
+                    std::process::exit(1);
+                }
+            } else {
+                invalid_prebuilt_platform(prebuilt_requested, preprocessed_host_path);
 
-            std::process::exit(1);
+                std::process::exit(1);
+            }
         }
 
         if linking_strategy == LinkingStrategy::Surgical {