add preprocess host cli option

crates/cli/src/lib.rs

@@ -48,6 +48,7 @@ pub const CMD_FORMAT: &str = "format";
 pub const CMD_TEST: &str = "test";
 pub const CMD_GLUE: &str = "glue";
 pub const CMD_GEN_STUB_LIB: &str = "gen-stub-lib";
+pub const CMD_PREPROCESS_HOST: &str = "preprocess-host";
 
 pub const FLAG_DEBUG: &str = "debug";
 pub const FLAG_BUNDLE: &str = "bundle";
@@ -345,6 +346,23 @@ pub fn build_app() -> Command {
                     .value_parser(value_parser!(PathBuf))
                     .required(true)
             )
+            .arg(
+                Arg::new(FLAG_TARGET)
+                    .long(FLAG_TARGET)
+                    .help("Choose a different target")
+                    .default_value(Into::<&'static str>::into(Target::default()))
+                    .value_parser(build_target_values_parser.clone())
+                    .required(false),
+            )
+        )
+        .subcommand(Command::new(CMD_PREPROCESS_HOST)
+            .about("Runs the surgical linker preprocessor to generate `.rh` and `.rm` files.")
+            .arg(
+                Arg::new(ROC_FILE)
+                    .help("The .roc file for an app using the platform")
+                    .value_parser(value_parser!(PathBuf))
+                    .required(true)
+            )
             .arg(
                 Arg::new(FLAG_TARGET)
                     .long(FLAG_TARGET)

crates/cli/src/main.rs

@@ -4,9 +4,10 @@ use roc_build::link::LinkType;
 use roc_build::program::{check_file, CodeGenBackend};
 use roc_cli::{
     build_app, format_files, format_src, test, BuildConfig, FormatMode, CMD_BUILD, CMD_CHECK,
-    CMD_DEV, CMD_DOCS, CMD_FORMAT, CMD_GEN_STUB_LIB, CMD_GLUE, CMD_REPL, CMD_RUN, CMD_TEST,
-    CMD_VERSION, DIRECTORY_OR_FILES, FLAG_CHECK, FLAG_DEV, FLAG_LIB, FLAG_NO_LINK, FLAG_OUTPUT,
-    FLAG_STDIN, FLAG_STDOUT, FLAG_TARGET, FLAG_TIME, GLUE_DIR, GLUE_SPEC, ROC_FILE,
+    CMD_DEV, CMD_DOCS, CMD_FORMAT, CMD_GEN_STUB_LIB, CMD_GLUE, CMD_PREPROCESS_HOST, CMD_REPL,
+    CMD_RUN, CMD_TEST, CMD_VERSION, DIRECTORY_OR_FILES, FLAG_CHECK, FLAG_DEV, FLAG_LIB,
+    FLAG_NO_LINK, FLAG_OUTPUT, FLAG_STDIN, FLAG_STDOUT, FLAG_TARGET, FLAG_TIME, GLUE_DIR,
+    GLUE_SPEC, ROC_FILE,
 };
 use roc_docs::generate_docs_html;
 use roc_error_macros::user_error;
@@ -131,7 +132,33 @@ fn main() -> io::Result<()> {
                 RocCacheDir::Persistent(cache::roc_cache_dir().as_path()),
                 &target.to_triple(),
                 function_kind,
-            )
+            );
+            Ok(0)
+        }
+        Some((CMD_PREPROCESS_HOST, matches)) => {
+            let input_path = matches.get_one::<PathBuf>(ROC_FILE).unwrap();
+            let target = matches
+                .get_one::<String>(FLAG_TARGET)
+                .and_then(|s| Target::from_str(s).ok())
+                .unwrap_or_default();
+
+            let triple = target.to_triple();
+            let function_kind = FunctionKind::LambdaSet;
+            let (platform_path, stub_lib, stub_dll_symbols) = roc_linker::generate_stub_lib(
+                &input_path,
+                RocCacheDir::Persistent(cache::roc_cache_dir().as_path()),
+                &triple,
+                function_kind,
+            );
+
+            roc_linker::preprocess_host(
+                &triple,
+                &platform_path.with_file_name("main.roc"),
+                &platform_path.with_file_name("linux-x64.rh"),
+                &stub_lib,
+                &stub_dll_symbols,
+            );
+            Ok(0)
         }
         Some((CMD_BUILD, matches)) => {
             let target = matches

crates/linker/src/lib.rs

@@ -90,7 +90,7 @@ pub fn generate_stub_lib(
     roc_cache_dir: RocCacheDir<'_>,
     triple: &Triple,
     function_kind: FunctionKind,
-) -> std::io::Result<i32> {
+) -> (PathBuf, PathBuf, Vec<String>) {
     // Note: this should theoretically just be able to load the host, I think.
     // Instead, I am loading an entire app because that was simpler and had example code.
     // If this was expected to stay around for the the long term, we should change it.
@@ -146,10 +146,10 @@ pub fn generate_stub_lib(
 
         let stub_dll_symbols = exposed_symbols.stub_dll_symbols();
         generate_dynamic_lib(triple, &stub_dll_symbols, &stub_lib);
+        (platform_path.into(), stub_lib, stub_dll_symbols)
     } else {
         unreachable!();
-    };
-    Ok(0)
+    }
 }
 
 pub fn generate_stub_lib_from_loaded(