fix(rust): remove RUNTIME_PATH (#274)

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### Description

Now if users want to have a module called `\0rolldown-runtime.js`, that's totally fine.
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### Test Plan

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---

crates/rolldown/src/bundler/module_loader/module_loader.rs

@@ -14,7 +14,7 @@ use crate::bundler::module::{Module, ModuleVec};
 use crate::bundler::module_loader::module_task_context::ModuleTaskCommonData;
 use crate::bundler::options::input_options::SharedInputOptions;
 use crate::bundler::plugin_driver::SharedPluginDriver;
-use crate::bundler::runtime::{Runtime, RUNTIME_PATH};
+use crate::bundler::runtime::Runtime;
 use crate::bundler::utils::ast_symbol::AstSymbol;
 use crate::bundler::utils::resolve_id::ResolvedRequestInfo;
 use crate::bundler::utils::symbols::Symbols;
@@ -31,6 +31,7 @@ pub struct ModuleLoader<T: FileSystem + Default> {
 #[derive(Debug, Default)]
 pub struct ModuleLoaderContext {
   visited: FxHashMap<FilePath, ModuleId>,
+  runtime_id: Option<ModuleId>,
   remaining: u32,
   intermediate_modules: IndexVec<ModuleId, Option<Module>>,
 }
@@ -91,21 +92,17 @@ impl<T: FileSystem + 'static + Default> ModuleLoader<T> {
   }
 
   pub fn try_spawn_runtime_module_task(&mut self) -> ModuleId {
-    match self.ctx.visited.entry(RUNTIME_PATH.to_string().into()) {
-      std::collections::hash_map::Entry::Occupied(visited) => *visited.get(),
-      std::collections::hash_map::Entry::Vacant(not_visited) => {
-        let id = self.ctx.intermediate_modules.push(None);
-        not_visited.insert(id);
-        self.ctx.remaining += 1;
-        let task = RuntimeNormalModuleTask::new(
-          // safety: Data in `ModuleTaskContext` are alive as long as the `NormalModuleTask`, but rustc doesn't know that.
-          unsafe { self.common_data.assume_static() },
-          id,
-        );
-        tokio::spawn(async move { task.run() });
-        id
-      }
-    }
+    *self.ctx.runtime_id.get_or_insert_with(|| {
+      let id = self.ctx.intermediate_modules.push(None);
+      self.ctx.remaining += 1;
+      let task = RuntimeNormalModuleTask::new(
+        // safety: Data in `ModuleTaskContext` are alive as long as the `NormalModuleTask`, but rustc doesn't know that.
+        unsafe { self.common_data.assume_static() },
+        id,
+      );
+      tokio::spawn(async move { task.run() });
+      id
+    })
   }
 
   pub async fn fetch_all_modules(

crates/rolldown/src/bundler/module_loader/runtime_normal_module_task.rs

@@ -9,7 +9,6 @@ use super::{module_task_context::ModuleTaskCommonData, Msg};
 use crate::bundler::{
   module::normal_module_builder::NormalModuleBuilder,
   module_loader::NormalModuleTaskResult,
-  runtime::RUNTIME_PATH,
   utils::{ast_scope::AstScope, ast_symbol::AstSymbol},
   visitors::scanner::{self, ScanResult},
 };
@@ -54,7 +53,9 @@ impl<'task, T: FileSystem + Default + 'static> RuntimeNormalModuleTask<'task, T>
     builder.id = Some(self.module_id);
     builder.ast = Some(ast);
     builder.unique_name = Some(unique_name);
-    builder.path = Some(ResourceId::new(RUNTIME_PATH.to_string().into()));
+    builder.path = Some(ResourceId::new(
+      "TODO: Runtime module should not have FilePath as source id".to_string().into(),
+    ));
     builder.named_imports = Some(named_imports);
     builder.named_exports = Some(named_exports);
     builder.stmt_infos = Some(stmt_infos);
@@ -65,7 +66,7 @@ impl<'task, T: FileSystem + Default + 'static> RuntimeNormalModuleTask<'task, T>
     builder.scope = Some(scope);
     builder.exports_kind = exports_kind;
     builder.namespace_symbol = Some(namespace_symbol);
-    builder.pretty_path = Some("rolldown-runtime.js".to_string());
+    builder.pretty_path = Some("<runtime>".to_string());
 
     self
       .common_data

crates/rolldown/src/bundler/runtime/mod.rs

@@ -4,8 +4,6 @@ use rustc_hash::FxHashMap;
 
 use super::module::NormalModule;
 
-pub static RUNTIME_PATH: &str = "rolldown-runtime.js";
-
 #[derive(Debug)]
 pub struct Runtime {
   id: ModuleId,