Reduce stripped ESM module load latency

Cargo.toml

@@ -111,6 +111,10 @@ harness = false
 name = "typescript_transform_latency"
 harness = false
 
+[[test]]
+name = "esm_module_load_phases"
+harness = false
+
 [[test]]
 name = "migrate_config_split"
 harness = false

crates/wasm-rquickjs/skeleton/src/internal/module_loading.rs

@@ -2184,6 +2184,10 @@ fn collect_declared_cjs_globals_in_esm(source: &str) -> Vec<String> {
     let mut i = 0usize;
     let mut declared = Vec::<String>::new();
     while i < bytes.len() {
+        if bytes[i].is_ascii_whitespace() {
+            i = skip_ascii_whitespace(source, i);
+            continue;
+        }
         if let Some(next) = skip_esm_cjs_global_scanner_span(source, i) {
             i = next;
             continue;
@@ -2241,6 +2245,10 @@ fn find_bare_cjs_global_in_esm_among(
             scopes.pop();
         }
 
+        if bytes[i].is_ascii_whitespace() {
+            i = skip_ascii_whitespace(source, i);
+            continue;
+        }
         if let Some(next) = skip_esm_cjs_global_scanner_span(source, i) {
             i = next;
             continue;
@@ -7207,6 +7215,14 @@ fn skip_ws_comments(source: &str, pos: usize) -> usize {
     skip_ws_comments_impl::<false>(source, pos).0
 }
 
+fn skip_ascii_whitespace(source: &str, mut pos: usize) -> usize {
+    let bytes = source.as_bytes();
+    while pos < bytes.len() && bytes[pos].is_ascii_whitespace() {
+        pos += 1;
+    }
+    pos
+}
+
 fn skip_ws_comments_with_line_terminator(source: &str, pos: usize) -> (usize, bool) {
     skip_ws_comments_impl::<true>(source, pos)
 }
@@ -11848,6 +11864,9 @@ pub(crate) async fn initialize_module_loading(rt: &AsyncRuntime, ctx: &AsyncCont
 }
 
 fn rewrite_import_meta_main(source: &str, replacement: &str) -> String {
+    if !source.contains("import") {
+        return source.to_string();
+    }
     let mut spans = Vec::new();
     let _ = scan_code_positions(source, true, |i, _| {
         if let Some(end) = parse_import_meta_main_span(source, i) {
@@ -12707,6 +12726,19 @@ import "./dep.js" withあ;
         );
     }
 
+    #[test]
+    fn dense_whitespace_keeps_esm_scanner_results() {
+        let padding = " \n\t\r".repeat(16_384);
+        let source = format!("{padding}export default 42;");
+
+        assert_cjs_global(&source, None);
+        assert!(collect_declared_cjs_globals_in_esm(&source).is_empty());
+        assert_eq!(rewrite_import_meta_main(&source, "false"), source);
+
+        let with_require = format!("{padding}export default require;");
+        assert_cjs_global(&with_require, Some("require"));
+    }
+
     #[test]
     fn package_type_diagnostics_ignore_local_exports_binding() {
         assert!(

examples/runtime/esm-module-load-phases/src/esm-module-load-phases.js

@@ -0,0 +1,49 @@
+import fs from 'node:fs';
+import { stripTypeScriptTypes } from 'node:module';
+import { runJavaScript } from 'wasm-rquickjs:execution';
+
+const ROOT = '/esm-module-load-phases';
+
+function typedPrefix(sourceBytes) {
+    const lines = [];
+    let length = 0;
+    for (let index = 0; length < sourceBytes; index++) {
+        const line = `type Padding${index} = { value: number; next?: Padding${index} };\n`;
+        lines.push(line);
+        length += line.length;
+    }
+    return lines.join('');
+}
+
+export async function measureCase(sourceBytes, sample) {
+    fs.mkdirSync(ROOT, { recursive: true });
+    const path = `${ROOT}/prepared-${sourceBytes}-${sample}.mjs`;
+    const typedSource = `${typedPrefix(Number(sourceBytes))}
+globalThis.__esmPhaseMarks.evaluationStart = performance.now();
+export default function run(): number { return 42; }
+globalThis.__esmPhaseMarks.evaluationEnd = performance.now();`;
+    const preparedSource = stripTypeScriptTypes(typedSource, { mode: 'strip' });
+    fs.writeFileSync(path, preparedSource);
+
+    const started = performance.now();
+    const execution = await runJavaScript({ source: `
+        globalThis.__esmPhaseMarks = {};
+        globalThis.__esmPhaseMarks.importStart = performance.now();
+        const loaded = await import(${JSON.stringify(path)});
+        globalThis.__esmPhaseMarks.importResolved = performance.now();
+        return {
+            value: loaded.default(),
+            marks: globalThis.__esmPhaseMarks,
+        };
+    ` });
+    return JSON.stringify({
+        requestedSourceBytes: Number(sourceBytes),
+        actualSourceBytes: typedSource.length,
+        preparedSourceBytes: preparedSource.length,
+        elapsedMs: performance.now() - started,
+        value: execution.value.value,
+        marks: execution.value.marks,
+        overflowed: execution.overflowed,
+        profile: execution.profile,
+    });
+}

examples/runtime/esm-module-load-phases/wit/esm-module-load-phases.wit

@@ -0,0 +1,5 @@
+package quickjs:esm-module-load-phases;
+
+world esm-module-load-phases {
+  export measure-case: func(source-bytes: u64, sample: u64) -> string;
+}