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feat(unstable): support TC39 import defer proposal - #32360
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Adds support for the TC39 "Deferred Module Evaluation" proposal (https://github.com/tc39/proposal-defer-import-eval, Stage 3). Changes: - Enable V8 `--js-defer-import-eval` flag for parser/runtime support - Separate `ModuleImportPhase::Defer` handling from `Evaluation` phase: deferred modules are instantiated but not evaluated, letting V8 create a deferred namespace that triggers evaluation on first property access - Add integration test validating deferred evaluation semantics Ported from denoland/deno_core#1280 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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bartlomieju
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devsnek
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Waiting on v8/v8@f2016e1 in V8 14.7. |
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Upgrade rusty_v8 to 147.3.0 which includes the Module::EvaluateForImportDefer and GetModuleNamespace with phase APIs needed for dynamic import.defer() support. Fix merge conflict in map.rs: restore separate Defer arm in dynamic import handling (must NOT call dynamic_import_module_evaluate since defer imports should not be evaluated until namespace access). Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Point to local deno_graph (with ImportDefer variants) and deno_doc. Add ImportDefer handling to unfurl.rs. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Use the new V8 Module::evaluate_for_import_defer API for dynamic import.defer() to properly gather async transitive dependencies before resolving. Get the deferred namespace via get_module_namespace_with_phase (kDefer) so V8 creates a namespace that triggers evaluation on first property access. Handles three promise states: - Fulfilled: all async deps ready, resolve immediately with deferred ns - Rejected: propagate error to dynamic import reject - Pending: store for later resolution via pending_dyn_mod_evaluations Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
- Filter TS18060 diagnostic ("Deferred imports are only supported when
module is esnext or preserve") since Deno handles module resolution
natively and supports import defer.
- Add test for running import defer from TypeScript files.
- Add test for type-checking import defer TypeScript files.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Replace local path overrides with published crate versions that include import defer support. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
…o feat/import-defer-eval
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Jarred-Sumner
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May 19, 2026
Implements the static form of the Stage 3 [Deferred Module Evaluation](https://github.com/tc39/proposal-defer-import-eval) proposal: ```js import defer * as ns from "./mod.js"; // mod.js is fetched and linked now, but NOT evaluated. console.log("before"); console.log(ns.value); // <-- mod.js evaluates here, synchronously ``` JavaScriptCore already carries the full runtime for this (landed upstream in [WebKit/WebKit#64053](WebKit/WebKit#64053) and present in Bun's pinned WebKit `3167a44fb9`). This PR threads the `ModulePhase::Defer` flag through Bun's own parser → printer → `JSModuleRecord` bridge so JSC actually receives it, and flips on `useImportDefer`. > Supersedes #30015, which was written before the WebKit bump and gated everything behind `#ifdef JSC_HAS_IMPORT_DEFER` pending a separate `oven-sh/WebKit` patch. That indirection is no longer needed. This PR is also narrower in scope — it intentionally does **not** implement dynamic `import.defer()`. ### What changed **Parser / AST** — `src/js_parser/parse/parse_stmt.rs`, `src/ast/{s,import_record}.rs` - `defer` is a contextual phase keyword only when immediately followed by `*`. In every other position (`import defer from "x"`, `import defer, {a} from "x"`, `import { defer } from "x"`) it remains an ordinary identifier. - New `S::Import.phase_defer` and `ImportRecord::Flags::PHASE_DEFER`. **Printer / ModuleInfo** — `src/js_printer/lib.rs` - Emits `import defer` so JSC's own parser sees it during bytecode generation. - New `RecordKind::ImportInfoNamespaceDefer`; `requested_modules` now dedups by `(specifier, phase)` — matching JSC's `ModuleAnalyzer::appendRequestedModule` — and serialises a parallel `u8` phase array. Transpiler-cache version bumped (20 → 21) for the format change. - `finalize()`'s local→indirect export rewrite intentionally ignores deferred namespace imports so `import defer * as ns; export { ns }` stays a **local** export, matching `ModuleAnalyzer::exportVariable`. **JSC bridge** — `src/bundler/analyze_transpiled_module.rs`, `src/bundler_jsc/analyze_jsc.rs`, `src/jsc/bindings/BunAnalyzeTranspiledModule.cpp` - `appendRequestedModule(..., ModulePhase::Defer)` and `addImportEntry({ Namespace, phase: Defer, ... })` for deferred requests. - `dumpRecordInfo` includes phase so the `BUN_DEBUG` fallback-parse diff stays in agreement. **Runtime** — `src/jsc/bindings/ZigGlobalObject.cpp` - `JSC::Options::useImportDefer() = true`. **Unrelated build fix** — `src/jsc/bindings/wtf-bindings.cpp` - `wtf/Int128.h` dropped its transitive `<cassert>` in the latest WebKit bump, which was the only thing declaring `assert()` for `uv__tty_make_raw()` in that unified-build unit. Added the include directly. ### Tests (`test/js/bun/resolve/import-defer.test.ts`, 16 cases) - Evaluation deferred until first non-symbol property access; no re-eval on subsequent access - `ns[Symbol.toStringTag] === "Deferred Module"` without triggering evaluation - `in` / `Object.keys` trigger evaluation - Throwing module re-throws the same error on every access - Same specifier imported at both Evaluation and Defer phase - Transitive TLA dep evaluated eagerly (`GatherAsynchronousTransitiveDependencies`) so on-access eval stays synchronous - `import defer * as ns; export { ns }` exports the deferred namespace as a local binding - `import defer * as d from "./x.json" with { type: "json" }` - Works in `.ts` - `defer` still a valid identifier (`import defer from`, `import defer, {x} from`, `import { defer } from`) - `Bun.Transpiler` round-trips `import defer` - `import defer { x } from` / `import defer x from` are syntax errors ### Out of scope - Dynamic `import.defer(...)` (left for a follow-up; JSC's upstream PR also excluded it). - Bundler-side lazy-wrapping: when `bun build` inlines a deferred dependency into the same chunk the defer semantics are lost (module body runs at chunk load). External imports (`--external`) correctly preserve `import defer` in the output. ### References - Proposal: https://github.com/tc39/proposal-defer-import-eval - Spec: https://tc39.es/proposal-defer-import-eval/ - WebKit runtime: WebKit/WebKit#64053 - Deno: denoland/deno#32360 --------- Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
liooil
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Aug 7, 2026
Implements the static form of the Stage 3 [Deferred Module Evaluation](https://github.com/tc39/proposal-defer-import-eval) proposal: ```js import defer * as ns from "./mod.js"; // mod.js is fetched and linked now, but NOT evaluated. console.log("before"); console.log(ns.value); // <-- mod.js evaluates here, synchronously ``` JavaScriptCore already carries the full runtime for this (landed upstream in [WebKit/WebKit#64053](WebKit/WebKit#64053) and present in Bun's pinned WebKit `3167a44fb9`). This PR threads the `ModulePhase::Defer` flag through Bun's own parser → printer → `JSModuleRecord` bridge so JSC actually receives it, and flips on `useImportDefer`. > Supersedes #30015, which was written before the WebKit bump and gated everything behind `#ifdef JSC_HAS_IMPORT_DEFER` pending a separate `oven-sh/WebKit` patch. That indirection is no longer needed. This PR is also narrower in scope — it intentionally does **not** implement dynamic `import.defer()`. ### What changed **Parser / AST** — `src/js_parser/parse/parse_stmt.rs`, `src/ast/{s,import_record}.rs` - `defer` is a contextual phase keyword only when immediately followed by `*`. In every other position (`import defer from "x"`, `import defer, {a} from "x"`, `import { defer } from "x"`) it remains an ordinary identifier. - New `S::Import.phase_defer` and `ImportRecord::Flags::PHASE_DEFER`. **Printer / ModuleInfo** — `src/js_printer/lib.rs` - Emits `import defer` so JSC's own parser sees it during bytecode generation. - New `RecordKind::ImportInfoNamespaceDefer`; `requested_modules` now dedups by `(specifier, phase)` — matching JSC's `ModuleAnalyzer::appendRequestedModule` — and serialises a parallel `u8` phase array. Transpiler-cache version bumped (20 → 21) for the format change. - `finalize()`'s local→indirect export rewrite intentionally ignores deferred namespace imports so `import defer * as ns; export { ns }` stays a **local** export, matching `ModuleAnalyzer::exportVariable`. **JSC bridge** — `src/bundler/analyze_transpiled_module.rs`, `src/bundler_jsc/analyze_jsc.rs`, `src/jsc/bindings/BunAnalyzeTranspiledModule.cpp` - `appendRequestedModule(..., ModulePhase::Defer)` and `addImportEntry({ Namespace, phase: Defer, ... })` for deferred requests. - `dumpRecordInfo` includes phase so the `BUN_DEBUG` fallback-parse diff stays in agreement. **Runtime** — `src/jsc/bindings/ZigGlobalObject.cpp` - `JSC::Options::useImportDefer() = true`. **Unrelated build fix** — `src/jsc/bindings/wtf-bindings.cpp` - `wtf/Int128.h` dropped its transitive `<cassert>` in the latest WebKit bump, which was the only thing declaring `assert()` for `uv__tty_make_raw()` in that unified-build unit. Added the include directly. ### Tests (`test/js/bun/resolve/import-defer.test.ts`, 16 cases) - Evaluation deferred until first non-symbol property access; no re-eval on subsequent access - `ns[Symbol.toStringTag] === "Deferred Module"` without triggering evaluation - `in` / `Object.keys` trigger evaluation - Throwing module re-throws the same error on every access - Same specifier imported at both Evaluation and Defer phase - Transitive TLA dep evaluated eagerly (`GatherAsynchronousTransitiveDependencies`) so on-access eval stays synchronous - `import defer * as ns; export { ns }` exports the deferred namespace as a local binding - `import defer * as d from "./x.json" with { type: "json" }` - Works in `.ts` - `defer` still a valid identifier (`import defer from`, `import defer, {x} from`, `import { defer } from`) - `Bun.Transpiler` round-trips `import defer` - `import defer { x } from` / `import defer x from` are syntax errors ### Out of scope - Dynamic `import.defer(...)` (left for a follow-up; JSC's upstream PR also excluded it). - Bundler-side lazy-wrapping: when `bun build` inlines a deferred dependency into the same chunk the defer semantics are lost (module body runs at chunk load). External imports (`--external`) correctly preserve `import defer` in the output. ### References - Proposal: https://github.com/tc39/proposal-defer-import-eval - Spec: https://tc39.es/proposal-defer-import-eval/ - WebKit runtime: WebKit/WebKit#64053 - Deno: denoland/deno#32360 --------- Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
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This commit adds an experimental support for
import defer ...syntax,for https://github.com/tc39/proposal-defer-import-eval proposal.
Closes #30053