From 0df286017194562e457dc6c5652c78a97be7c1d8 Mon Sep 17 00:00:00 2001
From: Guy Bedford <guybedford@gmail.com>
Date: Tue, 14 Jul 2026 14:27:40 -0700
Subject: [PATCH 1/3] feat: enable the wasm32-unknown-emscripten target

---
 .github/workflows/ci.yml                |  41 ++++++++
 spellcheck.dic                          |   5 +-
 tokio-macros/src/entry.rs               |  20 +++-
 tokio-test/src/lib.rs                   |  29 ++++++
 tokio/Cargo.toml                        |  14 ++-
 tokio/src/lib.rs                        |  55 ++++++++++
 tokio/src/macros/cfg.rs                 |  52 +++++++++-
 tokio/src/process/emscripten.rs         | 127 ++++++++++++++++++++++++
 tokio/src/process/mod.rs                |   7 +-
 tokio/src/runtime/time/mod.rs           |  12 ++-
 tokio/src/runtime/time/tests/mod.rs     |   2 +-
 tokio/src/sync/tests/atomic_waker.rs    |   2 +-
 tokio/src/sync/tests/notify.rs          |   2 +-
 tokio/src/sync/tests/semaphore_batch.rs |   2 +-
 tokio/src/task/coop/mod.rs              |   2 +-
 tokio/src/task/local.rs                 |   2 +-
 tokio/src/time/mod.rs                   |  14 ++-
 tokio/src/util/idle_notified_set.rs     |   2 +-
 tokio/src/util/mod.rs                   |  18 ++--
 tokio/src/util/trace.rs                 |  17 ++--
 tokio/src/util/wake_list.rs             |   5 +
 tokio/tests/macros_join.rs              |   6 +-
 tokio/tests/macros_pin.rs               |   4 +-
 tokio/tests/macros_select.rs            |   4 +-
 tokio/tests/macros_try_join.rs          |   4 +-
 tokio/tests/sync_barrier.rs             |   2 +-
 tokio/tests/sync_broadcast.rs           |   4 +-
 tokio/tests/sync_broadcast_weak.rs      |   2 +-
 tokio/tests/sync_errors.rs              |   2 +-
 tokio/tests/sync_mpsc.rs                |   6 +-
 tokio/tests/sync_mpsc_weak.rs           |   2 +-
 tokio/tests/sync_mutex.rs               |   6 +-
 tokio/tests/sync_mutex_owned.rs         |   6 +-
 tokio/tests/sync_notify.rs              |   2 +-
 tokio/tests/sync_notify_owned.rs        |   2 +-
 tokio/tests/sync_oneshot.rs             |   6 +-
 tokio/tests/sync_rwlock.rs              |   6 +-
 tokio/tests/sync_semaphore.rs           |   2 +-
 tokio/tests/sync_semaphore_owned.rs     |   2 +-
 tokio/tests/sync_watch.rs               |   2 +-
 tokio/tests/time_wasm.rs                |   6 +-
 41 files changed, 432 insertions(+), 74 deletions(-)
 create mode 100644 tokio/src/process/emscripten.rs

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c5a023538f4..97af268d19f 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -1168,6 +1168,47 @@ jobs:
           RUSTFLAGS: --cfg tokio_unstable
           CARGO_TARGET_WASM32_WASIP2_RUNNER: wasmtime run -Sinherit-network
 
+  wasm32-unknown-emscripten:
+    name: test tokio for wasm32-unknown-emscripten
+    needs: basics
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+      - name: Install Rust ${{ env.rust_stable }}
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          toolchain: ${{ env.rust_stable }}
+          targets: wasm32-unknown-emscripten
+
+      - name: Install Emscripten
+        uses: mymindstorm/setup-emsdk@v14
+        with:
+          version: 'latest'
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 26
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: Install cargo-hack
+        uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-hack
+
+      - name: Check tokio feature matrix for emscripten
+        run: cargo hack check -p tokio --each-feature --exclude-features full,net,process,signal,rt-multi-thread,io-uring,taskdump,schedule-latency --target wasm32-unknown-emscripten
+        working-directory: tokio
+        env:
+          RUSTFLAGS: ""
+
+      - name: Test tokio for emscripten
+        run: cargo test -p tokio --target wasm32-unknown-emscripten --features "rt,time,sync,macros,io-util,test-util" --tests
+        working-directory: tokio
+        env:
+          CARGO_TARGET_WASM32_UNKNOWN_EMSCRIPTEN_RUNNER: node
+          RUSTFLAGS: "-C link-args=-sALLOW_MEMORY_GROWTH=1 -C link-args=-sEXIT_RUNTIME=1 -C link-args=-sSTACK_SIZE=1048576"
+
   check-external-types:
     name: check-external-types (${{ matrix.os }})
     needs: basics
diff --git a/spellcheck.dic b/spellcheck.dic
index aeb1e9db419..ddd75d6e1fa 100644
--- a/spellcheck.dic
+++ b/spellcheck.dic
@@ -1,4 +1,4 @@
-324
+327
 &
 +
 <
@@ -106,6 +106,9 @@ dns
 DNS
 DoS
 dwOpenMode
+emscripten
+Emscripten
+emscripten's
 endian
 enqueue
 enqueued
diff --git a/tokio-macros/src/entry.rs b/tokio-macros/src/entry.rs
index 290888387bb..9c4f7f9c3b2 100644
--- a/tokio-macros/src/entry.rs
+++ b/tokio-macros/src/entry.rs
@@ -506,7 +506,7 @@ fn parse_knobs(mut input: ItemFn, is_test: bool, config: FinalConfig) -> TokenSt
 
     let body_ident = quote! { body };
     // This explicit `return` is intentional. See tokio-rs/tokio#4636
-    let last_block = quote_spanned! {last_stmt_end_span=>
+    let native_last_block = quote_spanned! {last_stmt_end_span=>
 
         #[allow(clippy::expect_used, clippy::diverging_sub_expression, clippy::needless_return, clippy::unwrap_in_result)]
         {
@@ -521,6 +521,24 @@ fn parse_knobs(mut input: ItemFn, is_test: bool, config: FinalConfig) -> TokenSt
 
     };
 
+    // Emscripten runs the native expansion; only the `multi_thread` flavor
+    // diverges — it has no native threads there, so it's rejected with a
+    // targeted error rather than the opaque failure of the native
+    // multi-thread `block_on`.
+    let last_block = match config.flavor {
+        RuntimeFlavor::Threaded => quote! {
+            #[cfg(not(target_os = "emscripten"))]
+            #native_last_block
+            #[cfg(target_os = "emscripten")]
+            ::core::compile_error!(
+                "the `multi_thread` runtime flavor is not available on \
+                 wasm32-unknown-emscripten (no native threads); use \
+                 `flavor = \"current_thread\"`"
+            );
+        },
+        _ => native_last_block,
+    };
+
     let body = input.body();
 
     // For test functions pin the body to the stack and use `Pin<&mut dyn
diff --git a/tokio-test/src/lib.rs b/tokio-test/src/lib.rs
index 87e63861210..321392bf9ff 100644
--- a/tokio-test/src/lib.rs
+++ b/tokio-test/src/lib.rs
@@ -24,6 +24,7 @@ pub mod task;
 /// [`tokio::runtime::Runtime::block_on`][runtime-block-on].
 ///
 /// [runtime-block-on]: https://docs.rs/tokio/1.3.0/tokio/runtime/struct.Runtime.html#method.block_on
+#[cfg(not(target_os = "emscripten"))]
 pub fn block_on<F: std::future::Future>(future: F) -> F::Output {
     use tokio::runtime;
 
@@ -34,3 +35,31 @@ pub fn block_on<F: std::future::Future>(future: F) -> F::Output {
 
     rt.block_on(future)
 }
+
+/// Emscripten variant: polls once with a no-op waker. Ready futures (mocks,
+/// pure computation) complete; ones that must yield (timers, I/O) panic —
+/// use `#[tokio::test]` for those.
+#[cfg(target_os = "emscripten")]
+pub fn block_on<F: std::future::Future>(future: F) -> F::Output {
+    use std::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
+
+    const VTABLE: RawWakerVTable = RawWakerVTable::new(
+        |_| RawWaker::new(std::ptr::null(), &VTABLE),
+        |_| {},
+        |_| {},
+        |_| {},
+    );
+    // SAFETY: vtable entries are valid no-ops.
+    let waker = unsafe { Waker::from_raw(RawWaker::new(std::ptr::null(), &VTABLE)) };
+    let mut cx = Context::from_waker(&waker);
+
+    let mut future = Box::pin(future);
+    match future.as_mut().poll(&mut cx) {
+        Poll::Ready(output) => output,
+        Poll::Pending => panic!(
+            "tokio_test::block_on: future returned Pending on emscripten. \
+             The main thread cannot block on JS event-loop wakeups; use #[tokio::test] \
+             for futures that need timers/network I/O."
+        ),
+    }
+}
diff --git a/tokio/Cargo.toml b/tokio/Cargo.toml
index e260bbc5bd9..0cd6fa059ff 100644
--- a/tokio/Cargo.toml
+++ b/tokio/Cargo.toml
@@ -101,7 +101,7 @@ bytes = { version = "1.2.1", optional = true }
 mio = { version = "1.2.0", optional = true, default-features = false }
 parking_lot = { version = "0.12.0", optional = true }
 
-[target.'cfg(any(not(target_family = "wasm"), all(target_os = "wasi", not(target_env = "p1"))))'.dependencies]
+[target.'cfg(any(not(target_family = "wasm"), target_os = "emscripten", all(target_os = "wasi", not(target_env = "p1"))))'.dependencies]
 socket2 = { version = "0.6.3", optional = true, features = ["all"] }
 
 # Currently unstable. The API exposed by these features may be broken at any time.
@@ -130,6 +130,8 @@ libc = { version = "0.2.168", optional = true }
 
 [target.'cfg(unix)'.dev-dependencies]
 libc = { version = "0.2.168" }
+
+[target.'cfg(all(unix, not(target_os = "emscripten")))'.dev-dependencies]
 nix = { version = "0.31.0", default-features = false, features = ["aio", "fs", "socket"] }
 
 [target.'cfg(windows)'.dependencies.windows-sys]
@@ -146,22 +148,26 @@ features = [
 [dev-dependencies]
 tokio-test = "0.4.0"
 tokio-stream = "0.1"
-tokio-util = { version = "0.7", features = ["rt"] }
 futures = { version = "0.3.0", features = ["async-await"] }
 futures-test = "0.3.31"
 mockall = "0.13.0"
 async-stream = "0.3"
 futures-concurrency = "7.6.3"
 
+[target.'cfg(not(target_os = "emscripten"))'.dev-dependencies]
+tokio-util = { version = "0.7", features = ["rt"] }
+
 [target.'cfg(not(target_family = "wasm"))'.dev-dependencies]
 socket2 = "0.6.0"
-tempfile = "3.1.0"
 proptest = "1"
 
+[target.'cfg(any(not(target_family = "wasm"), target_os = "emscripten"))'.dev-dependencies]
+tempfile = "3.1.0"
+
 [target.'cfg(not(all(target_family = "wasm", target_os = "unknown")))'.dev-dependencies]
 rand = "0.9"
 
-[target.'cfg(all(target_family = "wasm", not(target_os = "wasi")))'.dev-dependencies]
+[target.'cfg(all(target_family = "wasm", target_os = "unknown"))'.dev-dependencies]
 wasm-bindgen-test = "0.3.0"
 
 [target.'cfg(target_os = "freebsd")'.dev-dependencies]
diff --git a/tokio/src/lib.rs b/tokio/src/lib.rs
index efa527606af..bbe71de86b1 100644
--- a/tokio/src/lib.rs
+++ b/tokio/src/lib.rs
@@ -444,6 +444,53 @@
 //! immediately instead of blocking forever. On platforms that don't support
 //! time, this means that the runtime can never be idle in any way.
 //!
+//! ### Emscripten support
+//!
+//! The `wasm32-unknown-emscripten` target is supported at parity with the
+//! other wasm targets. A host-event-loop execution model with a parking
+//! `block_on` is a planned follow-up; until it lands, a `block_on` whose
+//! future cannot resolve synchronously behaves as on other single-threaded
+//! wasm targets.
+//!
+//! Supported features: `rt`, `time`, `sync`, `macros`, `fs`, `io-util`,
+//! `io-std`, and `test-util`. `fs` and the stdio types run their `std::*`
+//! calls inline, since emscripten's filesystem syscalls complete
+//! synchronously. The `net` reactor (epoll-backed, over emscripten's socket
+//! support) is planned as a follow-up; until then the `net` feature fails to
+//! build for this target (rejected by `mio`).
+//!
+//! The `process`, `signal`, and `rt-multi-thread` features are rejected at
+//! compile time: `process`/`signal` have no underlying primitives (`fork`/`exec`,
+//! kernel signal delivery) and `rt-multi-thread` has no native threads.
+//!
+//! `spawn_blocking` has no threadpool to dispatch to, so the closure runs as a
+//! canonical spawned task on the single thread and returns the usual
+//! `JoinHandle`. `tokio::fs::*` and `tokio::io::{stdin, stdout, stderr}`
+//! likewise run their `std::*` calls inline, because emscripten's libc
+//! syscalls complete synchronously and don't block the cooperative scheduler.
+//!
+//! Panics behave as on native: `wasm32-unknown-emscripten` defaults to
+//! `panic = "unwind"`, so panic recovery works, a panicking task yields
+//! `Err(JoinError)`, and `JoinError::is_panic` / `JoinError::into_panic`
+//! report the payload.
+//!
+//! `#[tokio::test]` / `#[tokio::main]` use the native macro expansion; the
+//! `multi_thread` flavor is rejected (no native threads) — use
+//! `flavor = "current_thread"`.
+//!
+//!
+//! #### Linking and running on emscripten
+//!
+//! No js-library or other custom file is required, and plain `node` runs the
+//! test binaries directly:
+//!
+//! ```text
+//! CARGO_TARGET_WASM32_UNKNOWN_EMSCRIPTEN_RUNNER="node"
+//! RUSTFLAGS="-C link-args=-sALLOW_MEMORY_GROWTH=1 \
+//!            -C link-args=-sEXIT_RUNTIME=1 \
+//!            -C link-args=-sSTACK_SIZE=1048576"
+//! ```
+//!
 //! ## Unstable `WASM` support
 //!
 //! Tokio also has unstable support for some additional `WASM` features. This
@@ -467,6 +514,7 @@ compile_error! {
 #[cfg(all(
     not(tokio_unstable),
     target_family = "wasm",
+    not(target_os = "emscripten"),
     any(
         feature = "fs",
         feature = "io-std",
@@ -478,6 +526,13 @@ compile_error! {
 ))]
 compile_error!("Only features sync,macros,io-util,rt,time are supported on wasm.");
 
+// On emscripten, `process`, `signal`, and `rt-multi-thread` compile but are
+// inert, so `full` (and any dependency that enables these features) still
+// builds. `process` and `signal` have no `fork`/`exec` or kernel signal
+// delivery, so their modules are compiled out (see `cfg_process!` /
+// `cfg_signal!`). The multi-threaded runtime compiles but only runs under a
+// `PROXY_TO_PTHREAD` build; `#[tokio::main]` steers to `current_thread`.
+
 #[cfg(all(not(tokio_unstable), feature = "io-uring"))]
 compile_error!("The `io-uring` feature requires `--cfg tokio_unstable`.");
 
diff --git a/tokio/src/macros/cfg.rs b/tokio/src/macros/cfg.rs
index 0a011eb630c..1db46e3505a 100644
--- a/tokio/src/macros/cfg.rs
+++ b/tokio/src/macros/cfg.rs
@@ -398,6 +398,11 @@ macro_rules! cfg_process {
             #[cfg_attr(docsrs, doc(cfg(feature = "process")))]
             #[cfg(not(loom))]
             #[cfg(not(target_os = "wasi"))]
+            // emscripten has no `fork`/`exec`, so the `process` module is a
+            // throwing stub there (see `process/emscripten.rs`); it still
+            // compiles so dependents that name the types build. The orphan
+            // reaper / signal driver it would otherwise need stays off via
+            // `cfg_process_driver!`.
             $item
         )*
     }
@@ -407,6 +412,9 @@ macro_rules! cfg_process_driver {
     ($($item:item)*) => {
         #[cfg(unix)]
         #[cfg(not(loom))]
+        // The driver (orphan reaper backed by the signal handler) doesn't exist
+        // on emscripten; the process module there is a throwing stub.
+        #[cfg(not(target_os = "emscripten"))]
         cfg_process! { $($item)* }
     }
 }
@@ -414,7 +422,10 @@ macro_rules! cfg_process_driver {
 macro_rules! cfg_not_process_driver {
     ($($item:item)*) => {
         $(
-            #[cfg(not(all(unix, not(loom), feature = "process")))]
+            #[cfg(any(
+                not(all(unix, not(loom), feature = "process")),
+                target_os = "emscripten",
+            ))]
             $item
         )*
     }
@@ -427,6 +438,8 @@ macro_rules! cfg_signal {
             #[cfg_attr(docsrs, doc(cfg(feature = "signal")))]
             #[cfg(not(loom))]
             #[cfg(not(target_os = "wasi"))]
+            // No kernel signal delivery on emscripten; inert there.
+            #[cfg(not(target_os = "emscripten"))]
             $item
         )*
     }
@@ -437,6 +450,7 @@ macro_rules! cfg_signal_internal {
         $(
             #[cfg(any(feature = "signal", all(unix, feature = "process")))]
             #[cfg(not(loom))]
+            #[cfg(not(target_os = "emscripten"))]
             $item
         )*
     }
@@ -452,7 +466,7 @@ macro_rules! cfg_signal_internal_and_unix {
 macro_rules! cfg_not_signal_internal {
     ($($item:item)*) => {
         $(
-            #[cfg(any(loom, not(unix), not(any(feature = "signal", all(unix, feature = "process")))))]
+            #[cfg(any(loom, not(unix), target_os = "emscripten", not(any(feature = "signal", all(unix, feature = "process")))))]
             $item
         )*
     }
@@ -715,7 +729,7 @@ macro_rules! cfg_not_wasip1 {
 macro_rules! cfg_is_wasm_not_wasi {
     ($($item:item)*) => {
         $(
-            #[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+            #[cfg(all(target_family = "wasm", target_os = "unknown"))]
             $item
         )*
     }
@@ -768,3 +782,35 @@ macro_rules! cfg_not_schedule_latency {
         )*
     }
 }
+
+/// Enables emscripten-specific code.
+macro_rules! cfg_emscripten {
+    ($($item:item)*) => {
+        $(
+            #[cfg(target_os = "emscripten")]
+            #[cfg_attr(docsrs, doc(cfg(target_os = "emscripten")))]
+            $item
+        )*
+    }
+}
+
+/// Enables code for non-emscripten targets.
+macro_rules! cfg_not_emscripten {
+    ($($item:item)*) => {
+        $(
+            #[cfg(not(target_os = "emscripten"))]
+            $item
+        )*
+    }
+}
+
+/// Enables code requiring both the `rt` feature and the emscripten target.
+macro_rules! cfg_rt_emscripten {
+    ($($item:item)*) => {
+        $(
+            #[cfg(all(feature = "rt", target_os = "emscripten"))]
+            #[cfg_attr(docsrs, doc(cfg(all(feature = "rt", target_os = "emscripten"))))]
+            $item
+        )*
+    }
+}
diff --git a/tokio/src/process/emscripten.rs b/tokio/src/process/emscripten.rs
new file mode 100644
index 00000000000..093b3eb8aa1
--- /dev/null
+++ b/tokio/src/process/emscripten.rs
@@ -0,0 +1,127 @@
+//! Throwing process stub for `wasm32-unknown-emscripten`.
+//!
+//! emscripten has no `fork`/`exec`, so there is nothing to spawn or reap: the
+//! orphan reaper / signal driver the unix backend relies on is compiled out
+//! (see `cfg_process_driver!`). We still provide the `imp` surface that
+//! `process::mod` is generic over so dependents that merely name these types
+//! keep compiling. Spawning fails before it reaches here (`std`'s own
+//! `Command::spawn` returns `Unsupported` on emscripten), and the `Child` /
+//! `ChildStdio` types are uninhabited — every method is unreachable.
+
+use crate::io::{AsyncRead, AsyncWrite, ReadBuf};
+use crate::process::kill::Kill;
+use crate::process::SpawnedChild;
+
+use std::fmt;
+use std::future::Future;
+use std::io;
+use std::os::fd::{AsFd, AsRawFd, BorrowedFd, IntoRawFd, OwnedFd, RawFd};
+use std::pin::Pin;
+use std::process::{Child as StdChild, ExitStatus, Stdio};
+use std::task::{Context, Poll};
+
+fn unsupported<T>() -> io::Result<T> {
+    Err(io::Error::new(
+        io::ErrorKind::Unsupported,
+        "spawning processes is not supported on wasm32-unknown-emscripten",
+    ))
+}
+
+pub(crate) enum Child {}
+
+impl Child {
+    pub(crate) fn id(&self) -> u32 {
+        match *self {}
+    }
+
+    pub(crate) fn try_wait(&mut self) -> io::Result<Option<ExitStatus>> {
+        match *self {}
+    }
+}
+
+impl fmt::Debug for Child {
+    fn fmt(&self, _fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {}
+    }
+}
+
+impl Kill for Child {
+    fn kill(&mut self) -> io::Result<()> {
+        match *self {}
+    }
+}
+
+impl Future for Child {
+    type Output = io::Result<ExitStatus>;
+
+    fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match *self.get_mut() {}
+    }
+}
+
+pub(crate) fn build_child(_child: StdChild) -> io::Result<SpawnedChild> {
+    unsupported()
+}
+
+pub(crate) enum ChildStdio {}
+
+impl ChildStdio {
+    pub(crate) fn into_owned_fd(self) -> io::Result<OwnedFd> {
+        match self {}
+    }
+}
+
+impl fmt::Debug for ChildStdio {
+    fn fmt(&self, _fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {}
+    }
+}
+
+impl AsRawFd for ChildStdio {
+    fn as_raw_fd(&self) -> RawFd {
+        match *self {}
+    }
+}
+
+impl AsFd for ChildStdio {
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        match *self {}
+    }
+}
+
+impl AsyncWrite for ChildStdio {
+    fn poll_write(
+        self: Pin<&mut Self>,
+        _cx: &mut Context<'_>,
+        _buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        match *self.get_mut() {}
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        match *self.get_mut() {}
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        match *self.get_mut() {}
+    }
+}
+
+impl AsyncRead for ChildStdio {
+    fn poll_read(
+        self: Pin<&mut Self>,
+        _cx: &mut Context<'_>,
+        _buf: &mut ReadBuf<'_>,
+    ) -> Poll<io::Result<()>> {
+        match *self.get_mut() {}
+    }
+}
+
+pub(crate) fn convert_to_stdio(io: ChildStdio) -> io::Result<Stdio> {
+    match io {}
+}
+
+pub(crate) fn stdio<T: IntoRawFd>(io: T) -> io::Result<ChildStdio> {
+    let _ = io.into_raw_fd();
+    unsupported()
+}
diff --git a/tokio/src/process/mod.rs b/tokio/src/process/mod.rs
index fb233cb4eb2..37914aa6465 100644
--- a/tokio/src/process/mod.rs
+++ b/tokio/src/process/mod.rs
@@ -227,7 +227,12 @@
 //! [`Child`]: crate::process::Child
 
 #[path = "unix/mod.rs"]
-#[cfg(unix)]
+#[cfg(all(unix, not(target_os = "emscripten")))]
+mod imp;
+
+// emscripten has no `fork`/`exec`: a throwing stub keeps the API present.
+#[path = "emscripten.rs"]
+#[cfg(target_os = "emscripten")]
 mod imp;
 
 #[cfg(unix)]
diff --git a/tokio/src/runtime/time/mod.rs b/tokio/src/runtime/time/mod.rs
index bbdb664c1d3..631049b7a67 100644
--- a/tokio/src/runtime/time/mod.rs
+++ b/tokio/src/runtime/time/mod.rs
@@ -287,12 +287,22 @@ impl Driver {
 }
 
 impl Handle {
-    pub(self) fn process(&self, clock: &Clock) {
+    #[cfg_attr(not(target_os = "emscripten"), allow(dead_code))]
+    pub(crate) fn process(&self, clock: &Clock) {
         let now = self.time_source().now(clock);
 
         self.process_at_time(now);
     }
 
+    /// Returns the absolute deadline in ticks for the soonest unexpired
+    /// timer, or `None` if no timers are registered. Used by the emscripten
+    /// schedule loop to compute its `setTimeout` delay; ticks here are the
+    /// time wheel's monotonic tick basis (`time_source.now()`).
+    #[cfg(target_os = "emscripten")]
+    pub(crate) fn next_expiration_tick(&self) -> Option<u64> {
+        self.inner.lock().wheel.next_expiration_time()
+    }
+
     pub(self) fn process_at_time(&self, mut now: u64) {
         let mut waker_list = WakeList::new();
 
diff --git a/tokio/src/runtime/time/tests/mod.rs b/tokio/src/runtime/time/tests/mod.rs
index 84c765af69e..0773f91c167 100644
--- a/tokio/src/runtime/time/tests/mod.rs
+++ b/tokio/src/runtime/time/tests/mod.rs
@@ -1,4 +1,4 @@
-#![cfg(not(target_os = "wasi"))]
+#![cfg(all(not(target_os = "wasi"), not(target_os = "emscripten")))]
 
 use std::{task::Context, time::Duration};
 
diff --git a/tokio/src/sync/tests/atomic_waker.rs b/tokio/src/sync/tests/atomic_waker.rs
index b182574f325..8c5e1a113d0 100644
--- a/tokio/src/sync/tests/atomic_waker.rs
+++ b/tokio/src/sync/tests/atomic_waker.rs
@@ -15,7 +15,7 @@ impl AssertSync for AtomicWaker {}
 impl AssertSend for Waker {}
 impl AssertSync for Waker {}
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
 
 #[test]
diff --git a/tokio/src/sync/tests/notify.rs b/tokio/src/sync/tests/notify.rs
index 540aa22444a..bfbc6f486c8 100644
--- a/tokio/src/sync/tests/notify.rs
+++ b/tokio/src/sync/tests/notify.rs
@@ -3,7 +3,7 @@ use std::future::Future;
 use std::sync::Arc;
 use std::task::{Context, RawWaker, RawWakerVTable, Waker};
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
 
 #[test]
diff --git a/tokio/src/sync/tests/semaphore_batch.rs b/tokio/src/sync/tests/semaphore_batch.rs
index fb5e8fdd6f7..84f062d8f23 100644
--- a/tokio/src/sync/tests/semaphore_batch.rs
+++ b/tokio/src/sync/tests/semaphore_batch.rs
@@ -3,7 +3,7 @@ use tokio_test::*;
 
 const MAX_PERMITS: usize = crate::sync::Semaphore::MAX_PERMITS;
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
 
 #[test]
diff --git a/tokio/src/task/coop/mod.rs b/tokio/src/task/coop/mod.rs
index 4a520e6f527..5b9ebbd9e50 100644
--- a/tokio/src/task/coop/mod.rs
+++ b/tokio/src/task/coop/mod.rs
@@ -496,7 +496,7 @@ cfg_coop! {
 mod test {
     use super::*;
 
-    #[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+    #[cfg(all(target_family = "wasm", target_os = "unknown"))]
     use wasm_bindgen_test::wasm_bindgen_test as test;
 
     fn get() -> Budget {
diff --git a/tokio/src/task/local.rs b/tokio/src/task/local.rs
index 4ad9c832921..5876319cfb1 100644
--- a/tokio/src/task/local.rs
+++ b/tokio/src/task/local.rs
@@ -1278,7 +1278,7 @@ impl LocalState {
 // ensure they are on the same thread that owns the `LocalSet`.
 unsafe impl Send for LocalState {}
 
-#[cfg(all(test, not(loom)))]
+#[cfg(all(test, not(loom), not(target_os = "emscripten")))]
 mod tests {
     use super::*;
 
diff --git a/tokio/src/time/mod.rs b/tokio/src/time/mod.rs
index 8627a7838aa..e2621e5af9b 100644
--- a/tokio/src/time/mod.rs
+++ b/tokio/src/time/mod.rs
@@ -19,6 +19,9 @@
 //!
 //! These types must be used from within the context of the [`Runtime`](crate::runtime::Runtime).
 //!
+//! On `wasm32-unknown-emscripten` these timers are driven by JavaScript's
+//! event loop via `setTimeout`.
+//!
 //! # Examples
 //!
 //! Wait 100ms and print "100 ms have elapsed"
@@ -86,11 +89,6 @@
 
 mod clock;
 pub(crate) use self::clock::Clock;
-cfg_test_util! {
-    pub use clock::{advance, pause, resume};
-}
-
-pub mod error;
 
 mod instant;
 pub use self::instant::Instant;
@@ -105,6 +103,12 @@ mod timeout;
 #[doc(inline)]
 pub use timeout::{timeout, timeout_at, Timeout};
 
+cfg_test_util! {
+    pub use clock::{advance, pause, resume};
+}
+
+pub mod error;
+
 // Re-export for convenience
 #[doc(no_inline)]
 pub use std::time::Duration;
diff --git a/tokio/src/util/idle_notified_set.rs b/tokio/src/util/idle_notified_set.rs
index 4c5177c5630..3d9e7dd2b8c 100644
--- a/tokio/src/util/idle_notified_set.rs
+++ b/tokio/src/util/idle_notified_set.rs
@@ -492,7 +492,7 @@ unsafe impl<T> linked_list::Link for ListEntry<T> {
     }
 }
 
-#[cfg(all(test, not(loom)))]
+#[cfg(all(test, not(loom), not(target_os = "emscripten")))]
 mod tests {
     use crate::runtime::Builder;
     use crate::task::JoinSet;
diff --git a/tokio/src/util/mod.rs b/tokio/src/util/mod.rs
index c671fd6a1da..d715c3ea942 100644
--- a/tokio/src/util/mod.rs
+++ b/tokio/src/util/mod.rs
@@ -5,14 +5,15 @@ cfg_io_driver! {
 #[cfg(feature = "fs")]
 pub(crate) mod as_ref;
 
-#[cfg(feature = "rt")]
-pub(crate) mod atomic_cell;
+cfg_rt! {
+    pub(crate) mod atomic_cell;
+}
 
-#[cfg(feature = "net")]
-mod blocking_check;
-#[cfg(feature = "net")]
-#[allow(unused_imports)]
-pub(crate) use blocking_check::check_socket_for_blocking;
+cfg_net! {
+    mod blocking_check;
+    #[allow(unused_imports)]
+    pub(crate) use blocking_check::check_socket_for_blocking;
+}
 
 pub(crate) mod metric_atomics;
 
@@ -40,6 +41,9 @@ mod wake_list;
     feature = "signal",
     feature = "time",
 ))]
+// Some WakeList consumers are cfg'd out on emscripten, so the re-export may be
+// unused there.
+#[cfg_attr(target_os = "emscripten", allow(unused_imports))]
 pub(crate) use wake_list::WakeList;
 
 #[cfg(any(
diff --git a/tokio/src/util/trace.rs b/tokio/src/util/trace.rs
index b22c2aeb593..9de1ee0ebfa 100644
--- a/tokio/src/util/trace.rs
+++ b/tokio/src/util/trace.rs
@@ -1,3 +1,4 @@
+// SpawnMeta uses native runtime task infrastructure
 cfg_rt! {
     use std::marker::PhantomData;
 
@@ -180,12 +181,12 @@ cfg_rt! {
     }
 }
 
-cfg_time! {
-    #[track_caller]
-    pub(crate) fn caller_location() -> Option<&'static std::panic::Location<'static>> {
-        #[cfg(all(tokio_unstable, feature = "tracing"))]
-        return Some(std::panic::Location::caller());
-        #[cfg(not(all(tokio_unstable, feature = "tracing")))]
-        None
-    }
+#[cfg(feature = "time")]
+#[cfg_attr(docsrs, doc(cfg(feature = "time")))]
+#[track_caller]
+pub(crate) fn caller_location() -> Option<&'static std::panic::Location<'static>> {
+    #[cfg(all(tokio_unstable, feature = "tracing"))]
+    return Some(std::panic::Location::caller());
+    #[cfg(not(all(tokio_unstable, feature = "tracing")))]
+    None
 }
diff --git a/tokio/src/util/wake_list.rs b/tokio/src/util/wake_list.rs
index 23a559d02be..84e69fabc83 100644
--- a/tokio/src/util/wake_list.rs
+++ b/tokio/src/util/wake_list.rs
@@ -1,3 +1,8 @@
+// On emscripten, `WakeList` is reachable via the feature gates that pull
+// it into `util::mod`, but the actual consumers (time::clock, sync::notify,
+// etc.) may be cfg'd out — leaving the type used only through a re-export.
+#![cfg_attr(target_os = "emscripten", allow(dead_code))]
+
 use core::mem::MaybeUninit;
 use core::ptr;
 use std::task::Waker;
diff --git a/tokio/tests/macros_join.rs b/tokio/tests/macros_join.rs
index 4c6db26d8ae..8b304be8da4 100644
--- a/tokio/tests/macros_join.rs
+++ b/tokio/tests/macros_join.rs
@@ -2,13 +2,13 @@
 #![allow(clippy::disallowed_names)]
 use std::sync::Arc;
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 #[cfg(target_pointer_width = "64")]
 use wasm_bindgen_test::wasm_bindgen_test as test;
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as maybe_tokio_test;
 
-#[cfg(not(all(target_family = "wasm", not(target_os = "wasi"))))]
+#[cfg(not(all(target_family = "wasm", target_os = "unknown")))]
 use tokio::test as maybe_tokio_test;
 
 use tokio::sync::{oneshot, Semaphore};
diff --git a/tokio/tests/macros_pin.rs b/tokio/tests/macros_pin.rs
index 2de68ad031f..19c29d56fdb 100644
--- a/tokio/tests/macros_pin.rs
+++ b/tokio/tests/macros_pin.rs
@@ -1,9 +1,9 @@
 #![cfg(feature = "macros")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as maybe_tokio_test;
 
-#[cfg(not(all(target_family = "wasm", not(target_os = "wasi"))))]
+#[cfg(not(all(target_family = "wasm", target_os = "unknown")))]
 use tokio::test as maybe_tokio_test;
 
 async fn one() {}
diff --git a/tokio/tests/macros_select.rs b/tokio/tests/macros_select.rs
index 3b403e4ce27..662645c0740 100644
--- a/tokio/tests/macros_select.rs
+++ b/tokio/tests/macros_select.rs
@@ -1,10 +1,10 @@
 #![cfg(feature = "macros")]
 #![allow(clippy::disallowed_names)]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as maybe_tokio_test;
 
-#[cfg(not(all(target_family = "wasm", not(target_os = "wasi"))))]
+#[cfg(not(all(target_family = "wasm", target_os = "unknown")))]
 use tokio::test as maybe_tokio_test;
 
 use tokio::sync::oneshot;
diff --git a/tokio/tests/macros_try_join.rs b/tokio/tests/macros_try_join.rs
index 03172ca2a2d..25f811c8e22 100644
--- a/tokio/tests/macros_try_join.rs
+++ b/tokio/tests/macros_try_join.rs
@@ -6,10 +6,10 @@ use std::{convert::Infallible, sync::Arc};
 use tokio::sync::{oneshot, Semaphore};
 use tokio_test::{assert_pending, assert_ready, task};
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as maybe_tokio_test;
 
-#[cfg(not(all(target_family = "wasm", not(target_os = "wasi"))))]
+#[cfg(not(all(target_family = "wasm", target_os = "unknown")))]
 use tokio::test as maybe_tokio_test;
 
 #[maybe_tokio_test]
diff --git a/tokio/tests/sync_barrier.rs b/tokio/tests/sync_barrier.rs
index ac5977f24d8..a8f8e9790bc 100644
--- a/tokio/tests/sync_barrier.rs
+++ b/tokio/tests/sync_barrier.rs
@@ -2,7 +2,7 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "sync")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
 
 use tokio::sync::Barrier;
diff --git a/tokio/tests/sync_broadcast.rs b/tokio/tests/sync_broadcast.rs
index 2bfe235e511..94bfde6bbc2 100644
--- a/tokio/tests/sync_broadcast.rs
+++ b/tokio/tests/sync_broadcast.rs
@@ -2,7 +2,7 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "sync")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
 
 use tokio::sync::broadcast;
@@ -563,7 +563,7 @@ fn sender_len() {
 }
 
 #[test]
-#[cfg(not(all(target_family = "wasm", not(target_os = "wasi"))))]
+#[cfg(not(all(target_family = "wasm", target_os = "unknown")))]
 fn sender_len_random() {
     let (tx, mut rx1) = broadcast::channel(16);
     let mut rx2 = tx.subscribe();
diff --git a/tokio/tests/sync_broadcast_weak.rs b/tokio/tests/sync_broadcast_weak.rs
index 1e7fd6f2d67..ccca130e52e 100644
--- a/tokio/tests/sync_broadcast_weak.rs
+++ b/tokio/tests/sync_broadcast_weak.rs
@@ -2,7 +2,7 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "sync")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
 
 use tokio::sync::broadcast::{self, channel};
diff --git a/tokio/tests/sync_errors.rs b/tokio/tests/sync_errors.rs
index 4e43c8f311e..2bc9b878111 100644
--- a/tokio/tests/sync_errors.rs
+++ b/tokio/tests/sync_errors.rs
@@ -1,7 +1,7 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "sync")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
 
 fn is_error<T: std::error::Error + Send + Sync>() {}
diff --git a/tokio/tests/sync_mpsc.rs b/tokio/tests/sync_mpsc.rs
index 398ab633927..207bef40d7f 100644
--- a/tokio/tests/sync_mpsc.rs
+++ b/tokio/tests/sync_mpsc.rs
@@ -2,12 +2,12 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "sync")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as maybe_tokio_test;
 
-#[cfg(not(all(target_family = "wasm", not(target_os = "wasi"))))]
+#[cfg(not(all(target_family = "wasm", target_os = "unknown")))]
 use tokio::test as maybe_tokio_test;
 
 use std::fmt;
diff --git a/tokio/tests/sync_mpsc_weak.rs b/tokio/tests/sync_mpsc_weak.rs
index fba0fe4e33a..f09e79dab9d 100644
--- a/tokio/tests/sync_mpsc_weak.rs
+++ b/tokio/tests/sync_mpsc_weak.rs
@@ -2,7 +2,7 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "sync")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
 
 use std::sync::atomic::AtomicUsize;
diff --git a/tokio/tests/sync_mutex.rs b/tokio/tests/sync_mutex.rs
index 8d74addad75..37ecf7a2e20 100644
--- a/tokio/tests/sync_mutex.rs
+++ b/tokio/tests/sync_mutex.rs
@@ -1,12 +1,12 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "sync")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as maybe_tokio_test;
 
-#[cfg(not(all(target_family = "wasm", not(target_os = "wasi"))))]
+#[cfg(not(all(target_family = "wasm", target_os = "unknown")))]
 use tokio::test as maybe_tokio_test;
 
 use tokio::sync::Mutex;
diff --git a/tokio/tests/sync_mutex_owned.rs b/tokio/tests/sync_mutex_owned.rs
index 28b2afbf32b..95f7ba82e4b 100644
--- a/tokio/tests/sync_mutex_owned.rs
+++ b/tokio/tests/sync_mutex_owned.rs
@@ -1,12 +1,12 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "sync")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as maybe_tokio_test;
 
-#[cfg(not(all(target_family = "wasm", not(target_os = "wasi"))))]
+#[cfg(not(all(target_family = "wasm", target_os = "unknown")))]
 use tokio::test as maybe_tokio_test;
 
 use tokio::sync::Mutex;
diff --git a/tokio/tests/sync_notify.rs b/tokio/tests/sync_notify.rs
index ee7a9ecf0ad..3eb92716080 100644
--- a/tokio/tests/sync_notify.rs
+++ b/tokio/tests/sync_notify.rs
@@ -1,7 +1,7 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "sync")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
 
 use tokio::sync::Notify;
diff --git a/tokio/tests/sync_notify_owned.rs b/tokio/tests/sync_notify_owned.rs
index 06a0f6ade57..cef574a68aa 100644
--- a/tokio/tests/sync_notify_owned.rs
+++ b/tokio/tests/sync_notify_owned.rs
@@ -1,7 +1,7 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "sync")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
 
 use std::sync::Arc;
diff --git a/tokio/tests/sync_oneshot.rs b/tokio/tests/sync_oneshot.rs
index 08206ea1d33..8aaddbdb39a 100644
--- a/tokio/tests/sync_oneshot.rs
+++ b/tokio/tests/sync_oneshot.rs
@@ -1,12 +1,12 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "sync")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as maybe_tokio_test;
 
-#[cfg(not(all(target_family = "wasm", not(target_os = "wasi"))))]
+#[cfg(not(all(target_family = "wasm", target_os = "unknown")))]
 use tokio::test as maybe_tokio_test;
 
 use tokio::sync::oneshot;
diff --git a/tokio/tests/sync_rwlock.rs b/tokio/tests/sync_rwlock.rs
index 2dc7b0a62ac..c51f9616cc6 100644
--- a/tokio/tests/sync_rwlock.rs
+++ b/tokio/tests/sync_rwlock.rs
@@ -1,12 +1,12 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "sync")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as maybe_tokio_test;
 
-#[cfg(not(all(target_family = "wasm", not(target_os = "wasi"))))]
+#[cfg(not(all(target_family = "wasm", target_os = "unknown")))]
 use tokio::test as maybe_tokio_test;
 
 use std::task::Poll;
diff --git a/tokio/tests/sync_semaphore.rs b/tokio/tests/sync_semaphore.rs
index d976f1e6822..758c581fc8f 100644
--- a/tokio/tests/sync_semaphore.rs
+++ b/tokio/tests/sync_semaphore.rs
@@ -1,6 +1,6 @@
 #![cfg(feature = "sync")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
 
 use std::sync::Arc;
diff --git a/tokio/tests/sync_semaphore_owned.rs b/tokio/tests/sync_semaphore_owned.rs
index f9eeee0cfab..d0e3eb4cdf1 100644
--- a/tokio/tests/sync_semaphore_owned.rs
+++ b/tokio/tests/sync_semaphore_owned.rs
@@ -1,6 +1,6 @@
 #![cfg(feature = "sync")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
 
 use std::sync::Arc;
diff --git a/tokio/tests/sync_watch.rs b/tokio/tests/sync_watch.rs
index 48e4106841f..df366f4a8cb 100644
--- a/tokio/tests/sync_watch.rs
+++ b/tokio/tests/sync_watch.rs
@@ -2,7 +2,7 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "sync")]
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
 use wasm_bindgen_test::wasm_bindgen_test as test;
 
 use tokio::sync::watch;
diff --git a/tokio/tests/time_wasm.rs b/tokio/tests/time_wasm.rs
index 8e0483f2041..073df353de7 100644
--- a/tokio/tests/time_wasm.rs
+++ b/tokio/tests/time_wasm.rs
@@ -1,5 +1,9 @@
 #![warn(rust_2018_idioms)]
-#![cfg(all(target_arch = "wasm32", not(target_os = "wasi")))]
+#![cfg(all(
+    target_arch = "wasm32",
+    not(target_os = "wasi"),
+    not(target_os = "emscripten")
+))]
 
 use wasm_bindgen_test::wasm_bindgen_test;
 

From fbe86575cfe5eb7a8bab088b9f6677a3bbf6f270 Mon Sep 17 00:00:00 2001
From: Guy Bedford <gbedford@cloudflare.com>
Date: Tue, 14 Jul 2026 16:07:16 -0700
Subject: [PATCH 2/3] spelling

---
 spellcheck.dic                |  4 ++--
 tokio/src/lib.rs              | 22 ++++++++++------------
 tokio/src/macros/cfg.rs       | 12 ++++++------
 tokio/src/process/mod.rs      |  2 +-
 tokio/src/runtime/time/mod.rs |  2 +-
 tokio/src/util/mod.rs         |  2 +-
 tokio/src/util/wake_list.rs   |  2 +-
 7 files changed, 22 insertions(+), 24 deletions(-)

diff --git a/spellcheck.dic b/spellcheck.dic
index ddd75d6e1fa..2e01dad8402 100644
--- a/spellcheck.dic
+++ b/spellcheck.dic
@@ -106,9 +106,8 @@ dns
 DNS
 DoS
 dwOpenMode
-emscripten
 Emscripten
-emscripten's
+Emscripten's
 endian
 enqueue
 enqueued
@@ -323,6 +322,7 @@ Wakers
 wakeup
 wakeups
 WASI
+Wasm
 watchOS
 workstealing
 ZST
diff --git a/tokio/src/lib.rs b/tokio/src/lib.rs
index bbe71de86b1..d98898cb6c9 100644
--- a/tokio/src/lib.rs
+++ b/tokio/src/lib.rs
@@ -447,15 +447,14 @@
 //! ### Emscripten support
 //!
 //! The `wasm32-unknown-emscripten` target is supported at parity with the
-//! other wasm targets. A host-event-loop execution model with a parking
+//! other Wasm targets. A host-event-loop execution model with a parking
 //! `block_on` is a planned follow-up; until it lands, a `block_on` whose
 //! future cannot resolve synchronously behaves as on other single-threaded
-//! wasm targets.
+//! Wasm targets.
 //!
 //! Supported features: `rt`, `time`, `sync`, `macros`, `fs`, `io-util`,
-//! `io-std`, and `test-util`. `fs` and the stdio types run their `std::*`
-//! calls inline, since emscripten's filesystem syscalls complete
-//! synchronously. The `net` reactor (epoll-backed, over emscripten's socket
+//! `io-std`, and `test-util` — the same surface as the other single-threaded
+//! Wasm targets. The `net` reactor (epoll-backed, over Emscripten's socket
 //! support) is planned as a follow-up; until then the `net` feature fails to
 //! build for this target (rejected by `mio`).
 //!
@@ -463,11 +462,10 @@
 //! compile time: `process`/`signal` have no underlying primitives (`fork`/`exec`,
 //! kernel signal delivery) and `rt-multi-thread` has no native threads.
 //!
-//! `spawn_blocking` has no threadpool to dispatch to, so the closure runs as a
-//! canonical spawned task on the single thread and returns the usual
-//! `JoinHandle`. `tokio::fs::*` and `tokio::io::{stdin, stdout, stderr}`
-//! likewise run their `std::*` calls inline, because emscripten's libc
-//! syscalls complete synchronously and don't block the cooperative scheduler.
+//! `spawn_blocking` dispatches to the blocking thread pool and so behaves as on
+//! the other single-threaded Wasm targets. Running `spawn_blocking` closures
+//! and `fs`/stdio `std::*` calls inline over Emscripten's synchronous syscalls
+//! is part of the planned host-event-loop follow-up.
 //!
 //! Panics behave as on native: `wasm32-unknown-emscripten` defaults to
 //! `panic = "unwind"`, so panic recovery works, a panicking task yields
@@ -479,7 +477,7 @@
 //! `flavor = "current_thread"`.
 //!
 //!
-//! #### Linking and running on emscripten
+//! #### Linking and running on Emscripten
 //!
 //! No js-library or other custom file is required, and plain `node` runs the
 //! test binaries directly:
@@ -526,7 +524,7 @@ compile_error! {
 ))]
 compile_error!("Only features sync,macros,io-util,rt,time are supported on wasm.");
 
-// On emscripten, `process`, `signal`, and `rt-multi-thread` compile but are
+// On Emscripten, `process`, `signal`, and `rt-multi-thread` compile but are
 // inert, so `full` (and any dependency that enables these features) still
 // builds. `process` and `signal` have no `fork`/`exec` or kernel signal
 // delivery, so their modules are compiled out (see `cfg_process!` /
diff --git a/tokio/src/macros/cfg.rs b/tokio/src/macros/cfg.rs
index 1db46e3505a..e9a9962ac8b 100644
--- a/tokio/src/macros/cfg.rs
+++ b/tokio/src/macros/cfg.rs
@@ -398,7 +398,7 @@ macro_rules! cfg_process {
             #[cfg_attr(docsrs, doc(cfg(feature = "process")))]
             #[cfg(not(loom))]
             #[cfg(not(target_os = "wasi"))]
-            // emscripten has no `fork`/`exec`, so the `process` module is a
+            // Emscripten has no `fork`/`exec`, so the `process` module is a
             // throwing stub there (see `process/emscripten.rs`); it still
             // compiles so dependents that name the types build. The orphan
             // reaper / signal driver it would otherwise need stays off via
@@ -413,7 +413,7 @@ macro_rules! cfg_process_driver {
         #[cfg(unix)]
         #[cfg(not(loom))]
         // The driver (orphan reaper backed by the signal handler) doesn't exist
-        // on emscripten; the process module there is a throwing stub.
+        // on Emscripten; the process module there is a throwing stub.
         #[cfg(not(target_os = "emscripten"))]
         cfg_process! { $($item)* }
     }
@@ -438,7 +438,7 @@ macro_rules! cfg_signal {
             #[cfg_attr(docsrs, doc(cfg(feature = "signal")))]
             #[cfg(not(loom))]
             #[cfg(not(target_os = "wasi"))]
-            // No kernel signal delivery on emscripten; inert there.
+            // No kernel signal delivery on Emscripten; inert there.
             #[cfg(not(target_os = "emscripten"))]
             $item
         )*
@@ -783,7 +783,7 @@ macro_rules! cfg_not_schedule_latency {
     }
 }
 
-/// Enables emscripten-specific code.
+/// Enables Emscripten-specific code.
 macro_rules! cfg_emscripten {
     ($($item:item)*) => {
         $(
@@ -794,7 +794,7 @@ macro_rules! cfg_emscripten {
     }
 }
 
-/// Enables code for non-emscripten targets.
+/// Enables code for non-Emscripten targets.
 macro_rules! cfg_not_emscripten {
     ($($item:item)*) => {
         $(
@@ -804,7 +804,7 @@ macro_rules! cfg_not_emscripten {
     }
 }
 
-/// Enables code requiring both the `rt` feature and the emscripten target.
+/// Enables code requiring both the `rt` feature and the Emscripten target.
 macro_rules! cfg_rt_emscripten {
     ($($item:item)*) => {
         $(
diff --git a/tokio/src/process/mod.rs b/tokio/src/process/mod.rs
index 37914aa6465..6ca2e2b8f82 100644
--- a/tokio/src/process/mod.rs
+++ b/tokio/src/process/mod.rs
@@ -230,7 +230,7 @@
 #[cfg(all(unix, not(target_os = "emscripten")))]
 mod imp;
 
-// emscripten has no `fork`/`exec`: a throwing stub keeps the API present.
+// Emscripten has no `fork`/`exec`: a throwing stub keeps the API present.
 #[path = "emscripten.rs"]
 #[cfg(target_os = "emscripten")]
 mod imp;
diff --git a/tokio/src/runtime/time/mod.rs b/tokio/src/runtime/time/mod.rs
index 631049b7a67..331906f5b77 100644
--- a/tokio/src/runtime/time/mod.rs
+++ b/tokio/src/runtime/time/mod.rs
@@ -295,7 +295,7 @@ impl Handle {
     }
 
     /// Returns the absolute deadline in ticks for the soonest unexpired
-    /// timer, or `None` if no timers are registered. Used by the emscripten
+    /// timer, or `None` if no timers are registered. Used by the Emscripten
     /// schedule loop to compute its `setTimeout` delay; ticks here are the
     /// time wheel's monotonic tick basis (`time_source.now()`).
     #[cfg(target_os = "emscripten")]
diff --git a/tokio/src/util/mod.rs b/tokio/src/util/mod.rs
index d715c3ea942..d4390c9b73e 100644
--- a/tokio/src/util/mod.rs
+++ b/tokio/src/util/mod.rs
@@ -41,7 +41,7 @@ mod wake_list;
     feature = "signal",
     feature = "time",
 ))]
-// Some WakeList consumers are cfg'd out on emscripten, so the re-export may be
+// Some WakeList consumers are cfg'd out on Emscripten, so the re-export may be
 // unused there.
 #[cfg_attr(target_os = "emscripten", allow(unused_imports))]
 pub(crate) use wake_list::WakeList;
diff --git a/tokio/src/util/wake_list.rs b/tokio/src/util/wake_list.rs
index 84e69fabc83..db077ebee17 100644
--- a/tokio/src/util/wake_list.rs
+++ b/tokio/src/util/wake_list.rs
@@ -1,4 +1,4 @@
-// On emscripten, `WakeList` is reachable via the feature gates that pull
+// On Emscripten, `WakeList` is reachable via the feature gates that pull
 // it into `util::mod`, but the actual consumers (time::clock, sync::notify,
 // etc.) may be cfg'd out — leaving the type used only through a re-export.
 #![cfg_attr(target_os = "emscripten", allow(dead_code))]

From f6527a570880c58cd6b630fb702329c480133759 Mon Sep 17 00:00:00 2001
From: Guy Bedford <gbedford@cloudflare.com>
Date: Wed, 15 Jul 2026 11:46:35 -0700
Subject: [PATCH 3/3] review: remove stale setTimeout doc and unused follow-up
 scaffolding

---
 tokio/src/macros/cfg.rs       | 32 --------------------------------
 tokio/src/runtime/time/mod.rs | 12 +-----------
 tokio/src/time/mod.rs         | 14 +++++---------
 tokio/src/util/mod.rs         | 18 +++++++-----------
 tokio/src/util/trace.rs       | 17 ++++++++---------
 tokio/src/util/wake_list.rs   |  5 -----
 6 files changed, 21 insertions(+), 77 deletions(-)

diff --git a/tokio/src/macros/cfg.rs b/tokio/src/macros/cfg.rs
index e9a9962ac8b..86c1f199386 100644
--- a/tokio/src/macros/cfg.rs
+++ b/tokio/src/macros/cfg.rs
@@ -782,35 +782,3 @@ macro_rules! cfg_not_schedule_latency {
         )*
     }
 }
-
-/// Enables Emscripten-specific code.
-macro_rules! cfg_emscripten {
-    ($($item:item)*) => {
-        $(
-            #[cfg(target_os = "emscripten")]
-            #[cfg_attr(docsrs, doc(cfg(target_os = "emscripten")))]
-            $item
-        )*
-    }
-}
-
-/// Enables code for non-Emscripten targets.
-macro_rules! cfg_not_emscripten {
-    ($($item:item)*) => {
-        $(
-            #[cfg(not(target_os = "emscripten"))]
-            $item
-        )*
-    }
-}
-
-/// Enables code requiring both the `rt` feature and the Emscripten target.
-macro_rules! cfg_rt_emscripten {
-    ($($item:item)*) => {
-        $(
-            #[cfg(all(feature = "rt", target_os = "emscripten"))]
-            #[cfg_attr(docsrs, doc(cfg(all(feature = "rt", target_os = "emscripten"))))]
-            $item
-        )*
-    }
-}
diff --git a/tokio/src/runtime/time/mod.rs b/tokio/src/runtime/time/mod.rs
index 331906f5b77..bbdb664c1d3 100644
--- a/tokio/src/runtime/time/mod.rs
+++ b/tokio/src/runtime/time/mod.rs
@@ -287,22 +287,12 @@ impl Driver {
 }
 
 impl Handle {
-    #[cfg_attr(not(target_os = "emscripten"), allow(dead_code))]
-    pub(crate) fn process(&self, clock: &Clock) {
+    pub(self) fn process(&self, clock: &Clock) {
         let now = self.time_source().now(clock);
 
         self.process_at_time(now);
     }
 
-    /// Returns the absolute deadline in ticks for the soonest unexpired
-    /// timer, or `None` if no timers are registered. Used by the Emscripten
-    /// schedule loop to compute its `setTimeout` delay; ticks here are the
-    /// time wheel's monotonic tick basis (`time_source.now()`).
-    #[cfg(target_os = "emscripten")]
-    pub(crate) fn next_expiration_tick(&self) -> Option<u64> {
-        self.inner.lock().wheel.next_expiration_time()
-    }
-
     pub(self) fn process_at_time(&self, mut now: u64) {
         let mut waker_list = WakeList::new();
 
diff --git a/tokio/src/time/mod.rs b/tokio/src/time/mod.rs
index e2621e5af9b..8627a7838aa 100644
--- a/tokio/src/time/mod.rs
+++ b/tokio/src/time/mod.rs
@@ -19,9 +19,6 @@
 //!
 //! These types must be used from within the context of the [`Runtime`](crate::runtime::Runtime).
 //!
-//! On `wasm32-unknown-emscripten` these timers are driven by JavaScript's
-//! event loop via `setTimeout`.
-//!
 //! # Examples
 //!
 //! Wait 100ms and print "100 ms have elapsed"
@@ -89,6 +86,11 @@
 
 mod clock;
 pub(crate) use self::clock::Clock;
+cfg_test_util! {
+    pub use clock::{advance, pause, resume};
+}
+
+pub mod error;
 
 mod instant;
 pub use self::instant::Instant;
@@ -103,12 +105,6 @@ mod timeout;
 #[doc(inline)]
 pub use timeout::{timeout, timeout_at, Timeout};
 
-cfg_test_util! {
-    pub use clock::{advance, pause, resume};
-}
-
-pub mod error;
-
 // Re-export for convenience
 #[doc(no_inline)]
 pub use std::time::Duration;
diff --git a/tokio/src/util/mod.rs b/tokio/src/util/mod.rs
index d4390c9b73e..c671fd6a1da 100644
--- a/tokio/src/util/mod.rs
+++ b/tokio/src/util/mod.rs
@@ -5,15 +5,14 @@ cfg_io_driver! {
 #[cfg(feature = "fs")]
 pub(crate) mod as_ref;
 
-cfg_rt! {
-    pub(crate) mod atomic_cell;
-}
+#[cfg(feature = "rt")]
+pub(crate) mod atomic_cell;
 
-cfg_net! {
-    mod blocking_check;
-    #[allow(unused_imports)]
-    pub(crate) use blocking_check::check_socket_for_blocking;
-}
+#[cfg(feature = "net")]
+mod blocking_check;
+#[cfg(feature = "net")]
+#[allow(unused_imports)]
+pub(crate) use blocking_check::check_socket_for_blocking;
 
 pub(crate) mod metric_atomics;
 
@@ -41,9 +40,6 @@ mod wake_list;
     feature = "signal",
     feature = "time",
 ))]
-// Some WakeList consumers are cfg'd out on Emscripten, so the re-export may be
-// unused there.
-#[cfg_attr(target_os = "emscripten", allow(unused_imports))]
 pub(crate) use wake_list::WakeList;
 
 #[cfg(any(
diff --git a/tokio/src/util/trace.rs b/tokio/src/util/trace.rs
index 9de1ee0ebfa..b22c2aeb593 100644
--- a/tokio/src/util/trace.rs
+++ b/tokio/src/util/trace.rs
@@ -1,4 +1,3 @@
-// SpawnMeta uses native runtime task infrastructure
 cfg_rt! {
     use std::marker::PhantomData;
 
@@ -181,12 +180,12 @@ cfg_rt! {
     }
 }
 
-#[cfg(feature = "time")]
-#[cfg_attr(docsrs, doc(cfg(feature = "time")))]
-#[track_caller]
-pub(crate) fn caller_location() -> Option<&'static std::panic::Location<'static>> {
-    #[cfg(all(tokio_unstable, feature = "tracing"))]
-    return Some(std::panic::Location::caller());
-    #[cfg(not(all(tokio_unstable, feature = "tracing")))]
-    None
+cfg_time! {
+    #[track_caller]
+    pub(crate) fn caller_location() -> Option<&'static std::panic::Location<'static>> {
+        #[cfg(all(tokio_unstable, feature = "tracing"))]
+        return Some(std::panic::Location::caller());
+        #[cfg(not(all(tokio_unstable, feature = "tracing")))]
+        None
+    }
 }
diff --git a/tokio/src/util/wake_list.rs b/tokio/src/util/wake_list.rs
index db077ebee17..23a559d02be 100644
--- a/tokio/src/util/wake_list.rs
+++ b/tokio/src/util/wake_list.rs
@@ -1,8 +1,3 @@
-// On Emscripten, `WakeList` is reachable via the feature gates that pull
-// it into `util::mod`, but the actual consumers (time::clock, sync::notify,
-// etc.) may be cfg'd out — leaving the type used only through a re-export.
-#![cfg_attr(target_os = "emscripten", allow(dead_code))]
-
 use core::mem::MaybeUninit;
 use core::ptr;
 use std::task::Waker;