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Merged
rust-bors merged 1 commit into from
May 8, 2026
Merged

Support -Cpanic=unwind on WASI targets #156061

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8 changes: 8 additions & 0 deletions compiler/rustc_target/src/spec/base/wasm.rs
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Expand Up @@ -110,6 +110,14 @@ pub(crate) fn options() -> TargetOptions {
// representation, so this is disabled.
generate_arange_section: false,

// Differ from LLVM's default to use the legacy exception-handling
// proposal instructions and use the standard exception-handling
// instructions. Note that this is only applicable when unwinding is
// actually turned on, which it's not by default on this target. For
// `-Zbuild-std` builds, however, this affects when rebuilding libstd
// with unwinding.
llvm_args: cvs!["-wasm-use-legacy-eh=false"],
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@bjorn3 bjorn3 May 3, 2026 •
edited
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Isn't

rust/compiler/rustc_codegen_ssa/src/base.rs

Lines 369 to 375 in 54f67d2

// Returns `true` if this session's target will use native wasm
// exceptions. This means that the VM does the unwinding for
// us
pub fn wants_wasm_eh(sess: &Session) -> bool {
sess.target.is_like_wasm
&& (sess.target.os != Os::Emscripten || sess.opts.unstable_opts.emscripten_wasm_eh)
}
already supposed to do this?

Edit: Might be confused with wasm vs js exception handling.

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@alexcrichton alexcrichton May 4, 2026

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I believe, yeah, that's related to the Emscripten-specific scheme of exceptions pre-wasm-exceptions of using JS instead. Effectively there's 3 modes of exceptions on Wasm:

  1. Historical JS-based support for Emscripten. That's used when wants_wasm_eh is false and is only applicable to historical versions of the Emscripten target (or -Zemscripten-wasm-eh=false)
  2. Historical wasm support. This uses the now-legacy exception-handling proposal that shipped in browsers but was never standardized. This uses wasm instructions and has wants_wasm_eh is true. LLVM by default emits this today.
  3. Modern wasm support. This uses the now-standard exception-handling proposal as the successor to the "legacy exceptions" of above. This is wants_wasm_eh is true plus an option to LLVM to use the new instructions.

For (1) the LLVM IR is different, and for (2) and (3) they're the same IR. I don't know much about (1) myself. For WASI this PR only exposes (3)


..Default::default()
}
}
5 changes: 5 additions & 0 deletions compiler/rustc_target/src/spec/targets/wasm32_unknown_emscripten.rs
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Expand Up @@ -31,6 +31,11 @@ pub(crate) fn target() -> Target {
panic_strategy: PanicStrategy::Unwind,
no_default_libraries: false,
families: cvs!["unix", "wasm"],
// Explicitly override the `base::wasm`'s `llvm_args` back to empty. The
// base is to force using the most standard exception-handling
// instructions, when enabled, but this target is intended to follow
// Emscripten, which is whatever LLVM defaults to.
llvm_args: cvs![],
..base::wasm::options()
};
Target {
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1 change: 1 addition & 0 deletions library/unwind/src/lib.rs
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Expand Up @@ -33,6 +33,7 @@ cfg_select! {
target_os = "psp",
target_os = "solid_asp3",
all(target_vendor = "fortanix", target_env = "sgx"),
all(target_os = "wasi", panic = "unwind"),
) => {
mod libunwind;
pub use libunwind::*;
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9 changes: 8 additions & 1 deletion library/unwind/src/libunwind.rs
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Expand Up @@ -75,7 +75,7 @@ pub const unwinder_private_data_size: usize = 2;
#[cfg(all(target_arch = "wasm32", target_os = "emscripten"))]
pub const unwinder_private_data_size: usize = 20;

#[cfg(all(target_arch = "wasm32", target_os = "linux"))]
#[cfg(all(target_arch = "wasm32", any(target_os = "linux", target_os = "wasi")))]
pub const unwinder_private_data_size: usize = 2;

#[cfg(target_arch = "hexagon")]
Expand Down Expand Up @@ -111,6 +111,13 @@ pub type _Unwind_Exception_Cleanup_Fn =
),
link(name = "unwind", kind = "static", modifiers = "-bundle")
)]
// Explicitly link the `unwind` library on WASI targets.
//
// This is provided in the self-contained sysroot for WASI targets by default.
// Note that Rust defaults to `-Cpanic=abort` on WASI targets meaning that this
// doesn't end up getting used by default, but this does mean that with
// `-Zbuild-std` this'll automatically link it in.
#[cfg_attr(target_os = "wasi", link(name = "unwind"))]
unsafe extern "C-unwind" {
pub fn _Unwind_Resume(exception: *mut _Unwind_Exception) -> !;
}
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10 changes: 10 additions & 0 deletions src/bootstrap/src/core/build_steps/compile.rs
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Expand Up @@ -457,6 +457,16 @@ fn copy_self_contained_objects(
DependencyType::TargetSelfContained,
);
}
if srcdir.join("eh").exists() {
copy_and_stamp(
builder,
&libdir_self_contained,
&srcdir.join("eh"),
"libunwind.a",
&mut target_deps,
DependencyType::TargetSelfContained,
);
}
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} else if target.is_windows_gnu() || target.is_windows_gnullvm() {
for obj in ["crt2.o", "dllcrt2.o"].iter() {
let src = compiler_file(builder, &builder.cc(target), target, CLang::C, obj);
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2 changes: 1 addition & 1 deletion src/bootstrap/src/lib.rs
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Expand Up @@ -1500,7 +1500,7 @@ impl Build {
if let Some(path) = finder.maybe_have("wasmtime")
&& let Ok(mut path) = path.into_os_string().into_string()
{
path.push_str(" run -C cache=n --dir .");
path.push_str(" run -Wexceptions -C cache=n --dir .");
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@bjorn3 bjorn3 May 5, 2026 •
edited by rustbot
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There are no tests added that use exceptions, right?

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@alexcrichton alexcrichton May 5, 2026

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Almost yeah, but there's one test that does extern crate panic_unwind; which pulls in libunwind which pulls in a throw instruction which then needs this to validate

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@bjorn3 bjorn3 May 6, 2026

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I'm surprised that even works without //@ needs-unwind to only run the test for panic=unwind. I thought rustc refused to link a panic runtime that doesn't match the -Cpanic=..., but I guess that check is only done when the panic runtime dependency is injected by rustc, not when it is explicitly done by the user.

// Make sure that tests have access to RUSTC_BOOTSTRAP. This (for example) is
// required for libtest to work on beta/stable channels.
//
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4 changes: 2 additions & 2 deletions src/ci/docker/host-x86_64/dist-various-2/Dockerfile
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Expand Up @@ -87,9 +87,9 @@ RUN /tmp/build-fuchsia-toolchain.sh
COPY host-x86_64/dist-various-2/build-x86_64-fortanix-unknown-sgx-toolchain.sh /tmp/
RUN /tmp/build-x86_64-fortanix-unknown-sgx-toolchain.sh

RUN curl -L https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-32/wasi-sdk-32.0-x86_64-linux.tar.gz | \
RUN curl -L https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-33/wasi-sdk-33.0-x86_64-linux.tar.gz | \
tar -xz
ENV WASI_SDK_PATH=/tmp/wasi-sdk-32.0-x86_64-linux
ENV WASI_SDK_PATH=/tmp/wasi-sdk-33.0-x86_64-linux

COPY scripts/freebsd-toolchain.sh /tmp/
RUN /tmp/freebsd-toolchain.sh i686
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4 changes: 2 additions & 2 deletions src/ci/docker/host-x86_64/pr-check-2/Dockerfile
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Expand Up @@ -21,9 +21,9 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
mingw-w64 \
&& rm -rf /var/lib/apt/lists/*

RUN curl -L https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-32/wasi-sdk-32.0-x86_64-linux.tar.gz | \
RUN curl -L https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-33/wasi-sdk-33.0-x86_64-linux.tar.gz | \
tar -xz
ENV WASI_SDK_PATH=/wasi-sdk-32.0-x86_64-linux
ENV WASI_SDK_PATH=/wasi-sdk-33.0-x86_64-linux

ENV RUST_CONFIGURE_ARGS="--set rust.validate-mir-opts=3"

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8 changes: 4 additions & 4 deletions src/ci/docker/host-x86_64/test-various/Dockerfile
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Expand Up @@ -43,9 +43,9 @@ WORKDIR /
COPY scripts/sccache.sh /scripts/
RUN sh /scripts/sccache.sh

RUN curl -L https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-32/wasi-sdk-32.0-x86_64-linux.tar.gz | \
RUN curl -L https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-33/wasi-sdk-33.0-x86_64-linux.tar.gz | \
tar -xz
ENV WASI_SDK_PATH=/wasi-sdk-32.0-x86_64-linux
ENV WASI_SDK_PATH=/wasi-sdk-33.0-x86_64-linux

ENV RUST_CONFIGURE_ARGS="--musl-root-x86_64=/usr/local/x86_64-linux-musl \
--set rust.lld"
Expand All @@ -57,9 +57,9 @@ ENV RUST_CONFIGURE_ARGS="--musl-root-x86_64=/usr/local/x86_64-linux-musl \
ENV NO_DEBUG_ASSERTIONS=1
ENV NO_OVERFLOW_CHECKS=1

RUN curl -L https://github.com/bytecodealliance/wasmtime/releases/download/v38.0.4/wasmtime-v38.0.4-x86_64-linux.tar.xz | \
RUN curl -L https://github.com/bytecodealliance/wasmtime/releases/download/v44.0.1/wasmtime-v44.0.1-x86_64-linux.tar.xz | \
tar -xJ
ENV PATH="$PATH:/wasmtime-v38.0.4-x86_64-linux"
ENV PATH="$PATH:/wasmtime-v44.0.1-x86_64-linux"

ENV WASM_WASIP_TARGET=wasm32-wasip1
ENV WASM_WASIP_SCRIPT="python3 /checkout/x.py --stage 2 test --host= --target $WASM_WASIP_TARGET \
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16 changes: 8 additions & 8 deletions tests/assembly-llvm/wasm_exceptions.rs
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Expand Up @@ -33,11 +33,11 @@ pub fn test_cleanup() {
}

// CHECK-NOT: call
// CHECK: try
// CHECK: try_table (catch_all_ref 0)
// CHECK: call may_panic
// CHECK: catch_all
// CHECK: rethrow
// CHECK: end_try
// CHECK: end_try_table
// CHECK: call log_number
// CHECK: throw_ref
}

// CHECK-LABEL: test_rtry:
Expand All @@ -57,11 +57,11 @@ pub fn test_rtry() {
}

// CHECK-NOT: call
// CHECK: try
// CHECK: try_table (catch __cpp_exception 0)
// CHECK: call may_panic
// CHECK: catch
// CHECK: end_try_table
// CHECK: call log_number
// CHECK: call log_number
// CHECK-NOT: rethrow
// CHECK: end_try
// CHECK-NOT: throw_ref
// CHECK: end_function
}
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