C ABI Changes for wasm32-unknown-unknown

content/c-abi-changes-for-wasm32-unknown-unknown.md

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+layout = "post"
+date = 2025-04-04
+title = "C ABI Changes for `wasm32-unknown-unknown`"
+author = "Alex Crichton"
++++
+
+The `extern "C"` ABI for the `wasm32-unknown-unknown` target has been using a
+non-standard definition since the inception of the target in that it does not
+implement the [official C ABI of WebAssembly][tool-conventions] and it
+additionally [leaks internal compiler implementation details][leak-details] of
+both the Rust compiler and LLVM. This will change in a future version of the
+Rust compiler and the [official C ABI][tool-conventions] will be used instead.
+
+This post details some history behind this change and the rationale for why it's
+being announced here, but you can skip straight to ["Am I
+affected?"](#am-i-affected) as well.
+
+[tool-conventions]: https://github.com/WebAssembly/tool-conventions/blob/main/BasicCABI.md
+[leak-details]: https://github.com/rust-lang/rust/issues/115666
+
+## History of `wasm32-unknown-unknown`'s C ABI
+
+When the `wasm32-unknown-unknown` target [was originally added][inception] in
+2017, not much care was given to the exact definition of the `extern "C"` ABI at
+the time. In 2018 [an ABI definition was added just for wasm][orig-abi] and the
+target is still using this definition [to this day][current-abi]. This
+definitions has become more and more problematic over time and while some issues
+have been fixed, the root cause still remains.
+
+Notably this ABI definition does not match the [tool-conventions] definition of
+the C API, which is the current standard for how WebAssembly toolchains should
+talk to one another. Originally this non-standard definition was used for all
+WebAssembly based targets except Emscripten, but [this changed in 2021][fix-wasi]
+where the WASI targets for Rust use a corrected ABI definition. Still, however,
+the non-standard definition remained in use for `wasm32-unknown-unknown`.
+
+The time has now come to correct this historical mistake and the Rust compiler
+will soon be using a correct ABI definition for the `wasm32-unknown-unknown`
+target. This means, however, that generated WebAssembly binaries will be
+different than before.
+
+## What is a WebAssembly C ABI?
+
+The definition of an ABI answers questions along the lines of:
+
+* What registers are arguments passed in?
+* What registers are results passed in?
+* How is a 128-bit integers passed as an argument?
+* How is a `union` passed as a return value?
+* When are parameters passed through memory instead of registers?
+* What is the size and alignment of a type in memory?
+
+For WebAssembly these answers are a little different than native platforms.
+For example, WebAssembly does not have physical registers and functions must all
+be annotated with a type. What WebAssembly does have is types such as `i32`,
+`i64`, `f32`, and `f64`. This means that for WebAssembly an ABI needs to define
+how to represent values in these types.
+
+This is where the [tool-conventions] document comes in. That document provides a
+definition for how to represent primitives in C in the WebAssembly format, and
+additionally how function signatures in C are mapped to function signatures in
+WebAssembly. For example a Rust `u32` is represented by a WebAssembly `i32` and
+is passed directly as a parameter as a function argument. If the Rust structure
+`#[repr(C)] struct Pair(f32, f64)` is returned from a function then a return
+pointer is used which must have alignment 8 and size of 16 bytes.
+
+In essence, the WebAssembly C ABI is acting as a bridge between C's type system
+and the WebAssembly type system. This includes details such as in-memory layouts
+and translations of a C function signature to a WebAssembly function signature.
+
+## How is `wasm32-unknown-unknown` non-standard?
+
+Despite the ABI definition today being non-standard, many aspects of it are
+still the same as what [tool-conventions] specifies. For example, size/alignment
+of types is the same as it is in C. The main difference is how function
+signatures are calculated. An example (where you can follow along on [godbolt])
+is:
+
+```rust
+#[repr(C)]
+pub struct Pair {
+    x: u32,
+    y: u32,
+}
+
+#[unsafe(no_mangle)]
+pub extern "C" fn pair_add(pair: Pair) -> u32 {
+    pair.x + pair.y
+}
+```
+
+This will generate the following WebAssembly function:
+
+```wasm
+(func $pair_add (param i32 i32) (result i32)
+  local.get 1
+  local.get 0
+  i32.add
+)
+```
+
+Notably you can see here that the struct `Pair` was "splatted" into its two
+components so the actual `$pair_add` function takes two arguments, the `x` and
+`y` fields. The [tool-conventions], however specifically says that "other
+struct[s] or union[s]" are passed indirectly, notably through memory. We can see
+this by compiling this C code:
+
+```c
+struct Pair {
+    unsigned x;
+    unsigned y;
+};
+
+unsigned pair_add(struct Pair pair) {
+    return pair.x + pair.y;
+}
+```
+
+which yields the generated function:
+
+```wasm
+(func (param i32) (result i32)
+  local.get 0
+  i32.load offset=4
+  local.get 0
+  i32.load
+  i32.add
+)
+```
+
+Here we can see, sure enough, that `pair` is passed in linear memory and this
+function only has a single argument, not two. This argument is a pointer into
+linear memory which stores the `x` and `y` fields.
+
+The Diplomat project has [compiled a much more comprehensive overview][quirks]
+than this and it's recommended to check that out if you're curious for an even
+deeper dive.
+
+## Why hasn't this been fixed long ago already?
+
+For `wasm32-unknown-unknown` it was well-known at the time in 2021 when WASI's
+ABI was updated that the ABI was non-standard. Why then has the ABI not been
+fixed like with WASI?
+The main reason originally for this was the [wasm-bindgen
+project][wasm-bindgen].
+
+In `wasm-bindgen` the goal is to make it easy to integrate Rust into a web
+browser with WebAssembly. JavaScript is used to interact with host APIs and the
+Rust module itself. Naturally, this communication touches on a lot of ABI
+details! The problem was that `wasm-bindgen` relied on the above example,
+specifically having `Pair` "splatted" across arguments instead of passed
+indirectly. The generated JS wouldn't work correctly if the argument was passed
+in-memory.
+
+At the time this was discovered it was found to be significantly difficult to
+fix `wasm-bindgen` to not rely on this splatting behavior. At the time it also
+wasn't thought to be a widespread issue nor was it costly for the compiler to
+have a non-standard ABI. Over the years though the pressure has mounted. The
+Rust compiler is carrying an [ever-growing list of hacks][leak-details] to work
+around the non-standard C ABI on `wasm32-unknown-unknown`. Additionally more
+projects have started to rely on this "splatting" behavior and the risk has
+gotten greater that there are more unknown projects relying on the non-standard
+behavior.
+
+In late 2023 [the wasm-bindgen project fixed bindings generation][wbgfix] to be
+unaffected by the transition to the standard definition of `extern "C"`. In the following months
+a [future-incompat lint was added to rustc][fcw1] to specifically migrate users
+of old `wasm-bindgen` versions to a "fixed" version. This was in anticipation of
+changing the ABI of `wasm32-unknown-unknown` once enough time had passed. Since
+early 2025 users of old `wasm-bindgen` versions [will now receive a hard
+error][hard-error] asking them to upgrade.
+
+Despite all this heroic effort done by contributors, however, it has now come to
+light that there are more projects than `wasm-bindgen` relying on this
+non-standard ABI definition. Consequently this blog post is intended to serve as
+a notice to other users on `wasm32-unknown-unknown` that the ABI break is
+upcoming and projects may need to be changed.
+
+## Am I affected?
+
+If you don't use the `wasm32-unknown-unknown` target, you are not affected by
+this change. If you don't use `extern "C"` on the `wasm32-unknown-unknown`
+target, you are also not affected. If you fall into this bucket, however, you
+may be affected!
+
+To determine the impact to your project there are a few tools at your disposal:
+
+* A new [future-incompat warning][fcw2] has been added to the Rust compiler
+  which will issue a warning if it detects a signature that will change when the
+  ABI is changed.
+* In 2023 a [`-Zwasm-c-abi=(legacy|spec)` flag was added][specflag] to the Rust
+  compiler. This defaults to `-Zwasm-c-abi=legacy`, the non-standard definition.
+  Code can use `-Zwasm-c-abi=spec` to use the standard definition of the C ABI
+  for a crate to test out if changes work.
+
+The best way to test your crate is to compile with `nightly-2025-03-27`
+or later, ensure there are no warnings, and then test your project still works
+with `-Zwasm-c-abi=spec`. If all that passes then you're good to go and the
+upcoming change to the C ABI will not affect your project.
+
+## I'm affected, now what?
+
+So you're using `wasm32-unknown-unknown`, you're using `extern "C"`, and the
+nightly compiler is giving you warnings. Additionally your project is broken
+when compiled with` -Zwasm-c-abi=spec`. What now?
+
+At this time this will unfortunately be a somewhat rough transition period for
+you. There are a few options at your disposal but they all have their downsides:
+
+1. Pin your Rust compiler version to the current stable, don't update until the
+   ABI has changed. This means that you won't get any compiler warnings (as old
+   compilers don't warn) and additionally you won't get broken when the ABI
+   changes (as you're not changing compilers). Eventually when you update to a
+   stable compiler with `-Zwasm-c-abi=spec` as the default you'll have to port
+   your JS or bindings to work with the new ABI.
+
+2. Update to Rust nightly as your compiler and pass `-Zwasm-c-abi=spec`. This is
+   front-loading the work required in (1) for your target. You can get your
+   project compatible with `-Zwasm-c-abi=spec` today. The downside of this
+   approach is that your project will only work with a nightly compiler and
+   `-Zwasm-c-abi=spec` and you won't be able to use stable until the default is
+   switched.
+
+3. Update your project to not rely on the non-standard behavior of
+   `-Zwasm-c-abi=legacy`. This involves, for example, not passing
+   structs-by-value in parameters. You can pass `&Pair` above, for example,
+   instead of `Pair`. This is similar to (2) above where the work is done
+   immediately to update a project but has the benefit of continuing to work on
+   stable Rust. The downside of this, however, is that you may not be able to
+   easily change or update your C ABI in some situations.
+
+4. Update to Rust nightly as your compiler and pass `-Zwasm-c-abi=legacy`. This
+   will silence compiler warnings for now but be aware that the ABI will still
+   change in the future and the `-Zwasm-c-abi=legacy` option will be removed
+   entirely. When the `-Zwasm-c-abi=legacy` option is removed the only option
+   will be the standard C ABI, what `-Zwasm-c-abi=spec` today enables.
+
+If you have uncertainties, questions, or difficulties, feel free to reach out on
+[the tracking issue for the future-incompat warning][tracking] or on Zulip.
+
+## Timeline of ABI changes
+
+At this time there is not an exact timeline of how the default ABI is going to
+change. It's expected to take on the order of 3-6 months, however, and will look
+something roughly like this:
+
+* 2025 March: (soon) - a [future-incompat warning][fcw2] will be added to the
+  compiler to warn projects if they're affected by this ABI change.
+* 2025-05-15: this future-incompat warning will reach the stable Rust channel as
+  1.87.0.
+* 2025 Summer: (ish) - the `-Zwasm-c-abi` flag will be removed from the compiler
+  and the `legacy` option will be entirely removed.
+
+Exactly when `-Zwasm-c-abi` is removed will depend on feedback from the
+community and whether the future-incompat warning triggers much. It's hoped that
+soon after the Rust 1.87.0 is stable, though, that the old legacy ABI behavior
+can be removed.
+
+[wbgfix]: https://github.com/rustwasm/wasm-bindgen/pull/3595
+[specflag]: https://github.com/rust-lang/rust/pull/117919
+[fcw1]: https://github.com/rust-lang/rust/pull/117918
+[fcw2]: https://github.com/rust-lang/rust/pull/138601
+[hard-error]: https://github.com/rust-lang/rust/pull/133951
+[inception]: https://github.com/rust-lang/rust/pull/45905
+[orig-abi]: https://github.com/rust-lang/rust/pull/48959
+[current-abi]: https://github.com/rust-lang/rust/blob/78948ac259253ce89effca1e8bb64d16f4684aa4/compiler/rustc_target/src/callconv/wasm.rs#L76-L114
+[fix-wasi]: https://github.com/rust-lang/rust/pull/79998
+[godbolt]: https://godbolt.org/z/fExj4M4no
+[conventions-struct]: https://github.com/WebAssembly/tool-conventions/blob/main/BasicCABI.md#function-arguments-and-return-values
+[wasm-bindgen]: https://github.com/rustwasm/wasm-bindgen
+[tracking]: https://github.com/rust-lang/rust/issues/138762
+[quirks]: https://github.com/rust-diplomat/diplomat/blob/main/docs/wasm_abi_quirks.md