Wasm-opt fails on Rust 1.87.0+, LLVM 20+

[MolotovCherry]

Describe the bug

An update to Rust with LLVM 20 requires new wasm features in the resulting binary. The new wasm features caused downstream breakage for a lot of people.

Affected: Rust 1.87.0+, LLVM 20+

Log file of failing build:
out.txt

Adding these flags to wasm-opt fixes the compilation
--enable-bulk-memory --enable-threads --enable-nontrapping-float-to-int

Note: I am unsure how exactly the compilation takes place. Perhaps who's at fault here is wasm-pack? In either case, this bridge plugin is still affected though

Please see
rust-lang/rust#137315
rust-lang/rust#137322
https://doc.rust-lang.org/nightly/rustc/platform-support/wasm32-unknown-unknown.html#enabled-webassembly-features

Steps to reproduce

flutter_rust_bridge_codegen build-web --release
flutter build web --wasm --release

Logs

n/a (log is in first post)

Expected behavior

Compiles properly

Generated binding code

n/a

OS

Wasm

Version of flutter_rust_bridge_codegen

2.8.0

Flutter info

[✓] Flutter (Channel stable, 3.29.0, on Microsoft Windows [Version 10.0.26100.3323], locale en-US) [310ms]
    • Flutter version 3.29.0 on channel stable at C:\Users\cherry\flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 35c388afb5 (4 weeks ago), 2025-02-10 12:48:41 -0800
    • Engine revision f73bfc4522
    • Dart version 3.7.0
    • DevTools version 2.42.2

Version of clang++

n/a

Additional context

No response

[welcome]

Hi! Thanks for opening your first issue here! 😄

[fzyzcjy]

Hmm I see. Btw where is --enable-threads from? From the links it seems they mention nontrapping-fptoint and bulk-memory EDIT: See it from logs

[fzyzcjy]

Btw, I wonder whether if we enable these flags, will the resulting binary require more modern browser etc?

[MolotovCherry]

I have no idea. It was mentioned in the failed build as something that was needed, but as to why it wanted it, I don't know. I can say though that my project setup is vanilla (I just made it fresh from the tool, and nothing core was altered)

Edit: It's from the atomics

[fzyzcjy]

Hmm looks like not very modern, so maybe this is ok

• https://caniuse.com/?search=bulk-memory
• https://caniuse.com/wasm-threads
• https://caniuse.com/wasm-nontrapping-fptoint

[fzyzcjy]

Hmm I cannot reproduce it:

Details

cargo +nightly --version
cargo 1.87.0-nightly (2622e844b 2025-02-28)

flutter_rust_bridge_codegen build-web --release

Building package executable... 
Built flutter_rust_bridge:flutter_rust_bridge.
> which wasm-pack (pwd: null, env: null)
/Users/tom/.cargo/bin/wasm-pack
> cargo read-manifest (pwd: rust, env: null)
> wasm-pack build -t no-modules -d /Volumes/MyExternal/ExternalRefCode/flutter_rust_bridge/frb_example/flutter_via_create/web/pkg --no-typescript --out-name rust_lib_flutter_via_create rust -- -Z build-std=std,panic_abort (pwd: null, env: {RUSTUP_TOOLCHAIN: nightly, RUSTFLAGS: -C target-feature=+atomics,+bulk-memory,+mutable-globals, CARGO_TERM_COLOR: always})
[INFO]: 🎯  Checking for the Wasm target...
[INFO]: 🌀  Compiling to Wasm...
 Downloading crates ...
  Downloaded compiler_builtins v0.1.151
   Compiling compiler_builtins v0.1.151
   Compiling core v0.0.0 (/Users/tom/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core)
   Compiling libc v0.2.170
   Compiling object v0.36.7
   Compiling std v0.0.0 (/Users/tom/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std)
   Compiling proc-macro2 v1.0.70
   Compiling unicode-ident v1.0.12
   Compiling wasm-bindgen-shared v0.2.92
   Compiling log v0.4.20
   Compiling bumpalo v3.14.0
   Compiling version_check v0.9.4
   Compiling typenum v1.17.0
   Compiling once_cell v1.18.0
   Compiling futures-core v0.3.29
   Compiling wasm-bindgen v0.2.92
   Compiling generic-array v0.14.7
   Compiling autocfg v1.1.0
   Compiling slab v0.4.9
   Compiling futures-task v0.3.29
   Compiling futures-channel v0.3.29
   Compiling futures-util v0.3.29
   Compiling portable-atomic v1.11.0
   Compiling cfg-if v1.0.0
   Compiling anyhow v1.0.75
   Compiling build-target v0.4.0
   Compiling hex v0.4.3
   Compiling flutter_rust_bridge v2.8.0 (/Volumes/MyExternal/ExternalRefCode/flutter_rust_bridge/frb_rust)
   Compiling quote v1.0.33
   Compiling syn v2.0.39
   Compiling crypto-common v0.1.6
   Compiling block-buffer v0.10.4
   Compiling digest v0.10.7
   Compiling md-5 v0.10.6
   Compiling wasm-bindgen-backend v0.2.92
   Compiling futures-macro v0.3.29
   Compiling delegate-attr v0.3.0
   Compiling flutter_rust_bridge_macros v2.8.0 (/Volumes/MyExternal/ExternalRefCode/flutter_rust_bridge/frb_macros)
   Compiling wasm-bindgen-macro-support v0.2.92
   Compiling wasm-bindgen-macro v0.2.92
   Compiling rustc-std-workspace-core v1.99.0 (/Users/tom/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/rustc-std-workspace-core)
   Compiling alloc v0.0.0 (/Users/tom/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc)
   Compiling memchr v2.7.4
   Compiling adler2 v2.0.0
   Compiling rustc-demangle v0.1.24
   Compiling unwind v0.0.0 (/Users/tom/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/unwind)
   Compiling dlmalloc v0.2.7
   Compiling rustc-std-workspace-alloc v1.99.0 (/Users/tom/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/rustc-std-workspace-alloc)
   Compiling panic_unwind v0.0.0 (/Users/tom/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/panic_unwind)
   Compiling panic_abort v0.0.0 (/Users/tom/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/panic_abort)
   Compiling gimli v0.31.1
   Compiling miniz_oxide v0.8.3
   Compiling std_detect v0.1.5 (/Users/tom/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/stdarch/crates/std_detect)
   Compiling hashbrown v0.15.2
   Compiling addr2line v0.24.2
   Compiling proc_macro v0.0.0 (/Users/tom/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/proc_macro)
   Compiling pin-project-lite v0.2.13
   Compiling futures-sink v0.3.29
   Compiling futures-io v0.3.29
   Compiling memchr v2.6.4
   Compiling pin-utils v0.1.0
   Compiling lazy_static v1.4.0
   Compiling tokio v1.34.0
   Compiling bytemuck v1.14.0
   Compiling byteorder v1.5.0
   Compiling js-sys v0.3.69
   Compiling console_error_panic_hook v0.1.7
   Compiling futures-executor v0.3.29
   Compiling web-sys v0.3.66
   Compiling futures v0.3.29
   Compiling wasm-bindgen-futures v0.4.42
warning: unstable feature specified for `-Ctarget-feature`: `atomics`
  |
  = note: this feature is not stably supported; its behavior can change in the future

warning: `flutter_rust_bridge` (lib) generated 1 warning
   Compiling rust_lib_flutter_via_create v0.1.0 (/Volumes/MyExternal/ExternalRefCode/flutter_rust_bridge/frb_example/flutter_via_create/rust)
warning: `rust_lib_flutter_via_create` (lib) generated 1 warning (1 duplicate)
    Finished `release` profile [optimized] target(s) in 35.15s
[INFO]: ⬇️  Installing wasm-bindgen...
[INFO]: found wasm-opt at "/usr/local/bin/wasm-opt"
[INFO]: Optimizing wasm binaries with `wasm-opt`...
[INFO]: Optional fields missing from Cargo.toml: 'description', 'repository', and 'license'. These are not necessary, but recommended
[INFO]: ✨   Done in 35.86s
[INFO]: 📦   Your wasm pkg is ready to publish at /Volumes/MyExternal/ExternalRefCode/flutter_rust_bridge/frb_example/flutter_via_create/web/pkg.

lutter build web --wasm --release

Downloading Web SDK...                                              5.7s

┌─ New feature ────────────────────────────────────────────────────────────────────────────┐
│   WebAssembly compilation is new. Understand the details before deploying to production. │
│   See https://flutter.dev/to/wasm for more information.                                  │
└──────────────────────────────────────────────────────────────────────────────────────────┘

Font asset "CupertinoIcons.ttf" was tree-shaken, reducing it from 257628 to 1472 bytes (99.4% reduction). Tree-shaking can be disabled
by providing the --no-tree-shake-icons flag when building your app.
Font asset "MaterialIcons-Regular.otf" was tree-shaken, reducing it from 1645184 to 7736 bytes (99.5% reduction). Tree-shaking can be
disabled by providing the --no-tree-shake-icons flag when building your app.
Compiling lib/main.dart for the Web...                             40.5s
✓ Built build/web

[fzyzcjy]

Btw, frb's build-web already support:

    --wasm-bindgen-args             Arguments passed to wasm-bindgen
    --wasm-pack-rustup-toolchain    Override RUSTUP_TOOLCHAIN environment variable when running wasm-pack
    --wasm-pack-rustflags           Override RUSTFLAGS environment variable when running wasm-pack

Does passing these help?

Especially, the default rustflags are:

RUSTFLAGS: -C target-feature=+atomics,+bulk-memory,+mutable-globals

So I wonder whether adding those to it will help

[MolotovCherry]

This actually didn't happen to me before either, but abruptly started.* I'm trying to see if I can pinpoint the cause

I know I did change the features to remove wasm-start, but I don't think that's the cause(nevertheless, here's the config)

flutter_rust_bridge = { version = "=2.8.0", features = [
    "anyhow",
    "dart-opaque",
    "log",
    "portable-atomic",
    "rust-async",
    "thread-pool",
    "user-utils",
] }

Edit for clarity: Abruptly maybe isn't the right word. Debug builds work fine for me; this only happened to me on release

I'm currently also running
wasm-pack v0.13.1
wasm-opt-cdcb9e877b68d02e / 117 (in .wasm-pack folder; updating to 122 didn't help)

[fzyzcjy]

I see. Feel free to try the flags I mentioned above and see whether it works

[MolotovCherry]

I see. Feel free to try the flags I mentioned above and see whether it works

May I ask, what is the full arg command you'd like me to try? So far I've tried every combination of flags and args I can think of, including downgrading the compiler to your version (prob downgrading it even further would "fix it"). I still can't pinpoint the cause/solution that doesn't involve adding those wasm-opt flags :/ (well, I suppose removing all the rustflags might "work", but I also tried that in the flag, perhaps I didn't get the syntax right)

(I wonder if it's an OS difference, since I'm on Windows)

(I've made sure wasm-pack, wasm-bindgen, etc all is up to date too)

[MolotovCherry]

I tried a new project like yours and it worked perfectly. I could not get it to fail no matter what I tried. At that point I realized it may be some config difference between new projects and my custom project structure.

So I made a new empty project with my same project structure, and it reliably triggered the first time. I uploaded the failing repo for you.

Though I do find it quite odd that one works but not the other.

This was made by first making a cargo workspace + empty crates + empty crates/wasm dir, then using integrate with the flags

❯ cat Cargo.toml
[workspace]
resolver = "3"
members = [
    "crates/wasm",
]

[profile.release]
lto = true
codegen-units = 1
strip = true

[profile.dev]
opt-level = 1

❯ mkdir crates
❯ cd crates
❯ mkdir wasm
❯ cd ..
❯ flutter create my_name
❯ cd my_name
❯ flutter_rust_bridge_codegen integrate --rust-crate-dir ../crates/wasm --rust-crate-name wasm --template app
❯ flutter_rust_bridge_codegen build-web -c ../crates/wasm -o ../../my_name/build/web --release

Hopefully this repo will be able to reproduce it for you
https://github.com/MolotovCherry/flutter_rust_bridge_2601

On a somewhat aside note, the below command crashes. The culprit that triggers it is using a hyphen in the project name (wasm-foo) with the flag --rust-crate-name

❯ flutter_rust_bridge_codegen integrate --rust-crate-dir ../crates/wasm --rust-crate-name wasm-foo --template app
# ... Snipped output
[2025-03-08T22:49:13.427Z WARN C:\Users\cherry\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\flutter_rust_bridge_codegen-2.8.0\src\library\commands\command_runner.rs:161] command="powershell" "-noprofile" "-command" "& \"flutter\" \"pub\" \"add\" \"wasm-foo\" \"--path=rust_builder\"" stdout= stderr=Not a valid package name: "null"

Usage: dart pub add [options] [<section>:]<package>[:descriptor]
       [<section>:]<package2>[:descriptor] ...]
-h, --help               Print this usage information.
    --[no-]offline       Use cached packages instead of accessing the network.
-n, --dry-run            Report what dependencies would change but don't change
                         any.
    --[no-]precompile    Build executables in immediate dependencies.
-C, --directory=<dir>    Run this in the directory <dir>.

Run "dart help" to see global options.
See https://dart.dev/tools/pub/cmd/pub-add for detailed documentation.
Failed to update packages.

Error: Command execution failed: Not a valid package name: "null"

Usage: dart pub add [options] [<section>:]<package>[:descriptor]
       [<section>:]<package2>[:descriptor] ...]
-h, --help               Print this usage information.
    --[no-]offline       Use cached packages instead of accessing the network.
-n, --dry-run            Report what dependencies would change but don't change
                         any.
    --[no-]precompile    Build executables in immediate dependencies.
-C, --directory=<dir>    Run this in the directory <dir>.

Run "dart help" to see global options.
See https://dart.dev/tools/pub/cmd/pub-add for detailed documentation.
Failed to update packages.

[fzyzcjy]

May I ask, what is the full arg command you'd like me to try?

flutter_rust_bridge_codegen build-web --wasm-pack-rustflags blahblah

etc

i.e. try to add those required flags into the wasm-pack-rustflags or things like that

[MolotovCherry]

May I ask, what is the full arg command you'd like me to try?

flutter_rust_bridge_codegen build-web --wasm-pack-rustflags blahblah

etc

i.e. try to add those required flags into the wasm-pack-rustflags or things like that

It's telling me it doesn't recognize them (I tried various combinations, from a single flag, to many, commas, no commas, etc, all leading to it not working). Seems rustc doesn't like it

❯ flutter_rust_bridge_codegen build-web --release --wasm-pack-rustflags "--enable-bulk-memory --enable-threads --enable-nontrapping-float-to-int"
Error: `cargo metadata` exited with an error: error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `C:\Users\cherry\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\bin\rustc.exe - --crate-name ___ --print=file-names --enable-bulk-memory --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg -Wwarnings` (exit code: 1)
  --- stderr
  error: Unrecognized option: 'enable-bulk-memory'


Caused by: `cargo metadata` exited with an error: error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `C:\Users\cherry\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\bin\rustc.exe - --crate-name ___ --print=file-names --enable-bulk-memory --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg -Wwarnings` (exit code: 1)
  --- stderr
  error: Unrecognized option: 'enable-bulk-memory'

(I also tried --wasm-bindgen-args and --cargo-build-args much to the same effect)