1979820 - Firefox Build Compatibility Issues with LLVM 21.1.x

Status: UNCONFIRMED

(Core :: General, enhancement)

Description

[Matt Damon]

Steps to reproduce:

Firefox fails to build with LLVM 21.1.0-rc1 due to multiple C++ template compatibility issues in bindgen-generated code and vendored libraries. Build reaches 58% completion before failing.

Environment:

• macOS 15.0 (Sequoia)
• Apple M4 MacBook Pro
• LLVM 21.1.0-rc1 compiled with PGO optimizations
• Firefox source from mozilla-central

Build Configuration:

# .mozconfig
ac_add_options --enable-application=browser
ac_add_options --target=aarch64-apple-darwin
ac_add_options --host=aarch64-apple-darwin
ac_add_options --enable-release
ac_add_options --enable-lto=cross
ac_add_options --enable-optimize="-O3 -mcpu=apple-m4 -pipe"
ac_add_options --enable-macos-target=15.0
ac_add_options --disable-warnings-as-errors

export RUSTFLAGS="-Ctarget-cpu=apple-m4 -Copt-level=3 -Clinker-plugin-lto -Ccodegen-units=1"
export CC="/opt/llvm-21.1.0-rc1/bin/clang"
export CXX="/opt/llvm-21.1.0-rc1/bin/clang++"

Reproduction:

./mach build

ATTEMPTS TO RESOLVE

1. Style Crate bindgen Issues

Error:

error[E0412]: cannot find type `invoke_result_t` in module `root::std::__1`
     --> gecko/structs.rs:51072:29
      |
51072 |             root::std::__1::invoke_result_t<ResolveFunction, ResolveValueT>;
      |                             ^^^^^^^^^^^^^^^ not found in `root::std::__1`

Attempted Fix:
Added to layout/style/ServoBindings.toml:

hide-types = [
    ".*__invoke_result_t.*",
    ".*invoke_result_t.*", 
    ".*invoke_result.*",
    # ... existing entries
]

Result: Partial success but std::invoke_result_t still leaks through in some contexts.

2. Gecko Profiler bindgen Issues

Error:

error[E0412]: cannot find type `_CharT` in this scope
error[E0412]: cannot find type `_Traits` in this scope  
error[E0109]: type arguments are not allowed on builtin type `u8`

Successful Fix:
Added to tools/profiler/rust-api/build.rs:

.blocklist_type(".*basic_string_view.*")
.opaque_type(".*basic_string_view.*")
.blocklist_type("std::.*basic_string_view.*")
.opaque_type("std::.*basic_string_view.*")
.blocklist_type("std::__1::basic_string_view.*")
.opaque_type("std::__1::basic_string_view.*")
.blocklist_type(".*basic_string___self_view.*")

Result: Successfully resolved gecko-profiler compilation.

3. Highway SIMD Library Issues

Error:

error: no type named 'type' in 'hwy::EnableIfT<false>'
error: call to 'Combine' is ambiguous

Context: Located in third_party/highway/hwy/ops/arm_neon-inl.h - appears to be template SFINAE compatibility issues with LLVM 21's stricter template resolution.

Actual results:

1. bindgen Template Handling: LLVM 21's libc++ uses more complex template metaprogramming that bindgen cannot parse reliably
2. std::invoke_result_t Incompatibility: This C++17 template is problematic across multiple Firefox components
3. Vendored Library Issues: Highway SIMD library has template compatibility problems with LLVM 21
4. Successful Partial Build: 58% completion achieved, indicating most compatibility issues are solvable

Expected results:

REQUESTED ACTION

1. Update bindgen Configuration: Comprehensive audit of all bindgen .toml files to hide problematic LLVM 21 template types
2. Highway Library Update: Coordinate with upstream Highway project for LLVM 21 compatibility
3. Template Compatibility Strategy: Develop systematic approach for handling newer C++ standard library templates in bindgen
4. CI Testing: Add LLVM 21 compatibility testing to catch these issues early

Specific Files Needing Attention:

• layout/style/ServoBindings.toml - needs stronger invoke_result_t blocking
• third_party/highway/ - needs upstream compatibility fixes
• All bindgen build.rs files - systematic template hiding audit needed

Severity: This will block Firefox adoption of LLVM 21 when it reaches stable release.