1441425 - xpcom-gtest compilation fails at #[derive(xpcom)] expected i8, found u8 on Linux ARMv7

Status: RESOLVED DUPLICATE of bug 1453551

(Core :: XPCOM, defect)

Description

[Henri Sivonen (:hsivonen) (not reading bugmail until 2026-07-27)]

Steps to reproduce:
 1) Find a Linux ARMv7+NEON system with more than 2 GB of RAM (to let a 32-bit process get the max 2 GB).
 2) Build Firefox with --disable-debug --enable-optimize --disable-crashreporter --disable-debug-symbols --disable-av1 --disable-webrtc
 3) ./mach gtest

Actual results:
0:00.72 /usr/bin/make -C testing/gtest -j4 -s gtest
 0:01.62 force-cargo-library-build
 0:03.96    Compiling xpcom-gtest v0.1.0 (file:///var/host/media/removable/gecko/gecko/xpcom/rust/gtest/xpcom)
 0:05.23 error[E0308]: mismatched types
 0:05.23   --> /var/host/media/removable/gecko/gecko/xpcom/rust/gtest/xpcom/test.rs:22:14
 0:05.23    |
 0:05.23 22 |     #[derive(xpcom)]
 0:05.23    |              ^^^^^ expected i8, found u8
 0:05.23    |
 0:05.24    = note: expected type `*const i8`
 0:05.24               found type `*const u8`
 0:05.24 
 0:05.25 error[E0308]: mismatched types
 0:05.25   --> /var/host/media/removable/gecko/gecko/xpcom/rust/gtest/xpcom/test.rs:36:36
 0:05.25    |
 0:05.25 36 |             assert!(CStr::from_ptr(topic).to_str() == Ok("test-rust-observe"));
 0:05.25    |                                    ^^^^^ expected u8, found i8
 0:05.25    |
 0:05.25    = note: expected type `*const u8`
 0:05.25               found type `*const i8`
 0:05.26 
 0:05.30 error: aborting due to 2 previous errors
 0:05.30 
 0:05.96 error: Could not compile `xpcom-gtest`.

Expected results:
Expected gtest to compile and run as on x86_64 with the same revision.

Comment 1

[Henri Sivonen (:hsivonen) (not reading bugmail until 2026-07-27)]

Does derive(xpcom) depend on platform C++ char signedness? Does char signedness differ between ARMv7 and x86_64 Linux?

Comment 2

[Nika Layzell [:nika] (ni? for response)]

The problem here is that when I wrote the signature I incorrectly assumed c_char was i8 here: https://searchfox.org/mozilla-central/rev/056a4057575029f58eb69ae89bc7b6c3c11e2e28/xpcom/rust/gtest/xpcom/test.rs#32

To fix this, we just need to change that *const i8 to a *const libc::c_char or *const std::os::raw::c_char.

It seems like char signedness does differ between ARMv7 and x86_64 linux.

Comment 3

[Henri Sivonen (:hsivonen) (not reading bugmail until 2026-07-27)]

Attachment: Bug 1441425 - Avoid assuming c_char signedness in gtest for Rust bindings for XPCOM.

Review commit: https://reviewboard.mozilla.org/r/224250/diff/#index_header
See other reviews: https://reviewboard.mozilla.org/r/224250/

Comment 4

[Nika Layzell [:nika] (ni? for response)]

Comment on attachment 8955089 [details]
Bug 1441425 - Avoid assuming c_char signedness in gtest for Rust bindings for XPCOM.

https://reviewboard.mozilla.org/r/224250/#review230292