emcc and wasm-ld can't link our ui tests for `wasm32-unknown-emscripten`?

[workingjubilee]

Perhaps I'm holding it wrong? About a fifth of the rustc UI test suite to fail on wasm32-unknown-emscripten when trying to run ./x.py test --target wasm32-unknown-emscripten. In all cases we get an error like this:

error: linking with `emcc` failed: exit status: 1
   |
   = note: "emcc" "-s" "EXPORTED_FUNCTIONS=[\"_main\"]" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/test/ui/array-slice-vec/vector-no-ann-2/a.vector_no_ann_2.86c223511763191d-cgu.0.rcgu.o" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/test/ui/array-slice-vec/vector-no-ann-2/a.1t3t8w654vioaffrv2c1spxg2.rcgu.o" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/libstd-6499696fb99a9577.rlib" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/libpanic_unwind-0c3cc3cd32c2b4b3.rlib" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/libmemchr-e7a6f41ef20fcc54.rlib" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/librustc_demangle-d35e921e53f26e9e.rlib" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/libstd_detect-a7a77daaad248f5e.rlib" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/libhashbrown-e0e54cdb8e75fbf2.rlib" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/librustc_std_workspace_alloc-9f19c89c353a136f.rlib" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/libminiz_oxide-ffd1c27bbffc4236.rlib" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/libadler-4452dbe2564d6cb8.rlib" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/libunwind-684258862f7c352d.rlib" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/libcfg_if-0f4ad8f59845e58a.rlib" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/liblibc-2be890f74433405e.rlib" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/liballoc-48460e2c05fc2c62.rlib" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/librustc_std_workspace_core-841075126ac27f05.rlib" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/libcore-ee3c0a07d74de06f.rlib" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/libcompiler_builtins-c113413e38c4b290.rlib" "-l" "c" "-B/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/bin/gcc-ld" "--target=wasm32-unknown-emscripten" "-s" "DISABLE_EXCEPTION_CATCHING=0" "-L" "/home/jubilee/rust/rustc/build/wasm32-unknown-emscripten/native/rust-test-helpers" "-L" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/test/ui/array-slice-vec/vector-no-ann-2/auxiliary" "-L" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/wasm32-unknown-emscripten/lib/self-contained" "-o" "/home/jubilee/rust/rustc/build/x86_64-unknown-linux-gnu/test/ui/array-slice-vec/vector-no-ann-2/a.wasm" "-O2" "-g0" "-sABORTING_MALLOC=0"
   = note: wasm-ld: error: /home/jubilee/.emscripten_cache/sysroot/lib/wasm32-emscripten/libstandalonewasm.a(__main_void.o): undefined symbol: main

[jieyouxu]

Does this target use some different main symbol name?

[jieyouxu]

... do we need to pass --no-entry to lib builds? emscripten-core/emscripten#9640

@workingjubilee can you check if (some of) the failing tests are lib crates?

Actually what am I doing, I can check

[jieyouxu]

I can't get past the building test helpers for this target cross-compiling from msvc host, not sure what's up with that.

[workingjubilee]

Huh.

cc @juntyr Can you make it any further?

[jieyouxu]

Also I note that wasm32-unknown-emscripten has no platform support docs and I have no clue what's needed to be able to run the tests. Because there's an open PR #131582 for this.

[jieyouxu]

In the open platform support docs PR this says

This target is not extensively tested in CI for the rust-lang/rust repository.
It's recommended to test the wasm32-wasip1 target instead
for WebAssembly compatibility.

... so I don't know, is this expected failure?