wasm32-unknown-emscripten target shouldn't use -fno-exceptions

[sssxks]

Background

#1284 introduced f no exceptions for all wasm targets

rust-or/highs-sys#43 encountered this issue on wasm32-unknown-emscripten

Repro

use std::env;

fn main() {
    println!("cargo:rerun-if-changed=native/throw.cpp");
    println!("cargo:rerun-if-env-changed=REPRO_FIX");

    let target = env::var("TARGET").expect("TARGET not set by cargo");
    let mut build = cc::Build::new();
    build.cpp(true).file("native/throw.cpp");

    if env::var_os("REPRO_FIX").is_some() {
        build.no_default_flags(true);
        if target.contains("emscripten") {
            build.flag("-fexceptions");
        }
    }

    build.compile("throwlib");
}

native/throw.cpp

int throw_and_catch() {
  try {
    throw 42;
  } catch (...) {
    return 0;
  }
}

pin cc = "=1.2.56", output contains

warning: emrepro@0.1.0: native/throw.cpp:3:5: error: cannot use 'throw' with exceptions disabled
warning: emrepro@0.1.0:     3 |     throw 42;
warning: emrepro@0.1.0:       |     ^
warning: emrepro@0.1.0: native/throw.cpp:2:3: error: cannot use 'try' with exceptions disabled
warning: emrepro@0.1.0:     2 |   try {
warning: emrepro@0.1.0:       |   ^
warning: emrepro@0.1.0: 2 errors generated.
error: failed to run custom build command for `emrepro v0.1.0 (/home/xks/repos/highs-sys/repro/cc-rs-emscripten-exceptions)`

Caused by:
  process didn't exit successfully: `/home/xks/repos/highs-sys/repro/cc-rs-emscripten-exceptions/target/debug/build/emrepro-c20309397f2411a0/build-script-build` (exit status: 1)
  --- stdout
  cargo:rerun-if-changed=native/throw.cpp
  cargo:rerun-if-env-changed=REPRO_FIX
  OUT_DIR = Some(/home/xks/repos/highs-sys/repro/cc-rs-emscripten-exceptions/target/wasm32-unknown-emscripten/debug/build/emrepro-a032e2f52b0f0067/out)
  OPT_LEVEL = Some(0)
  TARGET = Some(wasm32-unknown-emscripten)
  CARGO_ENCODED_RUSTFLAGS = Some()
  HOST = Some(x86_64-unknown-linux-gnu)
  cargo:rerun-if-env-changed=CXX_wasm32-unknown-emscripten
  CXX_wasm32-unknown-emscripten = None
  cargo:rerun-if-env-changed=CXX_wasm32_unknown_emscripten
  CXX_wasm32_unknown_emscripten = None
  cargo:rerun-if-env-changed=TARGET_CXX
  TARGET_CXX = None
  cargo:rerun-if-env-changed=CXX
  CXX = None
  cargo:rerun-if-env-changed=CC_ENABLE_DEBUG_OUTPUT
  cargo:rerun-if-env-changed=CRATE_CC_NO_DEFAULTS
  CRATE_CC_NO_DEFAULTS = None
  DEBUG = Some(true)
  cargo:rerun-if-env-changed=CXXFLAGS
  CXXFLAGS = None
  cargo:rerun-if-env-changed=TARGET_CXXFLAGS
  TARGET_CXXFLAGS = None
  cargo:rerun-if-env-changed=CXXFLAGS_wasm32_unknown_emscripten
  CXXFLAGS_wasm32_unknown_emscripten = None
  cargo:rerun-if-env-changed=CXXFLAGS_wasm32-unknown-emscripten
  CXXFLAGS_wasm32-unknown-emscripten = None
  cargo:warning=native/throw.cpp:3:5: error: cannot use 'throw' with exceptions disabled
  cargo:warning=    3 |     throw 42;
  cargo:warning=      |     ^
  cargo:warning=native/throw.cpp:2:3: error: cannot use 'try' with exceptions disabled
  cargo:warning=    2 |   try {
  cargo:warning=      |   ^
  cargo:warning=2 errors generated.

  --- stderr


  error occurred in cc-rs: command did not execute successfully (status code exit status: 1): LC_ALL="C" "em++" "-O0" "-ffunction-sections" "-fdata-sections" "-fno-exceptions" "-g" "-fno-omit-frame-pointer" "--target=wasm32-unknown-emscripten" "-Wall" "-Wextra" "-o" "/home/xks/repos/highs-sys/repro/cc-rs-emscripten-exceptions/target/wasm32-unknown-emscripten/debug/build/emrepro-a032e2f52b0f0067/out/44f7d3c4c0a9780b-throw.o" "-c" "native/throw.cpp"

[madsmtm]

I'd accept a PR fixing it.

There's also been some talk on changing the default for wasm32-unknown-unknown (see https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Change.20default.20for.20wasm32-unknown-unknown.20to.20panic.3Dunwind.3F/with/569598835), so I'm a bit unsure what the right flag for us to specify actually should be? Maybe we can extract it from CARGO_CFG_TARGET_FEATURE?

[kolayne]

Note: the issue does not concern just wasm32-unknown-emscripten. I am working with wasm32-wasi*, and I want to use C++ exceptions there as well. This has become possible with wasi sdk since WebAssembly/wasi-sdk#606 (merged 3 weeks ago, to become available in the upcoming release).

I'd accept a PR fixing it.

@madsmtm, should we just allow exceptions on all the wasm platforms?