Add tests for 'impl Default for [T; N]'

[MikailBag]

Related: #71690.
This pull request adds two tests:

• Even it T::default() panics, no leaks occur.
• [T; 0] is Default even if T is not.

I believe at some moment Default impl for arrays will be rewritten to use const generics instead of macros, and these tests will help to prevent behavior changes.

[rust-highfive]

r? @shepmaster

(rust_highfive has picked a reviewer for you, use r? to override)

[shepmaster]

@bors r+ rollup

Thanks!

[bors]

📌 Commit 591584e has been approved by shepmaster

[JohnTitor]

Failed in #72673 (comment):

[RUSTC-TIMING] coretests test:true 64.186
    Finished release [optimized] target(s) in 1m 09s
     Running build/x86_64-unknown-linux-gnu/stage2-std/wasm32-unknown-unknown/release/deps/coretests-b7cfeee46333c4a6.wasm
RuntimeError: unreachable
    at __rust_start_panic (wasm-function[1775]:1)
    at rust_panic (wasm-function[1763]:39)
    at _ZN3std9panicking20rust_panic_with_hook17hb4e2fb8af4804e97E (wasm-function[1758]:279)
    at _ZN3std9panicking11begin_panic17hb9f43a2c430997d1E (wasm-function[1107]:55)
    at _ZN4core5array75_$LT$impl$u20$core..default..Default$u20$for$u20$$u5b$T$u3b$$u20$_$u5d$$GT$7default17h75356a07fe8c5952E (wasm-function[410]:287)
    at _ZN4core3ops8function6FnOnce9call_once17h16c8e7c1f7d576cdE (wasm-function[409]:25)
    at _ZN4test28__rust_begin_short_backtrace17h804c565a83fc8136E (wasm-function[1401]:3)
    at _ZN4core3ops8function6FnOnce40call_once$u7b$$u7b$vtable.shim$u7d$$u7d$17hd4fcf7a756fe71aeE (wasm-function[1400]:6)
    at _ZN4test8run_test14run_test_inner17h4c09558679e02456E (wasm-function[1517]:2059)
    at _ZN4test8run_test17h0760c0ff80bcda5cE (wasm-function[1514]:812)
    at _ZN4test9run_tests17ha8a597da42e5b48eE (wasm-function[1468]:14251)
    at _ZN4test7console17run_tests_console17h90e9ea459accc65eE (wasm-function[1466]:1038)
    at _ZN4test9test_main17hff62743d0c9cc2e9E (wasm-function[1510]:332)
    at _ZN4test16test_main_static17hb90aede56671ffa7E (wasm-function[1512]:356)
    at _ZN9coretests4main17hfd760802efbf9d18E (wasm-function[991]:10)
    at _ZN3std2rt10lang_start28_$u7b$$u7b$closure$u7d$$u7d$17hb3e0ac0d8e7d9f29E (wasm-function[325]:25)
    at _ZN3std10sys_common9backtrace28__rust_begin_short_backtrace17h4faef310d2c1b955E (wasm-function[1744]:8)
    at _ZN3std2rt19lang_start_internal17h08ff942ca8d003dfE (wasm-function[1764]:229)
    at main (wasm-function[992]:46)
    at Object.<anonymous> (/checkout/src/etc/wasm32-shim.js:20:20)
error: test failed, to rerun pass '-p core --test coretests'

@bors r-

[MikailBag]

Thanks! I reproduced failure locally, on 'wasm32-unknown-unknown'.
I'm pretty sure it is caused by some unwinding limitations in wasm (maybe, it does not support unwinding at all?). I will try to find a solution.

[MikailBag]

Yes, it looks like following test

#[test]
fn debugging() {
    std::panic::catch_unwind(|| panic!("hi")).ok();
}

fails on wasm32-unknown-unknown with same error.
I think that test should be skipped on platforms that does not support unwinding.

[JohnTitor]

I'm pretty sure it is caused by some unwinding limitations in wasm (maybe, it does not support unwinding at all?)

Yeah, right. In this case, I think adding #[cfg(not(target_os = "emscripten"))] is fine here.

[MikailBag]

I think I have found out alternate solution:

• mark test as #[should_panic(expected = "OK")]
• raise panic!("OK") at the end of test

It seems that way test will be automatically skipped on all targets without unwinding support.
However, it is more hacky.

Also, it seems that target_os = emsripten will not match on wasm32-unknown-unknown.

[JohnTitor]

Also, it seems that target_os = emsripten will not match on wasm32-unknown-unknown.

Ah indeed! Then #[cfg(not(target_arch = "wasm32"))] should be fine. Also, it'd be great if you could add FIXME comment with your workaround :)

[MikailBag]

I think panic=abort problem is fixed, because ./x.py test src/libcore --target wasm32-unknown-unknown now works on my machine.