Miri hangs when a test fails

[NathanSWard]

Given the simple library:

#[cfg(test)]
mod tests {

    #[test]
    fn failed() {
        assert_eq!(1, 2);
    }
}

and then running

RUST_BACKTRACE=full MIRIFLAGS="-Zmiri-disable-isolation" cargo +nightly miri test

the output results in:

    Finished test [unoptimized + debuginfo] target(s) in 0.00s
     Running unittests (target/x86_64-unknown-linux-gnu/debug/deps/test_miri-72fa64ca114eaddd)

running 1 test
test tests::failed ... FAILED

failures:

and then just infinitely hangs.

[RalfJung]

Lol, I don't think I ever ran Miri on a test suite that failed in a well-defined way, i.e., without causing Miri to stop during UB.^^

-Zmiri-disable-isolation is known to make libtest very slow (#1702); what happens when you remove that flag?

[RalfJung]

This might be related to backtrace printing; Cc @Aaron1011

[Aaron1011]

This hangs for me with RUST_BACKTRACE=0 MIRIFLAGS="-Zmiri-disable-isolation" cargo +nightly miri test, so I think it's caused by -Zmiri-disable-isolation

@Aaron1011 Are you sure? This did not hang for me with exactly RUST_BACKTRACE=0 MIRIFLAGS="-Zmiri-disable-isolation" (with Miri 36176cd). It did hang with RUST_BACKTRACE=1 MIRIFLAGS="-Zmiri-disable-isolation" cargo miri test instead.

This is not related to -Zmiri-disable-isolation but related to Stacked Borrows:

$ cat hang.rs
#[test]
fn foo() {
    std::env::set_var("RUST_BACKTRACE", "1");
    panic!();
}

$ time miri --sysroot ~/.cache/miri/HOST --test hang.rs -Zmiri-disable-stacked-borrows

running 1 test
test foo ... FAILED

failures:

---- foo stdout ----
thread 'main' panicked at 'explicit panic', hang.rs:4:5
stack backtrace:
   0: std::rt::begin_panic::<&str>
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:519:12
   1: foo
             at hang.rs:4:5
   2: foo::{closure#0}
             at hang.rs:2:1
   3: <[closure@hang.rs:2:1: 5:2] as std::ops::FnOnce<()>>::call_once - shim
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
   4: <fn() as std::ops::FnOnce<()>>::call_once - shim(fn())
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
   5: test::run_test::{closure#2}
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:568:30
   6: <[closure@test::run_test::{closure#2}] as std::ops::FnOnce<()>>::call_once - shim(vtable)
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
   7: <std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send> as std::ops::FnOnce<()>>::call_once
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1546:9
   8: <std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>> as std::ops::FnOnce<()>>::call_once
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:344:9
   9: std::panicking::try::do_call::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:379:40
  10: std::panicking::try::<(), std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>>
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:343:19
  11: std::panic::catch_unwind::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:431:14
  12: test::run_test_in_process
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:600:18
  13: test::run_test::run_test_inner::{closure#0}
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:492:39
  14: test::run_test::run_test_inner
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:530:13
  15: test::run_test
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:564:28
  16: test::run_tests::<[closure@test::run_tests_console::{closure#2}]>
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:305:17
  17: test::run_tests_console
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/console.rs:289:5
  18: test::test_main
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:122:15
  19: test::test_main_static
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:141:5
  20: main
             at hang.rs:1:1
  21: <fn() as std::ops::FnOnce<()>>::call_once - shim(fn())
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
  22: std::rt::lang_start::<()>::{closure#0}
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:49:18
  23: std::ops::function::impls::<impl std::ops::FnOnce<()> for &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>::call_once
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:259:13
  24: std::panicking::try::do_call::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:379:40
  25: std::panicking::try::<i32, &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:343:19
  26: std::panic::catch_unwind::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:431:14
  27: std::rt::lang_start_internal
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:34:21
  28: std::rt::lang_start::<()>
             at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:48:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.


failures:
    foo

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out


real	0m5.681s
user	0m5.627s
sys	0m0.052s

$ time miri --sysroot ~/.cache/miri/HOST --test hang.rs

running 1 test
test foo ... FAILED

failures:
^C

real	2m2.036s
user	2m1.527s
sys	0m0.488s

This is probably not related to backtrace printing because this also makes Miri hang (without -Zmiri-disable-stacked-borrows and does not make it hang with -Zmiri-disable-stacked-borrows):

const BACKTRACE_TEXT: &str = "fake stack backtrace:
0: std::rt::begin_panic::<&str>
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:519:12
1: foo
          at hang.rs:4:5
2: foo::{closure#0}
          at hang.rs:2:1
3: <[closure@hang.rs:2:1: 5:2] as std::ops::FnOnce<()>>::call_once - shim
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
4: <fn() as std::ops::FnOnce<()>>::call_once - shim(fn())
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
5: test::run_test::{closure#2}
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:568:30
6: <[closure@test::run_test::{closure#2}] as std::ops::FnOnce<()>>::call_once - shim(vtable)
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
7: <std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send> as std::ops::FnOnce<()>>::call_once
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1546:9
8: <std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>> as std::ops::FnOnce<()>>::call_once
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:344:9
9: std::panicking::try::do_call::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:379:40
10: std::panicking::try::<(), std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>>
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:343:19
11: std::panic::catch_unwind::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:431:14
12: test::run_test_in_process
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:600:18
13: test::run_test::run_test_inner::{closure#0}
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:492:39
14: test::run_test::run_test_inner
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:530:13
15: test::run_test
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:564:28
16: test::run_tests::<[closure@test::run_tests_console::{closure#2}]>
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:305:17
17: test::run_tests_console
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/console.rs:289:5
18: test::test_main
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:122:15
19: test::test_main_static
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:141:5
20: main
          at hang.rs:1:1
21: <fn() as std::ops::FnOnce<()>>::call_once - shim(fn())
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
22: std::rt::lang_start::<()>::{closure#0}
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:49:18
23: std::ops::function::impls::<impl std::ops::FnOnce<()> for &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>::call_once
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:259:13
24: std::panicking::try::do_call::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:379:40
25: std::panicking::try::<i32, &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:343:19
26: std::panic::catch_unwind::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:431:14
27: std::rt::lang_start_internal
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:34:21
28: std::rt::lang_start::<()>
          at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:48:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.";

#[test]
fn foo() {
    eprintln!("{}", BACKTRACE_TEXT);
    panic!();
}

So it's probably related to output capturing (maybe here?) in libtest, and indeed adding --nocapture can fix the hang.

String::from_utf8_lossy() can indeed make Miri hang, while std::str::from_utf8() can even finish in the playground:

fn main() {
    String::from_utf8_lossy("stack backtrace:
0: std::rt::begin_panic::<&str>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:519:12
1: foo
            at hang.rs:4:5
2: foo::{closure#0}
            at hang.rs:2:1
3: <[closure@hang.rs:2:1: 5:2] as std::ops::FnOnce<()>>::call_once - shim
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
4: <fn() as std::ops::FnOnce<()>>::call_once - shim(fn())
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
5: test::run_test::{closure#2}
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:568:30
6: <[closure@test::run_test::{closure#2}] as std::ops::FnOnce<()>>::call_once - shim(vtable)
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
7: <std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send> as std::ops::FnOnce<()>>::call_once
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1546:9
8: <std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>> as std::ops::FnOnce<()>>::call_once
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:344:9
9: std::panicking::try::do_call::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:379:40
10: std::panicking::try::<(), std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:343:19
11: std::panic::catch_unwind::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:431:14
12: test::run_test_in_process
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:600:18
13: test::run_test::run_test_inner::{closure#0}
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:492:39
14: test::run_test::run_test_inner
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:530:13
15: test::run_test
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:564:28
16: test::run_tests::<[closure@test::run_tests_console::{closure#2}]>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:305:17
17: test::run_tests_console
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/console.rs:289:5
18: test::test_main
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:122:15
19: test::test_main_static
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:141:5
20: main
            at hang.rs:1:1
21: <fn() as std::ops::FnOnce<()>>::call_once - shim(fn())
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
22: std::rt::lang_start::<()>::{closure#0}
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:49:18
23: std::ops::function::impls::<impl std::ops::FnOnce<()> for &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>::call_once
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:259:13
24: std::panicking::try::do_call::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:379:40
25: std::panicking::try::<i32, &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:343:19
26: std::panic::catch_unwind::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:431:14
27: std::rt::lang_start_internal
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:34:21
28: std::rt::lang_start::<()>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:48:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.".as_bytes());
}

Minified from String::from_utf8_lossy()

fn main() {
    let source = b"stack backtrace:
0: std::rt::begin_panic::<&str>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:519:12
1: foo
            at hang.rs:4:5
2: foo::{closure#0}
            at hang.rs:2:1
3: <[closure@hang.rs:2:1: 5:2] as std::ops::FnOnce<()>>::call_once - shim
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
4: <fn() as std::ops::FnOnce<()>>::call_once - shim(fn())
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
5: test::run_test::{closure#2}
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:568:30
6: <[closure@test::run_test::{closure#2}] as std::ops::FnOnce<()>>::call_once - shim(vtable)
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
7: <std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send> as std::ops::FnOnce<()>>::call_once
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1546:9
8: <std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>> as std::ops::FnOnce<()>>::call_once
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:344:9
9: std::panicking::try::do_call::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:379:40
10: std::panicking::try::<(), std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:343:19
11: std::panic::catch_unwind::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:431:14
12: test::run_test_in_process
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:600:18
13: test::run_test::run_test_inner::{closure#0}
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:492:39
14: test::run_test::run_test_inner
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:530:13
15: test::run_test
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:564:28
16: test::run_tests::<[closure@test::run_tests_console::{closure#2}]>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:305:17
17: test::run_tests_console
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/console.rs:289:5
18: test::test_main
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:122:15
19: test::test_main_static
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:141:5
20: main
            at hang.rs:1:1
21: <fn() as std::ops::FnOnce<()>>::call_once - shim(fn())
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
22: std::rt::lang_start::<()>::{closure#0}
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:49:18
23: std::ops::function::impls::<impl std::ops::FnOnce<()> for &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>::call_once
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:259:13
24: std::panicking::try::do_call::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:379:40
25: std::panicking::try::<i32, &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:343:19
26: std::panic::catch_unwind::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:431:14
27: std::rt::lang_start_internal
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:34:21
28: std::rt::lang_start::<()>
            at /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:48:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.";
    let mut i = 0;
    while i < source.len() {
        unsafe {
            source.get_unchecked(i);
        }
        i += 1;
    }
}

Using [i] to replace get_unchecked() can make it fast again.

[RalfJung]

Okay, closing as a duplicate of #1367 then.