add empty line before backtrace, to separate it from help text

[RalfJung]

I think this improves how things look, and makes it easier to see the help text (it currently kind of drowns next to the backtrace).

Before:

error: unsupported operation: Miri does not support threading
  --> /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys/unix/thread.rs:68:19
   |
68 |         let ret = libc::pthread_create(&mut native, &attr, thread_start, &*p as *const _ as *mut _);
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Miri does not support threading
   |
   = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support
   = note: inside `std::sys::unix::thread::Thread::new` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys/unix/thread.rs:68:19
   = note: inside `std::thread::Builder::spawn_unchecked::<[closure@tests/compile-fail/thread-spawn.rs:6:19: 6:24], ()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/thread/mod.rs:492:26
   = note: inside `std::thread::Builder::spawn::<[closure@tests/compile-fail/thread-spawn.rs:6:19: 6:24], ()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/thread/mod.rs:386:18
   = note: inside `std::thread::spawn::<[closure@tests/compile-fail/thread-spawn.rs:6:19: 6:24], ()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/thread/mod.rs:619:5
note: inside `main` at tests/compile-fail/thread-spawn.rs:6:5
  --> tests/compile-fail/thread-spawn.rs:6:5
   |
6  |     thread::spawn(|| {});
   |     ^^^^^^^^^^^^^^^^^^^^
   = note: inside closure at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:67:34
   = note: inside closure at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:52:73
   = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<[closure@DefId(1:6030 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys_common/backtrace.rs:130:5
   = note: inside closure at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:52:13
   = note: inside `std::panicking::try::do_call::<[closure@DefId(1:6029 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/panicking.rs:331:40
   = note: inside `std::panicking::try::<i32, [closure@DefId(1:6029 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe]>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/panicking.rs:274:15
   = note: inside `std::panic::catch_unwind::<[closure@DefId(1:6029 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/panic.rs:394:14
   = note: inside `std::rt::lang_start_internal` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:51:25
   = note: inside `std::rt::lang_start::<()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:67:5

After:

error: unsupported operation: Miri does not support threading
  --> /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys/unix/thread.rs:68:19
   |
68 |         let ret = libc::pthread_create(&mut native, &attr, thread_start, &*p as *const _ as *mut _);
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Miri does not support threading
   |
   = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support
           
   = note: inside `std::sys::unix::thread::Thread::new` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys/unix/thread.rs:68:19
   = note: inside `std::thread::Builder::spawn_unchecked::<[closure@tests/compile-fail/thread-spawn.rs:6:19: 6:24], ()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/thread/mod.rs:492:26
   = note: inside `std::thread::Builder::spawn::<[closure@tests/compile-fail/thread-spawn.rs:6:19: 6:24], ()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/thread/mod.rs:386:18
   = note: inside `std::thread::spawn::<[closure@tests/compile-fail/thread-spawn.rs:6:19: 6:24], ()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/thread/mod.rs:619:5
note: inside `main` at tests/compile-fail/thread-spawn.rs:6:5
  --> tests/compile-fail/thread-spawn.rs:6:5
   |
6  |     thread::spawn(|| {});
   |     ^^^^^^^^^^^^^^^^^^^^
   = note: inside closure at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:67:34
   = note: inside closure at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:52:73
   = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<[closure@DefId(1:6030 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys_common/backtrace.rs:130:5
   = note: inside closure at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:52:13
   = note: inside `std::panicking::try::do_call::<[closure@DefId(1:6029 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/panicking.rs:331:40
   = note: inside `std::panicking::try::<i32, [closure@DefId(1:6029 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe]>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/panicking.rs:274:15
   = note: inside `std::panic::catch_unwind::<[closure@DefId(1:6029 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/panic.rs:394:14
   = note: inside `std::rt::lang_start_internal` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:51:25
   = note: inside `std::rt::lang_start::<()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:67:5

What do you think?

[RalfJung]

@oli-obk any opinion on this?

[RalfJung]

@bors r+

[bors]

📌 Commit 95ea03c has been approved by RalfJung

[bors]

⌛ Testing commit 95ea03c with merge 2e82a8d...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: RalfJung
Pushing 2e82a8d to master...