aaea63e9e2
Improve comments for the default backtrace printer
The existing comments were misleading, confusing, and outdated.
Take this comment for example:
```
// Any frames between `__rust_begin_short_backtrace` and `__rust_end_short_backtrace`
// are omitted from the backtrace in short mode, `__rust_end_short_backtrace` will be
// called before the panic hook, so we won't ignore any frames if there is no
// invoke of `__rust_begin_short_backtrace`.
```
this is just wrong. here is an example (full) backtrace:
<details>
```
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.01s
Running `/home/jyn/.local/lib/cargo/target/debug/example`
called `Option::unwrap()` on a `None` value
stack backtrace:
0: 0x56499698c595 - std::backtrace_rs::backtrace::libunwind::trace::h5ef2cc16e9a7415a
1: 0x56499698c595 - std::backtrace_rs::backtrace::trace_unsynchronized::h9b5e016e9075f714
2: 0x56499698c595 - std::sys_common::backtrace::_print_fmt::h2f62c7f9ff224e93
3: 0x56499698c595 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hbe51682735731910
4: 0x5649969aa26b - core::fmt::rt::Argument::fmt::h1994ab2b310d665e
5: 0x5649969aa26b - core::fmt::write::hade58a36d63468d7
6: 0x56499698a43f - std::io::Write::write_fmt::h16145587d801a9ab
7: 0x56499698c36e - std::sys_common::backtrace::_print::ha8082e56201dadb4
8: 0x56499698c36e - std::sys_common::backtrace::print::he30f96b4e7f6cbfd
9: 0x56499698d709 - std::panicking::default_hook::{{closure}}::hf0801f6b18a968d3
10: 0x56499698d4ac - std::panicking::default_hook::hd2defec7eda5aeb0
11: 0x56499698dc31 - std::panicking::rust_panic_with_hook::hde93283600065c53
12: 0x56499698daf3 - std::panicking::begin_panic_handler::{{closure}}::h5e151adbdb7ec0c1
13: 0x56499698ca59 - std::sys_common::backtrace::__rust_end_short_backtrace::he36a1407e0f77700
14: 0x56499698d7d4 - rust_begin_unwind
15: 0x5649969a9503 - core::panicking::panic_fmt::h2380d41365f95412
16: 0x5649969a958c - core::panicking::panic::h38cf8db80e8c6e67
17: 0x5649969a93e9 - core::option::unwrap_failed::he72696e53ff29a05
18: 0x5649969722b6 - core::option::Option<T>::unwrap::hb574dc0dc1703062
19: 0x5649969722b6 - example::main::h7a867aafacd93d75
20: 0x5649969721db - core::ops::function::FnOnce::call_once::h734f99a5e57291b7
21: 0x56499697226e - std::sys_common::backtrace::__rust_begin_short_backtrace::h02f5d58c351c4756
22: 0x564996972241 - std::rt::lang_start::{{closure}}::h8b134fe2c31a4355
23: 0x564996988662 - core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once::h88d7bb571ee2aaf4
24: 0x564996988662 - std::panicking::try::do_call::hfb78dfb6599c871d
25: 0x564996988662 - std::panicking::try::habd041c8c4c8e50c
27: 0x564996988662 - std::rt::lang_start_internal::{{closure}}::h227591a6f9c0879e
28: 0x564996988662 - std::panicking::try::do_call::h3c5878333c38916a
29: 0x564996988662 - std::panicking::try::h5af7b3a127cdae70
31: 0x564996988662 - std::rt::lang_start_internal::hbc85e809eeace0dd
32: 0x56499697221a - std::rt::lang_start::ha1eb16922c9cb224
33: 0x5649969722ee - main
34: 0x7f031962a1ca - __libc_start_call_main
35: 0x7f031962a28b - __libc_start_main_impl
36: 0x5649969720a5 - _start
37: 0x0 - <unknown>
```
</details>
note particularly frames 13-21, from start_backtrace to end_backtrace. with PrintFmt::Short, these are the *only* frames that are printed; i.e. we are doing the exact opposite of the comment.
r? ``@saethlin``
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