Format symbols under shared frames

[KodrAus]

Part of #71706
Also related to #72981

This changes the format used for backtraces in the currently unstable std::backtrace to align them with backtraces produced by panic!. It involves un-nesting symbols that share frames in std::backtrace to appear as if they belong under individual frames.

before:

  14: std::rt::lang_start_internal::{{closure}}
             at src/libstd/rt.rs:52
      std::panicking::try::do_call
             at src/libstd/panicking.rs:297
      std::panicking::try
             at src/libstd/panicking.rs:274
      std::panic::catch_unwind
             at src/libstd/panic.rs:394
      std::rt::lang_start_internal
             at src/libstd/rt.rs:51

after:

  14: std::rt::lang_start_internal::{{closure}}
             at src/libstd/rt.rs:52
  15: std::panicking::try::do_call
             at src/libstd/panicking.rs:297
  16: std::panicking::try
             at src/libstd/panicking.rs:274
  17: std::panic::catch_unwind
             at src/libstd/panic.rs:394
  18: std::rt::lang_start_internal
             at src/libstd/rt.rs:51

Since we don't share too much code between these formats right now or really check the Display formats for backtraces, they might end up diverging again.

[rust-highfive]

r? @cramertj

(rust-highfive has picked a reviewer for you, use r? to override)

[KodrAus]

Ah! Looks like we do have a bit of coverage for the panic format afterall 👀

[cramertj]

This looks reasonable to me, but it looks like CI is unhappy? r=me with that fixed.

[KodrAus]

We discussed this at a recent Libs meeting and thought making these formats consistent the other way was preferable. So instead of changing the way frames from panic!s are printed to show symbols that share a frame differently, we change the currently unstable Backtrace format to not show symbols that share a frame differently.

[KodrAus]

I've gone ahead and updated the description here to reflect the actual change being made, which is to std::backtrace, and not to panic! backtraces.

This should be ready for another review now.

Also cc @rust-lang/project-error-handling

[KodrAus]

@cramertj Are you still happy with this with the change made to Backtrace instead of panic!?

[KodrAus]

r? @sfackler

[sfackler]

I'm a bit confused as to the state of the PR - it looks like now it's just a purely stylistic change to the code, with no actual behavioral difference?

[KodrAus]

@sfackler it’s a bit subtle, but the difference is that we move the call to .frame() within the loop so it’s called for each symbol, instead of for each frame. It slightly changes the output format of std::backtrace backtraces to look like panic! backtraces. There’s an example in the OP.

[KodrAus]

I no longer have the bandwidth to carry this forward but if somebody else would like to pick it up sometime in the future then please feel free!

[yaahc]

I'll take this one over