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Miri backtrace improvements
Nicer pretty-printing of the `RUST_CTFE_BACKTRACE`-backtraces:
```
0: backtrace::backtrace::libunwind::trace::hc410fcb66fe85b11
at /home/r/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.9/src/backtrace/libunwind.rs:53
backtrace::backtrace::trace::h2106294a22648407
at /home/r/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.9/src/backtrace/mod.rs:42
1: backtrace::capture::Backtrace::new_unresolved::h5d8d98b993d092ba
at /home/r/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.9/src/capture.rs:88
2: <rustc::mir::interpret::error::EvalError<'tcx> as core::convert::From<rustc::mir::interpret::error::EvalErrorKind<'tcx, u64>>>::from::h6355269b2a661412
at librustc/mir/interpret/error.rs:236
3: <T as core::convert::Into<U>>::into::h70fcb917509539bd
at /home/r/src/rust/rustc.2/src/libcore/convert.rs:455
4: <rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, miri::Evaluator<'tcx>> as miri::fn_call::EvalContextExt<'tcx, 'mir>>::emulate_foreign_item::h9cde0e3ce7455a4a
at src/fn_call.rs:292
5: <rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, miri::Evaluator<'tcx>> as miri::fn_call::EvalContextExt<'tcx, 'mir>>::find_fn::h83f89524b9d1a49a
at src/fn_call.rs:74
6: <miri::Evaluator<'tcx> as rustc_mir::interpret::machine::Machine<'a, 'mir, 'tcx>>::find_fn::hf9980473c4775f0c
at src/lib.rs:345
rustc_mir::interpret::terminator::<impl rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::eval_fn_call::h401dec4a687f96e9
at /home/r/src/rust/rustc.2/src/librustc_mir/interpret/terminator.rs:285
```
Indentation is now consistent with `RUST_BACKTRACE`, and the frame number is not repeated when there are multiple symbols for a frame.
Also preserve the `ty::Instance` for the internal backtrace (showing which frames in the user code where on the interpreter stack when the error happened), used by miri to avoid printing spans for libstd internals:
```
error[E0080]: constant evaluation error: the evaluated program panicked
--> /home/r/src/rust/rustc.2/src/libstd/panicking.rs:525:9
|
525 | __rust_start_panic(obj as usize)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked
|
= note: inside call to `std::panicking::rust_panic` at /home/r/src/rust/rustc.2/src/libstd/panicking.rs:496:5
= note: inside call to `std::panicking::rust_panic_with_hook` at /home/r/src/rust/rustc.2/src/libstd/panicking.rs:390:5
= note: inside call to `std::panicking::continue_panic_fmt` at /home/r/src/rust/rustc.2/src/libstd/panicking.rs:345:5
note: inside call to `std::rt::begin_panic_fmt` at <::std::macros::panic macros>:8:1
--> tests/compile-fail/panic.rs:4:5
|
4 | assert_eq!(5, 6);
| ^^^^^^^^^^^^^^^^^
= note: inside call to `main` at /home/r/src/rust/rustc.2/src/libstd/rt.rs:74:34
= note: inside call to closure at /home/r/src/rust/rustc.2/src/libstd/rt.rs:59:75
= note: inside call to closure at /home/r/src/rust/rustc.2/src/libstd/sys_common/backtrace.rs:136:5
= note: inside call to `std::sys_common::backtrace::__rust_begin_short_backtrace::<[closure@DefId(1/1:1913 ~ std[78f0]::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/src/rust/rustc.2/src/libstd/rt.rs:59:13
= note: inside call to closure at /home/r/src/rust/rustc.2/src/libstd/panicking.rs:310:40
= note: inside call to `std::panicking::try::do_call::<[closure@DefId(1/1:1912 ~ std[78f0]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/src/rust/rustc.2/src/libstd/panicking.rs:306:5
= note: inside call to `std::panicking::try::<i32, [closure@DefId(1/1:1912 ~ std[78f0]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe]>` at /home/r/src/rust/rustc.2/src/libstd/panic.rs:398:9
= note: inside call to `std::panic::catch_unwind::<[closure@DefId(1/1:1912 ~ std[78f0]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/src/rust/rustc.2/src/libstd/rt.rs:58:25
= note: inside call to `std::rt::lang_start_internal` at /home/r/src/rust/rustc.2/src/libstd/rt.rs:74:5
= note: inside call to `std::rt::lang_start::<()>`
```
Also notice that we show filenames and line numbers here now.
r? @oli-obk
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