Add std::error::Report type

[yaahc]

This is a continuation of #90174, split into a separate PR since I cannot push to @seanchen1991 's fork

[rust-highfive]

r? @kennytm

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

[yaahc]

r? @m-ou-se

[yaahc]

Vendoring my most recent comment from the previous PR

---

Almost done resolving the recent round of comments but I have a couple of style questions that came up during the implementation that I could use some input on.

First, I want to make sure we have the right level of indentation setup for our multiline errors so they match up nicely with the output of Backtrace, and I'm wondering what you think would be best @m-ou-se.

Here's the current version with the changes I made and one source error:

Error: The source of the error

Caused by:
    Error with backtrace

Stack backtrace:
   0: std::error::tests::error_with_backtrace_outputs_correctly
   1: test::__rust_begin_short_backtrace
   2: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
   3: std::panicking::try
   4: test::run_test_in_process
   5: std::sys_common::backtrace::__rust_begin_short_backtrace
   6: std::panicking::try
   7: core::ops::function::FnOnce::call_once{{vtable.shim}}
   8: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
   9: std::sys::unix::thread::Thread::new::thread_start
  10: start_thread
             at /build/glibc-eX1tMB/glibc-2.31/nptl/pthread_create.c:477:8
  11: clone

And with 2 or more sources

Error: Error with two sources

Caused by:
    0: The source of the error
    1: Error with backtrace

Stack backtrace:
   0: std::error::tests::error_with_backtrace_outputs_correctly_with_two_sources
   1: test::__rust_begin_short_backtrace
   2: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
   3: std::panicking::try
   4: test::run_test_in_process
   5: std::sys_common::backtrace::__rust_begin_short_backtrace
   6: std::panicking::try
   7: core::ops::function::FnOnce::call_once{{vtable.shim}}
   8: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
   9: std::sys::unix::thread::Thread::new::thread_start
  10: start_thread
             at /build/glibc-eX1tMB/glibc-2.31/nptl/pthread_create.c:477:8

This is identical to the formatting rules for anyhow, as far as I can tell. Right now it seems to want to line up the error messages so that each message starts on the same column, but my instinct is to line it up so that the causes start on the same column as the symbols in the backtrace. The case where the errors aren't numbered already doesn't line up with the outermost source error message, so either way I'd like it to be changed for consistency, whether or not that ends up being based on the first error message or the backtrace.

My preferred version would look like this

Error: Error with two sources

Caused by:
   0: The source of the error
   1: Error with backtrace

Stack backtrace:
   0: std::error::tests::error_with_backtrace_outputs_correctly_with_two_sources
   1: test::__rust_begin_short_backtrace
   2: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
   3: std::panicking::try
   4: test::run_test_in_process
   5: std::sys_common::backtrace::__rust_begin_short_backtrace
   6: std::panicking::try
   7: core::ops::function::FnOnce::call_once{{vtable.shim}}
   8: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
   9: std::sys::unix::thread::Thread::new::thread_start
  10: start_thread
             at /build/glibc-eX1tMB/glibc-2.31/nptl/pthread_create.c:477:8

and

Error: The source of the error

Caused by:
      Error with backtrace

Stack backtrace:
   0: std::error::tests::error_with_backtrace_outputs_correctly
   1: test::__rust_begin_short_backtrace
   2: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
   3: std::panicking::try
   4: test::run_test_in_process
   5: std::sys_common::backtrace::__rust_begin_short_backtrace
   6: std::panicking::try
   7: core::ops::function::FnOnce::call_once{{vtable.shim}}
   8: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
   9: std::sys::unix::thread::Thread::new::thread_start
  10: start_thread
             at /build/glibc-eX1tMB/glibc-2.31/nptl/pthread_create.c:477:8
  11: clone

[yaahc]

The current issues I have with the proposed API, note that I don't think these should block it landing, but I will document them in the tracking issue and want to work towards resolving them.

• show_backtrace currently has no effect if the pretty format isn't enabled, we won't display backtraces ever in the single line format. I dislike this aspect, and am wondering if setting show_backtrace should implicitly also enable pretty, which isn't necessarily much better, but at least it's less error prone.
• I don't like the name of pretty, in the docs we refer to it as the multi-line format, should it just be multiline?
• I dislike how requiring the boolean for pretty/show_backtrace means you can't write something like let val = my_result.map_err(Report::new).map_err(Report::pretty).unwrap();, though I'm leaning towards this being the lesser evil compared to the issues with lifetimes and if statements on alternative builder API styles. It's not that big of a deal to introduce a closure like .map_err(|r| r.pretty(should_be_pretty))
• The current design of Report does not work well as a return value for main, since it needs to be something like fn main() -> Result<(), Report<MyError>>, and Box<dyn Error> wont work since it doesn't impl Error and it would require two simultaneous conversions via From
  • I think the best solution to this is to use similar type erasure tricks to those used in anyhow/extracterr so that we can first store the error in a monomorphized struct with a custom vtable for accessing it's error field, then erase that parameter and only access the error field via the lookup method which returns it as a &dyn Error.