RFC: Backtrace in core

0000-backtrace-in-core.md

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+- Feature Name: Backtrace in `core`
+- Start Date: (fill me in with today's date, YYYY-MM-DD)
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+This RFC will move the `Backtrace` type to `core`, making it available for usage in `no_std` contexts. This is a stepping stone for the `Error` type in `core` and eventually both types will end there. 
+
+It is still unclear whether `Backtrace` itself does need to be moved to `core` and in time this RFC will be unanimous in this matter.
+
+# Motivation
+[motivation]: #motivation
+
+The main reason behind moving `Backtrace` to `core` is to have essential types available for wider usage without the need to import `std`. While `Error` had a valid reason for not being in `core` (it relies on `Box` type for different conversions), `Backtrace` does not have similar blockers apart from its frame-allocating API
+
+Additionally, having this type in `core` will allow its users to provide their own implementations of the backtrace collection and reporting and not rely on the `std`-provided one if they don't want to.
+
+The outcome of this RFC will be a `Backtrace` type in `core` with implementation defined in `std` and compiler-generated implementation when `std` is not linked and user did not provide their own implementation for the reporting functions.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Backtraces are an essential part of the Rust ecosystem, being close coupled with error handling and reporting, be it to the user or the developer. They are usually of a common form:
+```rust
+   Compiling playground v0.0.1 (/playground)
+    Finished dev [unoptimized + debuginfo] target(s) in 1.66s
+     Running `target/debug/playground`
+thread 'main' panicked at 'index out of bounds: the len is 4 but the index is 4', src/main.rs:5:5
+stack backtrace:
+   0: rust_begin_unwind
+             at /rustc/53cb7b09b00cbea8754ffb78e7e3cb521cb8af4b/library/std/src/panicking.rs:493:5
+   1: core::panicking::panic_fmt
+             at /rustc/53cb7b09b00cbea8754ffb78e7e3cb521cb8af4b/library/core/src/panicking.rs:92:14
+   2: core::panicking::panic_bounds_check
+             at /rustc/53cb7b09b00cbea8754ffb78e7e3cb521cb8af4b/library/core/src/panicking.rs:69:5
+   3: <usize as core::slice::index::SliceIndex<[T]>>::index_mut
+             at /rustc/53cb7b09b00cbea8754ffb78e7e3cb521cb8af4b/library/core/src/slice/index.rs:190:14
+   4: core::slice::index::<impl core::ops::index::IndexMut<I> for [T]>::index_mut
+             at /rustc/53cb7b09b00cbea8754ffb78e7e3cb521cb8af4b/library/core/src/slice/index.rs:26:9
+   5: <alloc::vec::Vec<T,A> as core::ops::index::IndexMut<I>>::index_mut
+             at /rustc/53cb7b09b00cbea8754ffb78e7e3cb521cb8af4b/library/alloc/src/vec/mod.rs:2396:9
+   6: playground::main
+             at ./src/main.rs:5:5
+   7: core::ops::function::FnOnce::call_once
+             at /rustc/53cb7b09b00cbea8754ffb78e7e3cb521cb8af4b/library/core/src/ops/function.rs:227:5
+note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
+```
+
+They allow for a detailed inspection of what crashed at runtime and which libraries and functions were involved in the process. Obviously reporting and formatting it is also of a big importance, so `Error` allows for backtrace printing with help of a `backtrace()` method.
+
+Currently, `Backtrace` is an essential part of the `std` library and user can control whether the backtrace is enabled or disabled using the environmental variable: `RUST_BACKTRACE`.
+
+In terms of Guide-level changes, there is not much to be discussed - only that it is moved to `core` and if `std` is not linked, automatic backtrace handlers will be generated. Otherwise, the regular implementation of `Backtrace` is present.
+
+TODO: what else should be here?
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The following changes need to be made to implement this proposal:
+
+## Move the `Backtrace` to `core` and add a thin wrapper in `std`
+
+This way, a `StdBacktrace` struct is introduced in `std` that allows for `Debug` and `Display` formatting. 
+
+The regular API of `Backtrace` comprising `enabled()`, `create()` and `status()` would be left in the `std` as free-standing functions. Since, they are lang items they need to have a default implementation in case `std` is not linked, so they will be provided in such a form in the `core` library (and overwritten once `std` is linked):
+
+```rust
+/// Global implementation of backtrace functionality. Called to create
+/// `RawBacktrace` trait objects.
+#[cfg(not(bootstrap))]
+extern "Rust" {
+    #[lang = "backtrace_create"]
+    fn backtrace_create(ip: usize) -> *mut dyn RawBacktrace;
+
+    #[lang = "backtrace_enabled"]
+    fn backtrace_enabled() -> bool;
+
+    #[lang = "backtrace_status"]
+    fn backtrace_status(raw: *mut dyn RawBacktrace) -> BacktraceStatus;
+}
+
+#[cfg(bootstrap)]
+unsafe fn backtrace_create(_ip: usize) -> *mut dyn RawBacktrace {
+    UnsupportedBacktrace::create().inner
+}
+
+#[cfg(bootstrap)]
+unsafe fn backtrace_enabled() -> bool {
+    false
+}
+
+#[cfg(bootstrap)]
+unsafe fn backtrace_status(_raw: *mut dyn RawBacktrace) -> BacktraceStatus {
+    BacktraceStatus::Unsupported
+}
+
+```
+
+This change will make the `Backtrace` an optional part of the library and 
+This way, if the `Backtrace` is not enabled (on the library level) there is no need for it to report 
+
+Implementation-wise, `Backtrace` is **declared** in the `core` module and **defined** in `std` via *lang_items* which act as function hooks which are resolved during link-time. Special type `StdBacktrace` is introduced for the `std` part of implementation which acts as a proxy for `Debug` and `Display` trait impls.
+
+// TODO: add examples of how would one implement these functions themselves like panic hooks
+
+
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+There are actually many drawbacks, most important of them being `Backtrace` using a lot of `std` for the actual backtrace capturing. 
+
+The other one is a potential code bloat in `no_std` contexts, so a possible alternative may be only enabling the `Backtrace` conditionally via `cfg` settings. (not so sure about this though)
+
+
+`no_std` code bloat - Mara mentioned out
+`Error:backtrace` also seems to be blocking??
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+The proposed solution is the one which is currently implementable. However, if the Generic Member Access RFC was implemented (link!) we would not have to move the `Backtrace` to `core` at all. In the alternative solution, we would leave the `Backtrace` as it is and instead the `Error` trait will provide a `backtrace()` method which will use the Generic Access to extract the concrete `Backtrace` out of the propagated error.
+
+Alternatively, since the actual implementation of `Backtrace` uses allocating structures like `Box` for backtrace creation, it would be prudent to move it to where this type resides. This will in turn allow users of `alloc` module in embedded and `no_std` contexts to use this functionality without `std`. _should we go for this though instead of the core when we introduce such big changes?_
+
+A viable solution to allocating functions of `Backtrace` might be adding an API where the users could provide themselves the memory in which the backtrace should reside and truncate/report a failure in case the backtrace does not fit this preallocated space. 
+
+# Prior art
+[prior-art]: #prior-art
+
+This type is already implemented, but it seems like no type was moved from `std` to `core` previously so we have no point of reference on this one.
+
+As for `no_std` and embedded contexts, there exists the [mini-backtrace](https://github.com/amanieu/mini-backtrace) library that provides backtrace support via LLVM's libunwind.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+