stable mechanism to specify the behavior of panic! in no-std applications

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+- Feature Name: panic_implementation
+- Start Date: 2017-07-19
+- RFC PR: https://github.com/rust-lang/rfcs/pull/2070
+- Rust Issue: https://github.com/rust-lang/rust/issues/44489
+
+# Summary
+[summary]: #summary
+
+Provide a stable mechanism to specify the behavior of `panic!` in no-std
+applications.
+
+# Motivation
+[motivation]: #motivation
+
+The `#![no_std]` attribute was stabilized some time ago and it made possible to
+build no-std libraries on stable. However, to this day no-std applications
+still require a nightly compiler to be built. The main cause of this is that
+the behavior of `panic!` is left undefined in no-std context, and the only way
+to specify a panicking behavior is through the unstable `panic_fmt` [language
+item].
+
+[language item]: https://doc.rust-lang.org/unstable-book/language-features/lang-items.html
+
+This document proposes a stable mechanism to specify the behavior of `panic!` in
+no-std context. This would be a step towards enabling development of no-std
+applications like device firmware, kernels and operating systems on the stable
+channel.
+
+# Detailed design
+[design]: #detailed-design
+
+## Constraints
+
+`panic!` in no-std environments must continue to be free of memory allocations
+and [its API] can only be changed in a backward compatible way.
+
+[its API]: https://doc.rust-lang.org/core/macro.panic.html
+
+Although not a hard constraint, the cognitive load of the mechanism would be
+greatly reduced if it mimicked the existing [custom panic hook] mechanism as
+much as possible.
+
+[custom panic hook]: https://doc.rust-lang.org/std/panic/fn.set_hook.html
+
+## `PanicInfo`
+
+The types [`std::panic::PanicInfo`] and [`std::panic::Location`] will be moved
+into the `core` crate, and `PanicInfo` will gain a new method:
+
+[`std::panic::PanicInfo`]: https://doc.rust-lang.org/std/panic/struct.PanicInfo.html
+[`std::panic::Location`]: https://doc.rust-lang.org/std/panic/struct.Location.html
+
+``` rust
+impl PanicInfo {
+    pub fn message(&self) -> Option<&fmt::Arguments> { .. }
+}
+```
+
+This method returns `Some` if the `panic!` invocation needs to do any formatting
+like `panic!("{}: {}", key , value)` does.
+
+### `fmt::Display`
+
+For convenience, `PanicInfo` will gain an implementation of the `fmt::Display`
+trait that produces a message very similar to the one that the standard `panic!`
+hook produces. For instance, this program:
+
+``` rust
+use std::panic::{self, PanicInfo};
+
+fn panic_handler(pi: &PanicInfo) {
+    println!("the application {}", pi);
+}
+
+fn main() {
+    panic::set_hook(Box::new(panic_handler));
+
+    panic!("Hello, {}!", "world");
+}
+```
+
+Would print:
+
+``` console
+$ cargo run
+the application panicked at 'Hello, world!', src/main.rs:27:4
+```
+
+## `#[panic_implementation]`
+
+A `#[panic_implementation]` attribute will be added to the language. This
+attribute can be used to specify the behavior of `panic!` in no-std context.
+Only functions with signature `fn(&PanicInfo) -> !` can be annotated with this
+attribute, and only one item can be annotated with this attribute in the whole
+dependency graph of a crate.
+
+Here's an example of how to replicate the panic messages one gets on std
+programs on a no-std program:
+
+``` rust
+use core::fmt;
+use core::panic::PanicInfo;
+
+// prints: "program panicked at 'reason', src/main.rs:27:4"
+#[panic_implementation]
+fn my_panic(pi: &PanicInfo) -> ! {
+    let _ = writeln!(&MY_STDERR, "program {}", pi);
+
+    abort()
+}
+```
+
+The `#[panic_implementation]` item will roughly expand to:
+
+``` rust
+fn my_panic(pi: &PanicInfo) -> ! {
+    // same as before
+}
+
+// Generated by the compiler
+// This will always use the correct ABI and will work on the stable channel
+#[lang = "panic_fmt"]
+#[no_mangle]
+pub extern fn rust_begin_panic(msg: ::core::fmt::Arguments,
+                               file: &'static str,
+                               line: u32,
+                               col: u32) -> ! {
+    my_panic(&PanicInfo::__private_unstable_constructor(msg, file, line, col))
+}
+```
+
+## Payload
+
+The `core` version of the `panic!` macro will gain support for *payloads*, as in
+`panic!(42)`. When invoked with a payload `PanicInfo.payload()` will return the
+payload as an `&Any` trait object just like it does in std context with custom
+panic hooks.
+
+When using `core::panic!` with formatting, e.g. `panic!("{}", 42)`, the payload
+will be uninspectable: it won't be downcastable to any known type. This is where
+`core::panic!` diverges from `std::panic!`. The latter returns a `String`,
+behind the `&Any` trait object, from the `payload()` method in this situation.
+
+## Feature gate
+
+The initial implementation of the `#[panic_implementation]` mechanism as well as
+the `core::panic::Location` and `core::panic::PanicInfo` types will be feature
+gated. `std::panic::Location` and `std::panic::PanicInfo` will continue to be
+stable except for the new `PanicInfo.message` method.
+
+## Unwinding
+
+The `#[panic_implementation]` mechanism can only be used with no-std
+applications compiled with `-C panic=abort`. Applications compiled with `-C
+panic=unwind` additionally require the `eh_personality` language item which this
+proposal doesn't cover.
+
+## `std::panic!`
+
+This proposal doesn't affect how the selection of the panic runtime in `std`
+applications works (`panic_abort`, `panic_unwind`, etc.). Using
+`#[panic_implementation]` in `std` programs will cause a compiler error.
+
+# How We Teach This
+[how-we-teach-this]: #how-we-teach-this
+
+Currently, no-std applications are only possible on nightly so there's not much
+official documentation on this topic given its dependency on several unstable
+features. Hopefully once no-std applications are minimally possible on stable we
+can have a detailed chapter on the topic in ["The Rust Programming Language"]
+book. In the meantime, this feature can be documented in [the unstable book].
+
+["The Rust Programming Language"]: https://doc.rust-lang.org/book/second-edition/
+[the unstable book]: https://doc.rust-lang.org/unstable-book/
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+## Slight deviation from std
+
+Although both `#[panic_implementation]` (no-std) and custom panic hooks (std)
+use the same `PanicInfo` type. The behavior of the `PanicInfo.payload()` method
+changes depending on which context it is used: given `panic!("{}", 42)`,
+`payload()` will return a `String`, behind an `Any` trait object, in std context
+but it will return an opaque `Any` trait object in no-std context.
+
+# Alternatives
+[alternatives]: #alternatives
+
+## Not doing this
+
+Not providing a stable alternative to the `panic_fmt` language item means that
+no-std applications will continue to be tied to the nightly channel.
+
+## Two `PanicInfo` types
+
+An alternative design is to have two different `PanicInfo` types, one in `core`
+and one in `std`. The difference between these two types would be in their APIs:
+
+``` rust
+// core
+impl PanicInfo {
+    pub fn location(&self) -> Option<Location> { .. }
+    pub fn message(&self) -> Option<&fmt::Arguments> { .. }
+
+    // Not available
+    // pub fn payload(&self) -> &(Any + Send) { .. }
+}
+
+// std
+impl PanicInfo {
+    pub fn location(&self) -> Option<Location> { .. }
+    pub fn message(&self) -> Option<&fmt::Arguments> { .. }
+    pub fn payload(&self) -> &(Any + Send) { .. }
+}
+```
+
+In this alternative design the signature of the `#[panic_implementation]`
+function would be enforced to be `fn(&core::panic::PanicInfo) -> !`. Custom
+panic hooks will continue to use the `std::panic::PanicInfo` type.
+
+This design precludes supporting payloads in `core::panic!` but also eliminates
+the difference between `core::PanicInfo.payload()` in no-std vs std by
+eliminating the method in the former context.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+## `fmt::Display`
+
+Should the `Display` of `PanicInfo` format the panic information as `"panicked
+at 'reason', src/main.rs:27:4"`, as `"'reason', src/main.rs:27:4"`, or simply as
+`"reason"`.
+
+## Unwinding in no-std
+
+Is this design compatible, or can it be extended to work, with unwinding
+implementations for no-std environments?