Auto merge of #51366 - japaric:stable-panic-impl, r=Mark-Simulacrum

stabilize #[panic_handler]

closes #44489

### Update(2018-09-07)

This was proposed for stabilization in https://github.com/rust-lang/rust/issues/44489#issuecomment-398965881 and its FCP with disposition to merge / accept is nearly over. The summary of what's being stabilized can be found in https://github.com/rust-lang/rust/issues/44489#issuecomment-416645946

Documentation PRs:

- Reference. https://github.com/rust-lang-nursery/reference/pull/362
- Nomicon. https://github.com/rust-lang-nursery/nomicon/pull/75

---

`#[panic_implementation]` was implemented recently in #50338. `#[panic_implementation]` is basically the old `panic_fmt` language item but in a less error prone (\*) shape. There are still some issues and questions to sort out around this feature (cf. #44489) but this PR is meant to start a discussion about those issues / questions with the language team.

(\*) `panic_fmt` was not type checked; changes in its function signature caused serious, silent binary size regressions like the one observed in #43054

Some unresolved questions from #44489:

> 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".

The current implementation formats `PanicInfo` as the first alternative, which is how panic messages are formatted by the `std` panic handler. The `Display` implementation is more than a convenience: `PanicInfo.message` is unstable so it's not possible to replicate the `Display` implementation on stable.

> Is this design compatible, or can it be extended to work, with unwinding implementations for
> no-std environments?

I believe @whitequark made more progress with unwinding in no-std since their last comment in #44489. Perhaps they can give us an update?

---

Another unresolved question is where this feature should be documented. The feature currently doesn't have any documentation.

cc @rust-lang/lang
cc @jackpot51 @alevy @phil-opp

14 files changed, 9 insertions, 17 deletions

diff --git a/src/test/ui/panic-handler/auxiliary/some-panic-impl.rs b/src/test/ui/panic-handler/auxiliary/some-panic-impl.rs
index e3b4fba176e..090123198d1 100644
--- a/src/test/ui/panic-handler/auxiliary/some-panic-impl.rs
+++ b/src/test/ui/panic-handler/auxiliary/some-panic-impl.rs
@@ -11,7 +11,6 @@
 // no-prefer-dynamic
 
 #![crate_type = "rlib"]
-#![feature(panic_handler)]
 #![no_std]
 
 use core::panic::PanicInfo;
diff --git a/src/test/ui/panic-handler/panic-handler-bad-signature-1.rs b/src/test/ui/panic-handler/panic-handler-bad-signature-1.rs
index cc7e337fc9e..8d53c66adbd 100644
--- a/src/test/ui/panic-handler/panic-handler-bad-signature-1.rs
+++ b/src/test/ui/panic-handler/panic-handler-bad-signature-1.rs
@@ -10,7 +10,6 @@
 
 // compile-flags:-C panic=abort
 
-#![feature(panic_handler)]
 #![no_std]
 #![no_main]
 
diff --git a/src/test/ui/panic-handler/panic-handler-bad-signature-1.stderr b/src/test/ui/panic-handler/panic-handler-bad-signature-1.stderr
index 5771f4277b4..72b85da028f 100644
--- a/src/test/ui/panic-handler/panic-handler-bad-signature-1.stderr
+++ b/src/test/ui/panic-handler/panic-handler-bad-signature-1.stderr
@@ -1,11 +1,11 @@
 error: return type should be `!`
-  --> $DIR/panic-handler-bad-signature-1.rs:22:6
+  --> $DIR/panic-handler-bad-signature-1.rs:21:6
    |
 LL | ) -> () //~ ERROR return type should be `!`
    |      ^^
 
 error: argument should be `&PanicInfo`
-  --> $DIR/panic-handler-bad-signature-1.rs:21:11
+  --> $DIR/panic-handler-bad-signature-1.rs:20:11
    |
 LL |     info: PanicInfo, //~ ERROR argument should be `&PanicInfo`
    |           ^^^^^^^^^
diff --git a/src/test/ui/panic-handler/panic-handler-bad-signature-2.rs b/src/test/ui/panic-handler/panic-handler-bad-signature-2.rs
index ec698903c84..0e8b3d5e855 100644
--- a/src/test/ui/panic-handler/panic-handler-bad-signature-2.rs
+++ b/src/test/ui/panic-handler/panic-handler-bad-signature-2.rs
@@ -10,7 +10,6 @@
 
 // compile-flags:-C panic=abort
 
-#![feature(panic_handler)]
 #![no_std]
 #![no_main]
 
diff --git a/src/test/ui/panic-handler/panic-handler-bad-signature-2.stderr b/src/test/ui/panic-handler/panic-handler-bad-signature-2.stderr
index 4823f8a2781..f2bcb6b6809 100644
--- a/src/test/ui/panic-handler/panic-handler-bad-signature-2.stderr
+++ b/src/test/ui/panic-handler/panic-handler-bad-signature-2.stderr
@@ -1,5 +1,5 @@
 error: argument should be `&PanicInfo`
-  --> $DIR/panic-handler-bad-signature-2.rs:21:11
+  --> $DIR/panic-handler-bad-signature-2.rs:20:11
    |
 LL |     info: &'static PanicInfo, //~ ERROR argument should be `&PanicInfo`
    |           ^^^^^^^^^^^^^^^^^^
diff --git a/src/test/ui/panic-handler/panic-handler-bad-signature-3.rs b/src/test/ui/panic-handler/panic-handler-bad-signature-3.rs
index 585716c7c75..ac1b19105b6 100644
--- a/src/test/ui/panic-handler/panic-handler-bad-signature-3.rs
+++ b/src/test/ui/panic-handler/panic-handler-bad-signature-3.rs
@@ -10,7 +10,6 @@
 
 // compile-flags:-C panic=abort
 
-#![feature(panic_handler)]
 #![no_std]
 #![no_main]
 
diff --git a/src/test/ui/panic-handler/panic-handler-bad-signature-3.stderr b/src/test/ui/panic-handler/panic-handler-bad-signature-3.stderr
index 5d0395e17f5..77227e0d5a6 100644
--- a/src/test/ui/panic-handler/panic-handler-bad-signature-3.stderr
+++ b/src/test/ui/panic-handler/panic-handler-bad-signature-3.stderr
@@ -1,5 +1,5 @@
 error: function should have one argument
-  --> $DIR/panic-handler-bad-signature-3.rs:20:1
+  --> $DIR/panic-handler-bad-signature-3.rs:19:1
    |
 LL | fn panic() -> ! { //~ ERROR function should have one argument
    | ^^^^^^^^^^^^^^^
diff --git a/src/test/ui/panic-handler/panic-handler-bad-signature-4.rs b/src/test/ui/panic-handler/panic-handler-bad-signature-4.rs
index 9cda37f27b3..f4728408a1a 100644
--- a/src/test/ui/panic-handler/panic-handler-bad-signature-4.rs
+++ b/src/test/ui/panic-handler/panic-handler-bad-signature-4.rs
@@ -10,7 +10,6 @@
 
 // compile-flags:-C panic=abort
 
-#![feature(panic_handler)]
 #![no_std]
 #![no_main]
 
diff --git a/src/test/ui/panic-handler/panic-handler-bad-signature-4.stderr b/src/test/ui/panic-handler/panic-handler-bad-signature-4.stderr
index a61b9b3743c..0dd47cafd75 100644
--- a/src/test/ui/panic-handler/panic-handler-bad-signature-4.stderr
+++ b/src/test/ui/panic-handler/panic-handler-bad-signature-4.stderr
@@ -1,5 +1,5 @@
 error: should have no type parameters
-  --> $DIR/panic-handler-bad-signature-4.rs:20:1
+  --> $DIR/panic-handler-bad-signature-4.rs:19:1
    |
 LL | / fn panic<T>(pi: &PanicInfo) -> ! {
 LL | |     //~^ ERROR should have no type parameters
diff --git a/src/test/ui/panic-handler/panic-handler-duplicate.rs b/src/test/ui/panic-handler/panic-handler-duplicate.rs
index 7d7fe251869..c4fb4903e00 100644
--- a/src/test/ui/panic-handler/panic-handler-duplicate.rs
+++ b/src/test/ui/panic-handler/panic-handler-duplicate.rs
@@ -11,7 +11,6 @@
 // compile-flags:-C panic=abort
 
 #![feature(lang_items)]
-#![feature(panic_handler)]
 #![no_std]
 #![no_main]
 
diff --git a/src/test/ui/panic-handler/panic-handler-duplicate.stderr b/src/test/ui/panic-handler/panic-handler-duplicate.stderr
index d8afaa27e26..e243d9079d8 100644
--- a/src/test/ui/panic-handler/panic-handler-duplicate.stderr
+++ b/src/test/ui/panic-handler/panic-handler-duplicate.stderr
@@ -1,5 +1,5 @@
 error[E0152]: duplicate lang item found: `panic_impl`.
-  --> $DIR/panic-handler-duplicate.rs:26:1
+  --> $DIR/panic-handler-duplicate.rs:25:1
    |
 LL | / fn panic2(info: &PanicInfo) -> ! { //~ ERROR duplicate lang item found: `panic_impl`.
 LL | |     loop {}
@@ -7,7 +7,7 @@ LL | | }
    | |_^
    |
 note: first defined here.
-  --> $DIR/panic-handler-duplicate.rs:21:1
+  --> $DIR/panic-handler-duplicate.rs:20:1
    |
 LL | / fn panic(info: &PanicInfo) -> ! {
 LL | |     loop {}
diff --git a/src/test/ui/panic-handler/panic-handler-requires-panic-info.rs b/src/test/ui/panic-handler/panic-handler-requires-panic-info.rs
index ec68a414f54..133d6c0557a 100644
--- a/src/test/ui/panic-handler/panic-handler-requires-panic-info.rs
+++ b/src/test/ui/panic-handler/panic-handler-requires-panic-info.rs
@@ -13,7 +13,6 @@
 
 #![feature(lang_items)]
 #![feature(no_core)]
-#![feature(panic_handler)]
 #![no_core]
 #![no_main]
 
diff --git a/src/test/ui/panic-handler/panic-handler-std.rs b/src/test/ui/panic-handler/panic-handler-std.rs
index 7cbe0a35bae..ff5995237cf 100644
--- a/src/test/ui/panic-handler/panic-handler-std.rs
+++ b/src/test/ui/panic-handler/panic-handler-std.rs
@@ -10,7 +10,6 @@
 
 // error-pattern: duplicate lang item found: `panic_impl`.
 
-#![feature(panic_handler)]
 
 use std::panic::PanicInfo;
 
diff --git a/src/test/ui/panic-handler/panic-handler-std.stderr b/src/test/ui/panic-handler/panic-handler-std.stderr
index b141a217164..4ec1c6df355 100644
--- a/src/test/ui/panic-handler/panic-handler-std.stderr
+++ b/src/test/ui/panic-handler/panic-handler-std.stderr
@@ -1,5 +1,5 @@
 error[E0152]: duplicate lang item found: `panic_impl`.
-  --> $DIR/panic-handler-std.rs:18:1
+  --> $DIR/panic-handler-std.rs:17:1
    |
 LL | / fn panic(info: PanicInfo) -> ! {
 LL | |     loop {}
@@ -9,7 +9,7 @@ LL | | }
    = note: first defined in crate `std`.
 
 error: argument should be `&PanicInfo`
-  --> $DIR/panic-handler-std.rs:18:16
+  --> $DIR/panic-handler-std.rs:17:16
    |
 LL | fn panic(info: PanicInfo) -> ! {
    |                ^^^^^^^^^