Rollup merge of rust-lang#51366 - japaric:stable-panic-impl, r=Mark-Simulacrum

stabilize #[panic_handler]

closes rust-lang#44489

### Update(2018-09-07)

This was proposed for stabilization in rust-lang#44489 (comment) and its FCP with disposition to merge / accept is nearly over. The summary of what's being stabilized can be found in rust-lang#44489 (comment)

Documentation PRs:

- Reference. rust-lang/reference#362
- Nomicon. rust-lang/nomicon#75

---

`#[panic_implementation]` was implemented recently in rust-lang#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. rust-lang#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 rust-lang#43054

Some unresolved questions from rust-lang#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 rust-lang#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

Author: kennytm

src/libstd/lib.rs

@@ -312,7 +312,6 @@
 #![feature(doc_keyword)]
 #![feature(panic_info_message)]
 #![cfg_attr(stage0, feature(panic_implementation))]
-#![cfg_attr(not(stage0), feature(panic_handler))]
 #![feature(non_exhaustive)]
 
 #![default_lib_allocator]

src/libsyntax/feature_gate.rs

@@ -487,7 +487,6 @@ declare_features! (
 
     // RFC 2070: #[panic_implementation] / #[panic_handler]
     (active, panic_implementation, "1.28.0", Some(44489), None),
-    (active, panic_handler, "1.30.0", Some(44489), None),
 
     // #[doc(keyword = "...")]
     (active, doc_keyword, "1.28.0", Some(51315), None),
@@ -674,6 +673,7 @@ declare_features! (
     (accepted, proc_macro_path_invoc, "1.30.0", Some(38356), None),
     // Allows all literals in attribute lists and values of key-value pairs.
     (accepted, attr_literals, "1.30.0", Some(34981), None),
+    (accepted, panic_handler, "1.30.0", Some(44489), None),
 );
 
 // If you change this, please modify src/doc/unstable-book as well. You must
@@ -1143,10 +1143,7 @@ pub const BUILTIN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeG
            cfg_fn!(panic_implementation))),
 
     // RFC 2070
-    ("panic_handler", Normal, Gated(Stability::Unstable,
-                                    "panic_handler",
-                                    "#[panic_handler] is an unstable feature",
-                                    cfg_fn!(panic_handler))),
+    ("panic_handler", Normal, Ungated),
 
     ("alloc_error_handler", Normal, Gated(Stability::Unstable,
                            "alloc_error_handler",

src/test/compile-fail/auxiliary/some-panic-impl.rs

@@ -11,7 +11,6 @@
 // no-prefer-dynamic
 
 #![crate_type = "rlib"]
-#![feature(panic_handler)]
 #![no_std]
 
 use core::panic::PanicInfo;

src/test/compile-fail/panic-handler-twice.rs

@@ -10,7 +10,6 @@
 
 // aux-build:some-panic-impl.rs
 
-#![feature(panic_handler)]
 #![feature(lang_items)]
 #![no_std]
 #![no_main]

src/test/run-make-fulldeps/issue-51671/app.rs

@@ -10,7 +10,6 @@
 
 #![crate_type = "bin"]
 #![feature(lang_items)]
-#![feature(panic_handler)]
 #![no_main]
 #![no_std]
 

src/test/run-make-fulldeps/panic-impl-transitive/panic-impl-provider.rs

@@ -9,7 +9,6 @@
 // except according to those terms.
 
 #![crate_type = "rlib"]
-#![feature(panic_handler)]
 #![no_std]
 
 use core::panic::PanicInfo;

src/test/ui/consts/const-eval/const_panic_libcore_main.rs

@@ -10,7 +10,6 @@
 
 #![crate_type = "bin"]
 #![feature(lang_items)]
-#![feature(panic_handler)]
 #![feature(const_panic)]
 #![no_main]
 #![no_std]

src/test/ui/consts/const-eval/const_panic_libcore_main.stderr

@@ -1,31 +1,31 @@
 error: this constant cannot be used
-  --> $DIR/const_panic_libcore_main.rs:20:1
+  --> $DIR/const_panic_libcore_main.rs:19:1
    |
 LL | const Z: () = panic!("cheese");
    | ^^^^^^^^^^^^^^----------------^
    |               |
-   |               the evaluated program panicked at 'cheese', $DIR/const_panic_libcore_main.rs:20:15
+   |               the evaluated program panicked at 'cheese', $DIR/const_panic_libcore_main.rs:19:15
    |
    = note: #[deny(const_err)] on by default
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: this constant cannot be used
-  --> $DIR/const_panic_libcore_main.rs:23:1
+  --> $DIR/const_panic_libcore_main.rs:22:1
    |
 LL | const Y: () = unreachable!();
    | ^^^^^^^^^^^^^^--------------^
    |               |
-   |               the evaluated program panicked at 'internal error: entered unreachable code', $DIR/const_panic_libcore_main.rs:23:15
+   |               the evaluated program panicked at 'internal error: entered unreachable code', $DIR/const_panic_libcore_main.rs:22:15
    |
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: this constant cannot be used
-  --> $DIR/const_panic_libcore_main.rs:26:1
+  --> $DIR/const_panic_libcore_main.rs:25:1
    |
 LL | const X: () = unimplemented!();
    | ^^^^^^^^^^^^^^----------------^
    |               |
-   |               the evaluated program panicked at 'not yet implemented', $DIR/const_panic_libcore_main.rs:26:15
+   |               the evaluated program panicked at 'not yet implemented', $DIR/const_panic_libcore_main.rs:25:15
    |
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 

src/test/ui/feature-gates/feature-gate-panic-handler.rs

@@ -11,7 +11,6 @@
 // no-prefer-dynamic
 
 #![crate_type = "rlib"]
-#![feature(panic_handler)]
 #![no_std]
 
 use core::panic::PanicInfo;

src/test/ui/feature-gates/feature-gate-panic-handler.stderr

@@ -10,7 +10,6 @@
 
 // compile-flags:-C panic=abort
 
-#![feature(panic_handler)]
 #![no_std]
 #![no_main]
 

src/test/ui/panic-handler/auxiliary/some-panic-impl.rs

@@ -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`
    |           ^^^^^^^^^

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]
 

src/test/ui/panic-handler/panic-handler-bad-signature-1.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`
    |           ^^^^^^^^^^^^^^^^^^

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]
 

src/test/ui/panic-handler/panic-handler-bad-signature-2.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
    | ^^^^^^^^^^^^^^^

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]
 

src/test/ui/panic-handler/panic-handler-bad-signature-3.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

src/test/ui/panic-handler/panic-handler-bad-signature-4.rs

@@ -11,7 +11,6 @@
 // compile-flags:-C panic=abort
 
 #![feature(lang_items)]
-#![feature(panic_handler)]
 #![no_std]
 #![no_main]
 

src/test/ui/panic-handler/panic-handler-bad-signature-4.stderr

@@ -1,13 +1,13 @@
 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 {}
 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 {}

src/test/ui/panic-handler/panic-handler-duplicate.rs

@@ -13,7 +13,6 @@
 
 #![feature(lang_items)]
 #![feature(no_core)]
-#![feature(panic_handler)]
 #![no_core]
 #![no_main]
 

src/test/ui/panic-handler/panic-handler-duplicate.stderr

@@ -10,7 +10,6 @@
 
 // error-pattern: duplicate lang item found: `panic_impl`.
 
-#![feature(panic_handler)]
 
 use std::panic::PanicInfo;
 

src/test/ui/panic-handler/panic-handler-requires-panic-info.rs

@@ -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) -> ! {
    |                ^^^^^^^^^

src/test/ui/panic-handler/panic-handler-std.rs

@@ -12,7 +12,6 @@
 
 #![crate_type = "rlib"]
 #![no_std]
-#![feature(panic_handler)]
 
 #[panic_handler]
 pub fn panic_fmt(_: &::core::panic::PanicInfo) -> ! {