Auto merge of rust-lang#93179 - Urgau:unreachable-2021, r=m-ou-se,oli-obk

Fix invalid special casing of the unreachable! macro

This pull-request fix an invalid special casing of the `unreachable!` macro in the same way the `panic!` macro was solved, by adding two new internal only macros `unreachable_2015` and `unreachable_2021` edition dependent and turn `unreachable!` into a built-in macro that do dispatching. This logic is stolen from the `panic!` macro.

~~This pull-request also adds an internal feature `format_args_capture_non_literal` that allows capturing arguments from formatted string that expanded from macros. The original RFC rust-lang#2795 mentioned this as a future possibility. This feature is [required](rust-lang#92137 (comment)) because of concatenation that needs to be done inside the macro:~~
```rust
$crate::concat!("internal error: entered unreachable code: ", $fmt)
```

**In summary** the new behavior for the `unreachable!` macro with this pr is:

Edition 2021:
```rust
let x = 5;
unreachable!("x is {x}");
```
```
internal error: entered unreachable code: x is 5
```

Edition <= 2018:
```rust
let x = 5;
unreachable!("x is {x}");
```
```
internal error: entered unreachable code: x is {x}
```

Also note that the change in this PR are **insta-stable** and **breaking changes** but this a considered as being a [bug](rust-lang#92137 (comment)).
If someone could start a perf run and then a crater run this would be appreciated.

Fixes rust-lang#92137

Author: bors

core/src/macros/mod.rs

@@ -594,6 +594,22 @@ macro_rules! writeln {
 ///     unreachable!("The loop should always return");
 /// }
 /// ```
+#[cfg(not(bootstrap))]
+#[macro_export]
+#[rustc_builtin_macro(unreachable)]
+#[allow_internal_unstable(edition_panic)]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(test), rustc_diagnostic_item = "unreachable_macro")]
+macro_rules! unreachable {
+    // Expands to either `$crate::panic::unreachable_2015` or `$crate::panic::unreachable_2021`
+    // depending on the edition of the caller.
+    ($($arg:tt)*) => {
+        /* compiler built-in */
+    };
+}
+
+/// unreachable!() macro
+#[cfg(bootstrap)]
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "unreachable_macro")]

core/src/panic.rs

@@ -58,6 +58,39 @@ pub macro panic_2021 {
     ),
 }
 
+#[doc(hidden)]
+#[unstable(feature = "edition_panic", issue = "none", reason = "use unreachable!() instead")]
+#[allow_internal_unstable(core_panic)]
+#[rustc_diagnostic_item = "unreachable_2015_macro"]
+#[rustc_macro_transparency = "semitransparent"]
+pub macro unreachable_2015 {
+    () => (
+        $crate::panicking::panic("internal error: entered unreachable code")
+    ),
+    // Use of `unreachable_display` for non_fmt_panic lint.
+    // NOTE: the message ("internal error ...") is embeded directly in unreachable_display
+    ($msg:expr $(,)?) => (
+        $crate::panicking::unreachable_display(&$msg)
+    ),
+    ($fmt:expr, $($arg:tt)*) => (
+        $crate::panic!($crate::concat!("internal error: entered unreachable code: ", $fmt), $($arg)*)
+    ),
+}
+
+#[doc(hidden)]
+#[unstable(feature = "edition_panic", issue = "none", reason = "use unreachable!() instead")]
+#[allow_internal_unstable(core_panic)]
+#[rustc_diagnostic_item = "unreachable_2021_macro"]
+#[rustc_macro_transparency = "semitransparent"]
+pub macro unreachable_2021 {
+    () => (
+        $crate::panicking::panic("internal error: entered unreachable code")
+    ),
+    ($($t:tt)+) => (
+        $crate::panic!("internal error: entered unreachable code: {}", $crate::format_args!($($t)+))
+    ),
+}
+
 /// An internal trait used by libstd to pass data from libstd to `panic_unwind`
 /// and other panic runtimes. Not intended to be stabilized any time soon, do
 /// not use.

core/src/panicking.rs

@@ -50,12 +50,20 @@ pub const fn panic(expr: &'static str) -> ! {
 
 #[inline]
 #[track_caller]
-#[lang = "panic_str"] // needed for `non-fmt-panics` lint
+#[rustc_diagnostic_item = "panic_str"]
 #[rustc_const_unstable(feature = "core_panic", issue = "none")]
 pub const fn panic_str(expr: &str) -> ! {
     panic_display(&expr);
 }
 
+#[cfg(not(bootstrap))]
+#[inline]
+#[track_caller]
+#[rustc_diagnostic_item = "unreachable_display"] // needed for `non-fmt-panics` lint
+pub fn unreachable_display<T: fmt::Display>(x: &T) -> ! {
+    panic_fmt(format_args!("internal error: entered unreachable code: {}", *x));
+}
+
 #[inline]
 #[track_caller]
 #[lang = "panic_display"] // needed for const-evaluated panics