Handle panicking like rustc CTFE does

Instead of using `core::fmt::format` to format panic messages, which may in turn
panic too and cause recursive panics and other messy things, redirect
`panic_fmt` to `const_panic_fmt` like CTFE, which in turn goes to
`panic_display` and does the things normally. See the tests for the full
call stack.

crates/hir-ty/src/mir/eval/tests.rs

@@ -31,6 +31,7 @@ fn eval_main(db: &TestDB, file_id: FileId) -> Result<(String, String), MirEvalEr
             db.trait_environment(func_id.into()),
         )
         .map_err(|e| MirEvalError::MirLowerError(func_id, e))?;
+
     let (result, output) = interpret_mir(db, body, false, None);
     result?;
     Ok((output.stdout().into_owned(), output.stderr().into_owned()))
@@ -87,6 +88,42 @@ fn main() {
     );
 }
 
+#[test]
+fn panic_fmt() {
+    // panic!
+    // -> panic_2021 (builtin macro redirection)
+    //   -> #[lang = "panic_fmt"] core::panicking::panic_fmt (hooked by CTFE for redirection)
+    //   -> core::panicking::const_panic_fmt
+    //     -> #[rustc_const_panic_str] core::panicking::panic_display (hooked by CTFE for builtin panic)
+    //       -> Err(ConstEvalError::Panic)
+    check_pass(
+        r#"
+//- minicore: fmt, panic
+fn main() {
+    panic!("hello, world!");
+}
+    "#,
+    );
+    panic!("a");
+}
+
+#[test]
+fn panic_display() {
+    // panic!
+    // -> panic_2021 (builtin macro redirection)
+    //   -> #[rustc_const_panic_str] core::panicking::panic_display (hooked by CTFE for builtin panic)
+    //     -> Err(ConstEvalError::Panic)
+    check_pass(
+        r#"
+//- minicore: fmt, panic
+
+fn main() {
+    panic!("{}", "hello, world!");
+}
+    "#,
+    );
+}
+
 #[test]
 fn drop_basic() {
     check_pass(

crates/test-utils/src/minicore.rs

@@ -1356,18 +1356,36 @@ pub mod iter {
 // region:panic
 mod panic {
     pub macro panic_2021 {
-        () => (
-            $crate::panicking::panic("explicit panic")
-        ),
-        ($($t:tt)+) => (
-            $crate::panicking::panic_fmt($crate::const_format_args!($($t)+))
-        ),
+        () => ({
+            const fn panic_cold_explicit() -> ! {
+                $crate::panicking::panic_explicit()
+            }
+            panic_cold_explicit();
+        }),
+        // Special-case the single-argument case for const_panic.
+        ("{}", $arg:expr $(,)?) => ({
+            #[rustc_const_panic_str] // enforce a &&str argument in const-check and hook this by const-eval
+            const fn panic_cold_display<T: $crate::fmt::Display>(arg: &T) -> ! {
+                loop {}
+            }
+            panic_cold_display(&$arg);
+        }),
+        ($($t:tt)+) => ({
+            // Semicolon to prevent temporaries inside the formatting machinery from
+            // being considered alive in the caller after the panic_fmt call.
+            $crate::panicking::panic_fmt($crate::const_format_args!($($t)+));
+        }),
     }
 }
 
 mod panicking {
-    #[lang = "panic_fmt"]
-    pub const fn panic_fmt(_fmt: crate::fmt::Arguments<'_>) -> ! {
+    #[rustc_const_panic_str] // enforce a &&str argument in const-check and hook this by const-eval
+    pub const fn panic_display<T: fmt::Display>(x: &T) -> ! {
+        panic_fmt(format_args!("{}", *x));
+    }
+
+    #[lang = "panic_fmt"] // needed for const-evaluated panics
+    pub const fn panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
         loop {}
     }