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.rs

@@ -2317,7 +2317,7 @@ impl Evaluator<'_> {
 
     fn exec_fn_with_args(
         &mut self,
-        def: FunctionId,
+        mut def: FunctionId,
         args: &[IntervalAndTy],
         generic_args: Substitution,
         locals: &Locals,
@@ -2335,6 +2335,9 @@ impl Evaluator<'_> {
         )? {
             return Ok(None);
         }
+        if let Some(redirect_def) = self.detect_and_redirect_special_function(def)? {
+            def = redirect_def;
+        }
         let arg_bytes = args.iter().map(|it| IntervalOrOwned::Borrowed(it.interval));
         match self.get_mir_or_dyn_index(def, generic_args.clone(), locals, span)? {
             MirOrDynIndex::Dyn(self_ty_idx) => {

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

@@ -158,6 +158,25 @@ impl Evaluator<'_> {
         Ok(false)
     }
 
+    pub(super) fn detect_and_redirect_special_function(
+        &mut self,
+        def: FunctionId,
+    ) -> Result<Option<FunctionId>> {
+        // `PanicFmt` is redirected to `ConstPanicFmt`
+        if let Some(LangItem::PanicFmt) = self.db.lang_attr(def.into()) {
+            let resolver =
+                self.db.crate_def_map(self.crate_id).crate_root().resolver(self.db.upcast());
+
+            let Some(hir_def::lang_item::LangItemTarget::Function(const_panic_fmt)) =
+                self.db.lang_item(resolver.krate(), LangItem::ConstPanicFmt)
+            else {
+                not_supported!("const_panic_fmt lang item not found or not a function");
+            };
+            return Ok(Some(const_panic_fmt));
+        }
+        Ok(None)
+    }
+
     /// Clone has special impls for tuples and function pointers
     fn exec_clone(
         &mut self,
@@ -291,9 +310,14 @@ impl Evaluator<'_> {
         use LangItem::*;
         let candidate = self.db.lang_attr(def.into())?;
         // We want to execute these functions with special logic
-        if [PanicFmt, BeginPanic, SliceLen, DropInPlace].contains(&candidate) {
+        // `PanicFmt` is not detected here as it's redirected later.
+        if [BeginPanic, SliceLen, DropInPlace].contains(&candidate) {
             return Some(candidate);
         }
+        if self.db.attrs(def.into()).by_key("rustc_const_panic_str").exists() {
+            // `#[rustc_const_panic_str]` is treated like `lang = "begin_panic"` by rustc CTFE.
+            return Some(LangItem::BeginPanic);
+        }
         None
     }
 
@@ -309,43 +333,6 @@ impl Evaluator<'_> {
         let mut args = args.iter();
         match it {
             BeginPanic => Err(MirEvalError::Panic("<unknown-panic-payload>".to_owned())),
-            PanicFmt => {
-                let message = (|| {
-                    let resolver = self
-                        .db
-                        .crate_def_map(self.crate_id)
-                        .crate_root()
-                        .resolver(self.db.upcast());
-                    let Some(format_fn) = resolver.resolve_path_in_value_ns_fully(
-                        self.db.upcast(),
-                        &hir_def::path::Path::from_known_path_with_no_generic(
-                            ModPath::from_segments(
-                                hir_expand::mod_path::PathKind::Abs,
-                                [name![std], name![fmt], name![format]],
-                            ),
-                        ),
-                    ) else {
-                        not_supported!("std::fmt::format not found");
-                    };
-                    let hir_def::resolver::ValueNs::FunctionId(format_fn) = format_fn else {
-                        not_supported!("std::fmt::format is not a function")
-                    };
-                    let interval = self.interpret_mir(
-                        self.db
-                            .mir_body(format_fn.into())
-                            .map_err(|e| MirEvalError::MirLowerError(format_fn, e))?,
-                        args.map(|x| IntervalOrOwned::Owned(x.clone())),
-                    )?;
-                    let message_string = interval.get(self)?;
-                    let addr =
-                        Address::from_bytes(&message_string[self.ptr_size()..2 * self.ptr_size()])?;
-                    let size = from_bytes!(usize, message_string[2 * self.ptr_size()..]);
-                    Ok(std::string::String::from_utf8_lossy(self.read_memory(addr, size)?)
-                        .into_owned())
-                })()
-                .unwrap_or_else(|e| format!("Failed to render panic format args: {e:?}"));
-                Err(MirEvalError::Panic(message))
-            }
             SliceLen => {
                 let arg = args.next().ok_or(MirEvalError::InternalError(
                     "argument of <[T]>::len() is not provided".into(),

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 {}
     }