Auto merge of #86998 - m-ou-se:const-panic-fmt-as-str, r=oli-obk

Make const panic!("..") work in Rust 2021.

During const eval, this replaces calls to core::panicking::panic_fmt and std::panicking::being_panic_fmt with a call to a new const fn: core::panicking::const_panic_fmt. That function uses fmt::Arguments::as_str() to get the str and calls panic_str with that instead.

panic!() invocations with formatting arguments are still not accepted, as the creation of such a fmt::Arguments cannot be done in constant functions right now.

r? `@RalfJung`

compiler/rustc_mir/src/const_eval/machine.rs

@@ -30,7 +30,9 @@ impl<'mir, 'tcx> InterpCx<'mir, 'tcx, CompileTimeInterpreter<'mir, 'tcx>> {
         &mut self,
         instance: ty::Instance<'tcx>,
         args: &[OpTy<'tcx>],
-    ) -> InterpResult<'tcx> {
+    ) -> InterpResult<'tcx, Option<ty::Instance<'tcx>>> {
+        // The list of functions we handle here must be in sync with
+        // `is_lang_panic_fn` in `transform/check_consts/mod.rs`.
         let def_id = instance.def_id();
         if Some(def_id) == self.tcx.lang_items().panic_fn()
             || Some(def_id) == self.tcx.lang_items().panic_str()
@@ -43,10 +45,25 @@ impl<'mir, 'tcx> InterpCx<'mir, 'tcx, CompileTimeInterpreter<'mir, 'tcx>> {
             let msg = Symbol::intern(self.read_str(&msg_place)?);
             let span = self.find_closest_untracked_caller_location();
             let (file, line, col) = self.location_triple_for_span(span);
-            Err(ConstEvalErrKind::Panic { msg, file, line, col }.into())
-        } else {
-            Ok(())
+            return Err(ConstEvalErrKind::Panic { msg, file, line, col }.into());
+        } else if Some(def_id) == self.tcx.lang_items().panic_fmt()
+            || Some(def_id) == self.tcx.lang_items().begin_panic_fmt()
+        {
+            // For panic_fmt, call const_panic_fmt instead.
+            if let Some(const_panic_fmt) = self.tcx.lang_items().const_panic_fmt() {
+                return Ok(Some(
+                    ty::Instance::resolve(
+                        *self.tcx,
+                        ty::ParamEnv::reveal_all(),
+                        const_panic_fmt,
+                        self.tcx.intern_substs(&[]),
+                    )
+                    .unwrap()
+                    .unwrap(),
+                ));
+            }
         }
+        Ok(None)
     }
 }
 
@@ -241,10 +258,21 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
                 if !ecx.tcx.has_attr(def.did, sym::default_method_body_is_const) {
                     // Some functions we support even if they are non-const -- but avoid testing
                     // that for const fn!
-                    ecx.hook_panic_fn(instance, args)?;
-                    // We certainly do *not* want to actually call the fn
-                    // though, so be sure we return here.
-                    throw_unsup_format!("calling non-const function `{}`", instance)
+                    if let Some(new_instance) = ecx.hook_panic_fn(instance, args)? {
+                        // We call another const fn instead.
+                        return Self::find_mir_or_eval_fn(
+                            ecx,
+                            new_instance,
+                            _abi,
+                            args,
+                            _ret,
+                            _unwind,
+                        );
+                    } else {
+                        // We certainly do *not* want to actually call the fn
+                        // though, so be sure we return here.
+                        throw_unsup_format!("calling non-const function `{}`", instance)
+                    }
                 }
             }
         }

compiler/rustc_mir/src/transform/check_consts/mod.rs

@@ -74,9 +74,14 @@ impl ConstCx<'mir, 'tcx> {
 
 /// Returns `true` if this `DefId` points to one of the official `panic` lang items.
 pub fn is_lang_panic_fn(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
+    // We can allow calls to these functions because `hook_panic_fn` in
+    // `const_eval/machine.rs` ensures the calls are handled specially.
+    // Keep in sync with what that function handles!
     Some(def_id) == tcx.lang_items().panic_fn()
         || Some(def_id) == tcx.lang_items().panic_str()
         || Some(def_id) == tcx.lang_items().begin_panic_fn()
+        || Some(def_id) == tcx.lang_items().panic_fmt()
+        || Some(def_id) == tcx.lang_items().begin_panic_fmt()
 }
 
 pub fn rustc_allow_const_fn_unstable(