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.

Author: m-ou-se

compiler/rustc_hir/src/lang_items.rs

@@ -276,13 +276,16 @@ language_item_table! {
     // is required to define it somewhere. Additionally, there are restrictions on crates that use
     // a weak lang item, but do not have it defined.
     Panic,                   sym::panic,               panic_fn,                   Target::Fn;
+    PanicFmt,                sym::panic_fmt,           panic_fmt,                  Target::Fn;
     PanicStr,                sym::panic_str,           panic_str,                  Target::Fn;
+    ConstPanicFmt,           sym::const_panic_fmt,     const_panic_fmt,            Target::Fn;
     PanicBoundsCheck,        sym::panic_bounds_check,  panic_bounds_check_fn,      Target::Fn;
     PanicInfo,               sym::panic_info,          panic_info,                 Target::Struct;
     PanicLocation,           sym::panic_location,      panic_location,             Target::Struct;
     PanicImpl,               sym::panic_impl,          panic_impl,                 Target::Fn;
     /// libstd panic entry point. Necessary for const eval to be able to catch it
     BeginPanic,              sym::begin_panic,         begin_panic_fn,             Target::Fn;
+    BeginPanicFmt,           sym::begin_panic_fmt,     begin_panic_fmt,            Target::Fn;
 
     ExchangeMalloc,          sym::exchange_malloc,     exchange_malloc_fn,         Target::Fn;
     BoxFree,                 sym::box_free,            box_free_fn,                Target::Fn;

compiler/rustc_mir/src/const_eval/machine.rs

@@ -30,7 +30,7 @@ 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>>> {
         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 +43,25 @@ 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)
     }
 }
 
@@ -223,7 +238,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
 
     fn find_mir_or_eval_fn(
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
-        instance: ty::Instance<'tcx>,
+        mut instance: ty::Instance<'tcx>,
         _abi: Abi,
         args: &[OpTy<'tcx>],
         _ret: Option<(&PlaceTy<'tcx>, mir::BasicBlock)>,
@@ -241,10 +256,14 @@ 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.
+                        instance = new_instance;
+                    } 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

@@ -77,6 +77,8 @@ pub fn is_lang_panic_fn(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
     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(

compiler/rustc_span/src/symbol.rs

@@ -323,6 +323,7 @@ symbols! {
         await_macro,
         bang,
         begin_panic,
+        begin_panic_fmt,
         bench,
         bin,
         bind_by_move_pattern_guards,
@@ -420,6 +421,7 @@ symbols! {
         const_loop,
         const_mut_refs,
         const_panic,
+        const_panic_fmt,
         const_precise_live_drops,
         const_ptr,
         const_raw_ptr_deref,
@@ -586,6 +588,7 @@ symbols! {
         fmaf32,
         fmaf64,
         fmt,
+        fmt_as_str,
         fmt_internals,
         fmul_fast,
         fn_align,
@@ -881,6 +884,7 @@ symbols! {
         panic_2021,
         panic_abort,
         panic_bounds_check,
+        panic_fmt,
         panic_handler,
         panic_impl,
         panic_implementation,

library/core/src/fmt/mod.rs

@@ -337,7 +337,7 @@ impl<'a> Arguments<'a> {
     #[doc(hidden)]
     #[inline]
     #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
-    pub fn new_v1(pieces: &'a [&'static str], args: &'a [ArgumentV1<'a>]) -> Arguments<'a> {
+    pub const fn new_v1(pieces: &'a [&'static str], args: &'a [ArgumentV1<'a>]) -> Arguments<'a> {
         Arguments { pieces, fmt: None, args }
     }
 
@@ -350,7 +350,7 @@ impl<'a> Arguments<'a> {
     #[doc(hidden)]
     #[inline]
     #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
-    pub fn new_v1_formatted(
+    pub const fn new_v1_formatted(
         pieces: &'a [&'static str],
         args: &'a [ArgumentV1<'a>],
         fmt: &'a [rt::v1::Argument],

library/core/src/lib.rs

@@ -73,6 +73,7 @@
 #![feature(cfg_target_has_atomic)]
 #![feature(const_heap)]
 #![feature(const_alloc_layout)]
+#![feature(const_arguments_as_str)]
 #![feature(const_assert_type)]
 #![feature(const_discriminant)]
 #![feature(const_cell_into_inner)]

library/core/src/panicking.rs

@@ -74,6 +74,7 @@ fn panic_bounds_check(index: usize, len: usize) -> ! {
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
+#[cfg_attr(not(bootstrap), lang = "panic_fmt")] // needed for const-evaluated panics
 pub fn panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
     if cfg!(feature = "panic_immediate_abort") {
         super::intrinsics::abort()
@@ -92,6 +93,17 @@ pub fn panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
     unsafe { panic_impl(&pi) }
 }
 
+/// This function is used instead of panic_fmt in const eval.
+#[cfg(not(bootstrap))]
+#[lang = "const_panic_fmt"]
+pub const fn const_panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
+    if let Some(msg) = fmt.as_str() {
+        panic_str(msg);
+    } else {
+        panic_str("???");
+    }
+}
+
 #[derive(Debug)]
 #[doc(hidden)]
 pub enum AssertKind {

library/std/src/panicking.rs

@@ -448,6 +448,7 @@ pub fn panicking() -> bool {
 #[cfg_attr(not(feature = "panic_immediate_abort"), track_caller)]
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
+#[cfg_attr(all(not(bootstrap), not(test)), lang = "begin_panic_fmt")]
 pub fn begin_panic_fmt(msg: &fmt::Arguments<'_>) -> ! {
     if cfg!(feature = "panic_immediate_abort") {
         intrinsics::abort()