Clean up special function const checks

Mark them as const and `#[rustc_do_not_const_check]` instead of hard-coding
them in const-eval checks.

Author: nbdd0121

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -30,34 +30,25 @@ impl<'mir, 'tcx> InterpCx<'mir, 'tcx, CompileTimeInterpreter<'mir, 'tcx>> {
         &mut self,
         instance: ty::Instance<'tcx>,
         args: &[OpTy<'tcx>],
-        is_const_fn: bool,
     ) -> InterpResult<'tcx, Option<ty::Instance<'tcx>>> {
-        // The list of functions we handle here must be in sync with
-        // `is_lang_special_const_fn` in `transform/check_consts/mod.rs`.
+        // All `#[rustc_do_not_const_check]` functions should be hooked here.
         let def_id = instance.def_id();
 
-        if is_const_fn {
-            if Some(def_id) == self.tcx.lang_items().const_eval_select() {
-                // redirect to const_eval_select_ct
-                if let Some(const_eval_select) = self.tcx.lang_items().const_eval_select_ct() {
-                    return Ok(Some(
-                        ty::Instance::resolve(
-                            *self.tcx,
-                            ty::ParamEnv::reveal_all(),
-                            const_eval_select,
-                            instance.substs,
-                        )
-                        .unwrap()
-                        .unwrap(),
-                    ));
-                }
+        if Some(def_id) == self.tcx.lang_items().const_eval_select() {
+            // redirect to const_eval_select_ct
+            if let Some(const_eval_select) = self.tcx.lang_items().const_eval_select_ct() {
+                return Ok(Some(
+                    ty::Instance::resolve(
+                        *self.tcx,
+                        ty::ParamEnv::reveal_all(),
+                        const_eval_select,
+                        instance.substs,
+                    )
+                    .unwrap()
+                    .unwrap(),
+                ));
             }
-            return Ok(None);
-        }
-
-        if Some(def_id) == self.tcx.lang_items().panic_fn()
-            || Some(def_id) == self.tcx.lang_items().panic_str()
-            || Some(def_id) == self.tcx.lang_items().panic_display()
+        } else if Some(def_id) == self.tcx.lang_items().panic_display()
             || Some(def_id) == self.tcx.lang_items().begin_panic_fn()
         {
             // &str or &&str
@@ -286,19 +277,16 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
                 }
             }
 
-            // Some functions we support even if they are non-const -- but avoid testing
-            // that for const fn!
-            // `const_eval_select` is a const fn because it must use const trait bounds.
-            if let Some(new_instance) = ecx.hook_special_const_fn(instance, args, is_const_fn)? {
-                // We call another const fn instead.
-                return Self::find_mir_or_eval_fn(ecx, new_instance, _abi, args, _ret, _unwind);
-            }
-
             if !is_const_fn {
                 // 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_special_const_fn(instance, args)? {
+                // We call another const fn instead.
+                return Self::find_mir_or_eval_fn(ecx, new_instance, _abi, args, _ret, _unwind);
+            }
         }
         // This is a const fn. Call it.
         Ok(Some(ecx.load_mir(instance.def, None)?))

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -24,7 +24,7 @@ use std::ops::Deref;
 use super::ops::{self, NonConstOp, Status};
 use super::qualifs::{self, CustomEq, HasMutInterior, NeedsDrop, NeedsNonConstDrop};
 use super::resolver::FlowSensitiveAnalysis;
-use super::{is_lang_panic_fn, is_lang_special_const_fn, ConstCx, Qualif};
+use super::{ConstCx, Qualif};
 use crate::const_eval::is_unstable_const_fn;
 
 // We are using `MaybeMutBorrowedLocals` as a proxy for whether an item may have been mutated
@@ -918,31 +918,27 @@ impl Visitor<'tcx> for Checker<'mir, 'tcx> {
                 }
 
                 // At this point, we are calling a function, `callee`, whose `DefId` is known...
-                if is_lang_special_const_fn(tcx, callee) {
-                    // `begin_panic` and `panic_display` are generic functions that accept
-                    // types other than str. Check to enforce that only str can be used in
-                    // const-eval.
-
-                    // const-eval of the `begin_panic` fn assumes the argument is `&str`
-                    if Some(callee) == tcx.lang_items().begin_panic_fn() {
-                        match args[0].ty(&self.ccx.body.local_decls, tcx).kind() {
-                            ty::Ref(_, ty, _) if ty.is_str() => (),
-                            _ => self.check_op(ops::PanicNonStr),
-                        }
-                    }
 
-                    // const-eval of the `panic_display` fn assumes the argument is `&&str`
-                    if Some(callee) == tcx.lang_items().panic_display() {
-                        match args[0].ty(&self.ccx.body.local_decls, tcx).kind() {
-                            ty::Ref(_, ty, _) if matches!(ty.kind(), ty::Ref(_, ty, _) if ty.is_str()) =>
-                                {}
-                            _ => self.check_op(ops::PanicNonStr),
-                        }
+                // `begin_panic` and `panic_display` are generic functions that accept
+                // types other than str. Check to enforce that only str can be used in
+                // const-eval.
+
+                // const-eval of the `begin_panic` fn assumes the argument is `&str`
+                if Some(callee) == tcx.lang_items().begin_panic_fn() {
+                    match args[0].ty(&self.ccx.body.local_decls, tcx).kind() {
+                        ty::Ref(_, ty, _) if ty.is_str() => return,
+                        _ => self.check_op(ops::PanicNonStr),
                     }
+                }
 
-                    if is_lang_panic_fn(tcx, callee) {
-                        // run stability check on non-panic special const fns.
-                        return;
+                // const-eval of the `panic_display` fn assumes the argument is `&&str`
+                if Some(callee) == tcx.lang_items().panic_display() {
+                    match args[0].ty(&self.ccx.body.local_decls, tcx).kind() {
+                        ty::Ref(_, ty, _) if matches!(ty.kind(), ty::Ref(_, ty, _) if ty.is_str()) =>
+                        {
+                            return;
+                        }
+                        _ => self.check_op(ops::PanicNonStr),
                     }
                 }
 

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

@@ -72,24 +72,6 @@ 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 {
-    Some(def_id) == tcx.lang_items().panic_fn()
-        || Some(def_id) == tcx.lang_items().panic_str()
-        || Some(def_id) == tcx.lang_items().panic_display()
-        || Some(def_id) == tcx.lang_items().begin_panic_fn()
-        || Some(def_id) == tcx.lang_items().panic_fmt()
-}
-
-/// Returns `true` if this `DefId` points to one of the lang items that will be handled differently
-/// in const_eval.
-pub fn is_lang_special_const_fn(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
-    // We can allow calls to these functions because `hook_special_const_fn` in
-    // `const_eval/machine.rs` ensures the calls are handled specially.
-    // Keep in sync with what that function handles!
-    is_lang_panic_fn(tcx, def_id) || Some(def_id) == tcx.lang_items().const_eval_select()
-}
-
 pub fn rustc_allow_const_fn_unstable(
     tcx: TyCtxt<'tcx>,
     def_id: DefId,

compiler/rustc_const_eval/src/transform/promote_consts.rs

@@ -26,7 +26,7 @@ use rustc_index::vec::{Idx, IndexVec};
 use std::cell::Cell;
 use std::{cmp, iter, mem};
 
-use crate::transform::check_consts::{is_lang_special_const_fn, qualifs, ConstCx};
+use crate::transform::check_consts::{qualifs, ConstCx};
 use crate::transform::MirPass;
 
 /// A `MirPass` for promotion.
@@ -656,9 +656,7 @@ impl<'tcx> Validator<'_, 'tcx> {
         }
 
         let is_const_fn = match *fn_ty.kind() {
-            ty::FnDef(def_id, _) => {
-                self.tcx.is_const_fn_raw(def_id) || is_lang_special_const_fn(self.tcx, def_id)
-            }
+            ty::FnDef(def_id, _) => self.tcx.is_const_fn_raw(def_id),
             _ => false,
         };
         if !is_const_fn {

library/core/src/lib.rs

@@ -107,6 +107,7 @@
 #![feature(const_discriminant)]
 #![feature(const_float_bits_conv)]
 #![feature(const_float_classify)]
+#![feature(const_fmt_arguments_new)]
 #![feature(const_heap)]
 #![feature(const_inherent_unchecked_arith)]
 #![feature(const_int_unchecked_arith)]

library/core/src/panicking.rs

@@ -34,13 +34,10 @@ use crate::panic::{Location, PanicInfo};
 // never inline unless panic_immediate_abort to avoid code
 // bloat at the call sites as much as possible
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
 #[lang = "panic"] // needed by codegen for panic on overflow and other `Assert` MIR terminators
-pub fn panic(expr: &'static str) -> ! {
-    if cfg!(feature = "panic_immediate_abort") {
-        super::intrinsics::abort()
-    }
-
+pub const fn panic(expr: &'static str) -> ! {
     // Use Arguments::new_v1 instead of format_args!("{}", expr) to potentially
     // reduce size overhead. The format_args! macro uses str's Display trait to
     // write expr, which calls Formatter::pad, which must accommodate string
@@ -52,15 +49,16 @@ pub fn panic(expr: &'static str) -> ! {
 
 #[inline]
 #[track_caller]
-#[lang = "panic_str"] // needed for const-evaluated panics
-pub fn panic_str(expr: &str) -> ! {
-    panic_fmt(format_args!("{}", expr));
+#[lang = "panic_str"] // needed for `non-fmt-panics` lint
+pub const fn panic_str(expr: &str) -> ! {
+    panic_display(&expr);
 }
 
 #[inline]
 #[track_caller]
 #[lang = "panic_display"] // needed for const-evaluated panics
-pub fn panic_display<T: fmt::Display>(x: &T) -> ! {
+#[rustc_do_not_const_check] // hooked by const-eval
+pub const fn panic_display<T: fmt::Display>(x: &T) -> ! {
     panic_fmt(format_args!("{}", *x));
 }
 
@@ -89,7 +87,8 @@ fn panic_bounds_check(index: usize, len: usize) -> ! {
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
 #[lang = "panic_fmt"] // needed for const-evaluated panics
-pub fn panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
+#[rustc_do_not_const_check] // hooked by const-eval
+pub const fn panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
     if cfg!(feature = "panic_immediate_abort") {
         super::intrinsics::abort()
     }

library/std/src/lib.rs

@@ -257,6 +257,7 @@
 #![feature(const_cstr_unchecked)]
 #![feature(const_fn_floating_point_arithmetic)]
 #![feature(const_fn_fn_ptr_basics)]
+#![feature(const_fn_trait_bound)]
 #![feature(const_format_args)]
 #![feature(const_io_structs)]
 #![feature(const_ip)]

library/std/src/panicking.rs

@@ -512,7 +512,8 @@ pub fn begin_panic_handler(info: &PanicInfo<'_>) -> ! {
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[cold]
 #[track_caller]
-pub fn begin_panic<M: Any + Send>(msg: M) -> ! {
+#[rustc_do_not_const_check] // hooked by const-eval
+pub const fn begin_panic<M: Any + Send>(msg: M) -> ! {
     if cfg!(feature = "panic_immediate_abort") {
         intrinsics::abort()
     }