Require const stability attributes on intrinsics to be able to use them in constant contexts

src/libcore/intrinsics.rs

@@ -1,6 +1,22 @@
 //! Compiler intrinsics.
 //!
 //! The corresponding definitions are in `librustc_codegen_llvm/intrinsic.rs`.
+//! The corresponding const implementations are in `librustc_mir/interpret/intrinsics.rs`
+//!
+//! # Const intrinsics
+//!
+//! Note: any changes to the constness of intrinsics should be discussed with the language team.
+//! This includes changes in the stability of the constness.
+//!
+//! In order to make an intrinsic usable at compile-time, one needs to copy the implementation
+//! from https://github.com/rust-lang/miri/blob/master/src/shims/intrinsics.rs to
+//! `librustc_mir/interpret/intrinsics.rs` and add a
+//! `#[rustc_const_unstable(feature = "foo", issue = "01234")]` to the intrinsic.
+//!
+//! If an intrinsic is supposed to be used from a `const fn` with a `rustc_const_stable` attribute,
+//! the intrinsic's attribute must be `rustc_const_stable`, too. Such a change should not be done
+//! without T-lang consulation, because it bakes a feature into the language that cannot be
+//! replicated in user code without compiler support.
 //!
 //! # Volatiles
 //!
@@ -667,14 +683,17 @@ extern "rust-intrinsic" {
     ///
     /// The stabilized version of this intrinsic is
     /// [`std::mem::size_of`](../../std/mem/fn.size_of.html).
+    #[rustc_const_stable(feature = "const_size_of", since = "1.40.0")]
     pub fn size_of<T>() -> usize;
 
     /// Moves a value to an uninitialized memory location.
     ///
     /// Drop glue is not run on the destination.
     pub fn move_val_init<T>(dst: *mut T, src: T);
 
+    #[rustc_const_stable(feature = "const_min_align_of", since = "1.40.0")]
     pub fn min_align_of<T>() -> usize;
+    #[rustc_const_unstable(feature = "const_pref_align_of", issue = "0")]
     pub fn pref_align_of<T>() -> usize;
 
     /// The size of the referenced value in bytes.
@@ -685,18 +704,21 @@ extern "rust-intrinsic" {
     pub fn min_align_of_val<T: ?Sized>(_: &T) -> usize;
 
     /// Gets a static string slice containing the name of a type.
+    #[rustc_const_unstable(feature = "const_type_name", issue = "0")]
     pub fn type_name<T: ?Sized>() -> &'static str;
 
     /// Gets an identifier which is globally unique to the specified type. This
     /// function will return the same value for a type regardless of whichever
     /// crate it is invoked in.
+    #[rustc_const_unstable(feature = "const_type_id", issue = "0")]
     pub fn type_id<T: ?Sized + 'static>() -> u64;
 
     /// A guard for unsafe functions that cannot ever be executed if `T` is uninhabited:
     /// This will statically either panic, or do nothing.
     pub fn panic_if_uninhabited<T>();
 
     /// Gets a reference to a static `Location` indicating where it was called.
+    #[rustc_const_unstable(feature = "const_caller_location", issue = "47809")]
     pub fn caller_location() -> &'static crate::panic::Location<'static>;
 
     /// Creates a value initialized to zero.
@@ -951,6 +973,7 @@ extern "rust-intrinsic" {
     ///
     /// The stabilized version of this intrinsic is
     /// [`std::mem::needs_drop`](../../std/mem/fn.needs_drop.html).
+    #[rustc_const_stable(feature = "const_needs_drop", since = "1.40.0")]
     pub fn needs_drop<T>() -> bool;
 
     /// Calculates the offset from a pointer.
@@ -1150,6 +1173,7 @@ extern "rust-intrinsic" {
 
 
     /// Returns the number of bits set in an integer type `T`
+    #[rustc_const_stable(feature = "const_ctpop", since = "1.40.0")]
     pub fn ctpop<T>(x: T) -> T;
 
     /// Returns the number of leading unset bits (zeroes) in an integer type `T`.
@@ -1177,6 +1201,7 @@ extern "rust-intrinsic" {
     /// let num_leading = ctlz(x);
     /// assert_eq!(num_leading, 16);
     /// ```
+    #[rustc_const_stable(feature = "const_ctlz", since = "1.40.0")]
     pub fn ctlz<T>(x: T) -> T;
 
     /// Like `ctlz`, but extra-unsafe as it returns `undef` when
@@ -1193,6 +1218,7 @@ extern "rust-intrinsic" {
     /// let num_leading = unsafe { ctlz_nonzero(x) };
     /// assert_eq!(num_leading, 3);
     /// ```
+    #[rustc_const_unstable(feature = "constctlz", issue = "0")]
     pub fn ctlz_nonzero<T>(x: T) -> T;
 
     /// Returns the number of trailing unset bits (zeroes) in an integer type `T`.
@@ -1220,6 +1246,7 @@ extern "rust-intrinsic" {
     /// let num_trailing = cttz(x);
     /// assert_eq!(num_trailing, 16);
     /// ```
+    #[rustc_const_stable(feature = "const_cttz", since = "1.40.0")]
     pub fn cttz<T>(x: T) -> T;
 
     /// Like `cttz`, but extra-unsafe as it returns `undef` when
@@ -1236,30 +1263,36 @@ extern "rust-intrinsic" {
     /// let num_trailing = unsafe { cttz_nonzero(x) };
     /// assert_eq!(num_trailing, 3);
     /// ```
+    #[rustc_const_unstable(feature = "const_cttz", issue = "0")]
     pub fn cttz_nonzero<T>(x: T) -> T;
 
     /// Reverses the bytes in an integer type `T`.
+    #[rustc_const_stable(feature = "const_bswap", since = "1.40.0")]
     pub fn bswap<T>(x: T) -> T;
 
     /// Reverses the bits in an integer type `T`.
+    #[rustc_const_stable(feature = "const_bitreverse", since = "1.40.0")]
     pub fn bitreverse<T>(x: T) -> T;
 
     /// Performs checked integer addition.
     /// The stabilized versions of this intrinsic are available on the integer
     /// primitives via the `overflowing_add` method. For example,
     /// [`std::u32::overflowing_add`](../../std/primitive.u32.html#method.overflowing_add)
+    #[rustc_const_stable(feature = "const_int_overflow", since = "1.40.0")]
     pub fn add_with_overflow<T>(x: T, y: T) -> (T, bool);
 
     /// Performs checked integer subtraction
     /// The stabilized versions of this intrinsic are available on the integer
     /// primitives via the `overflowing_sub` method. For example,
     /// [`std::u32::overflowing_sub`](../../std/primitive.u32.html#method.overflowing_sub)
+    #[rustc_const_stable(feature = "const_int_overflow", since = "1.40.0")]
     pub fn sub_with_overflow<T>(x: T, y: T) -> (T, bool);
 
     /// Performs checked integer multiplication
     /// The stabilized versions of this intrinsic are available on the integer
     /// primitives via the `overflowing_mul` method. For example,
     /// [`std::u32::overflowing_mul`](../../std/primitive.u32.html#method.overflowing_mul)
+    #[rustc_const_stable(feature = "const_int_overflow", since = "1.40.0")]
     pub fn mul_with_overflow<T>(x: T, y: T) -> (T, bool);
 
     /// Performs an exact division, resulting in undefined behavior where
@@ -1275,9 +1308,11 @@ extern "rust-intrinsic" {
 
     /// Performs an unchecked left shift, resulting in undefined behavior when
     /// y < 0 or y >= N, where N is the width of T in bits.
+    #[rustc_const_stable(feature = "const_int_unchecked", since = "1.40.0")]
     pub fn unchecked_shl<T>(x: T, y: T) -> T;
     /// Performs an unchecked right shift, resulting in undefined behavior when
     /// y < 0 or y >= N, where N is the width of T in bits.
+    #[rustc_const_stable(feature = "const_int_unchecked", since = "1.40.0")]
     pub fn unchecked_shr<T>(x: T, y: T) -> T;
 
     /// Returns the result of an unchecked addition, resulting in
@@ -1296,39 +1331,46 @@ extern "rust-intrinsic" {
     /// The stabilized versions of this intrinsic are available on the integer
     /// primitives via the `rotate_left` method. For example,
     /// [`std::u32::rotate_left`](../../std/primitive.u32.html#method.rotate_left)
+    #[rustc_const_stable(feature = "const_int_rotate", since = "1.40.0")]
     pub fn rotate_left<T>(x: T, y: T) -> T;
 
     /// Performs rotate right.
     /// The stabilized versions of this intrinsic are available on the integer
     /// primitives via the `rotate_right` method. For example,
     /// [`std::u32::rotate_right`](../../std/primitive.u32.html#method.rotate_right)
+    #[rustc_const_stable(feature = "const_int_rotate", since = "1.40.0")]
     pub fn rotate_right<T>(x: T, y: T) -> T;
 
     /// Returns (a + b) mod 2<sup>N</sup>, where N is the width of T in bits.
     /// The stabilized versions of this intrinsic are available on the integer
     /// primitives via the `wrapping_add` method. For example,
     /// [`std::u32::wrapping_add`](../../std/primitive.u32.html#method.wrapping_add)
+    #[rustc_const_stable(feature = "const_int_wrapping", since = "1.40.0")]
     pub fn wrapping_add<T>(a: T, b: T) -> T;
     /// Returns (a - b) mod 2<sup>N</sup>, where N is the width of T in bits.
     /// The stabilized versions of this intrinsic are available on the integer
     /// primitives via the `wrapping_sub` method. For example,
     /// [`std::u32::wrapping_sub`](../../std/primitive.u32.html#method.wrapping_sub)
+    #[rustc_const_stable(feature = "const_int_wrapping", since = "1.40.0")]
     pub fn wrapping_sub<T>(a: T, b: T) -> T;
     /// Returns (a * b) mod 2<sup>N</sup>, where N is the width of T in bits.
     /// The stabilized versions of this intrinsic are available on the integer
     /// primitives via the `wrapping_mul` method. For example,
     /// [`std::u32::wrapping_mul`](../../std/primitive.u32.html#method.wrapping_mul)
+    #[rustc_const_stable(feature = "const_int_wrapping", since = "1.40.0")]
     pub fn wrapping_mul<T>(a: T, b: T) -> T;
 
     /// Computes `a + b`, while saturating at numeric bounds.
     /// The stabilized versions of this intrinsic are available on the integer
     /// primitives via the `saturating_add` method. For example,
     /// [`std::u32::saturating_add`](../../std/primitive.u32.html#method.saturating_add)
+    #[rustc_const_stable(feature = "const_int_saturating", since = "1.40.0")]
     pub fn saturating_add<T>(a: T, b: T) -> T;
     /// Computes `a - b`, while saturating at numeric bounds.
     /// The stabilized versions of this intrinsic are available on the integer
     /// primitives via the `saturating_sub` method. For example,
     /// [`std::u32::saturating_sub`](../../std/primitive.u32.html#method.saturating_sub)
+    #[rustc_const_stable(feature = "const_int_saturating", since = "1.40.0")]
     pub fn saturating_sub<T>(a: T, b: T) -> T;
 
     /// Returns the value of the discriminant for the variant in 'v',
@@ -1350,6 +1392,7 @@ extern "rust-intrinsic" {
     pub fn nontemporal_store<T>(ptr: *mut T, val: T);
 
     /// See documentation of `<*const T>::offset_from` for details.
+    #[rustc_const_unstable(feature = "const_ptr_offset_from", issue = "0")]
     pub fn ptr_offset_from<T>(ptr: *const T, base: *const T) -> isize;
 
     /// Internal hook used by Miri to implement unwinding.

src/libcore/lib.rs

@@ -128,6 +128,8 @@
 #![feature(maybe_uninit_slice)]
 #![feature(external_doc)]
 #![feature(associated_type_bounds)]
+#![feature(const_type_id)]
+#![feature(const_caller_location)]
 
 #[prelude_import]
 #[allow(unused)]

src/libcore/macros/mod.rs

@@ -5,6 +5,7 @@
     // the `caller_location` intrinsic, but once  `#[track_caller]` is implemented,
     // `panicking::{panic, panic_fmt}` can use that instead of a `Location` argument.
     core_intrinsics,
+    const_caller_location,
 )]
 #[stable(feature = "core", since = "1.6.0")]
 macro_rules! panic {

src/librustc/ty/constness.rs

@@ -2,7 +2,7 @@ use crate::ty::query::Providers;
 use crate::hir::def_id::DefId;
 use crate::hir;
 use crate::ty::TyCtxt;
-use syntax_pos::symbol::{sym, Symbol};
+use syntax_pos::symbol::Symbol;
 use rustc_target::spec::abi::Abi;
 use crate::hir::map::blocks::FnLikeNode;
 use syntax::attr;
@@ -41,51 +41,12 @@ impl<'tcx> TyCtxt<'tcx> {
         }
     }
 
-    /// Returns `true` if the `def_id` refers to an intrisic which we've whitelisted
-    /// for being called from stable `const fn`s (`min_const_fn`).
-    ///
-    /// Adding more intrinsics requires sign-off from @rust-lang/lang.
-    ///
-    /// This list differs from the list in `is_const_intrinsic` in the sense that any item on this
-    /// list must be on the `is_const_intrinsic` list, too, because if an intrinsic is callable from
-    /// stable, it must be callable at all.
-    fn is_intrinsic_min_const_fn(self, def_id: DefId) -> bool {
-        match self.item_name(def_id) {
-            | sym::size_of
-            | sym::min_align_of
-            | sym::needs_drop
-            // Arithmetic:
-            | sym::add_with_overflow // ~> .overflowing_add
-            | sym::sub_with_overflow // ~> .overflowing_sub
-            | sym::mul_with_overflow // ~> .overflowing_mul
-            | sym::wrapping_add // ~> .wrapping_add
-            | sym::wrapping_sub // ~> .wrapping_sub
-            | sym::wrapping_mul // ~> .wrapping_mul
-            | sym::saturating_add // ~> .saturating_add
-            | sym::saturating_sub // ~> .saturating_sub
-            | sym::unchecked_shl // ~> .wrapping_shl
-            | sym::unchecked_shr // ~> .wrapping_shr
-            | sym::rotate_left // ~> .rotate_left
-            | sym::rotate_right // ~> .rotate_right
-            | sym::ctpop // ~> .count_ones
-            | sym::ctlz // ~> .leading_zeros
-            | sym::cttz // ~> .trailing_zeros
-            | sym::bswap // ~> .swap_bytes
-            | sym::bitreverse // ~> .reverse_bits
-            => true,
-            _ => false,
-        }
-    }
-
     /// Returns `true` if this function must conform to `min_const_fn`
     pub fn is_min_const_fn(self, def_id: DefId) -> bool {
         // Bail out if the signature doesn't contain `const`
         if !self.is_const_fn_raw(def_id) {
             return false;
         }
-        if let Abi::RustIntrinsic = self.fn_sig(def_id).abi() {
-            return self.is_intrinsic_min_const_fn(def_id);
-        }
 
         if self.features().staged_api {
             // In order for a libstd function to be considered min_const_fn
@@ -134,62 +95,7 @@ pub fn provide(providers: &mut Providers<'_>) {
     fn is_const_intrinsic(tcx: TyCtxt<'_>, def_id: DefId) -> Option<bool> {
         match tcx.fn_sig(def_id).abi() {
             Abi::RustIntrinsic |
-            Abi::PlatformIntrinsic => {
-                // FIXME: deduplicate these two lists as much as possible
-                match tcx.item_name(def_id) {
-                    // Keep this list in the same order as the match patterns in
-                    // `librustc_mir/interpret/intrinsics.rs`
-
-                    // This whitelist is a list of intrinsics that have a miri-engine implementation
-                    // and can thus be called when enabling enough feature gates. The similar
-                    // whitelist in `is_intrinsic_min_const_fn` (in this file), exists for allowing
-                    // the intrinsics to be called by stable const fns.
-                    | sym::caller_location
-
-                    | sym::min_align_of
-                    | sym::pref_align_of
-                    | sym::needs_drop
-                    | sym::size_of
-                    | sym::type_id
-                    | sym::type_name
-
-                    | sym::ctpop
-                    | sym::cttz
-                    | sym::cttz_nonzero
-                    | sym::ctlz
-                    | sym::ctlz_nonzero
-                    | sym::bswap
-                    | sym::bitreverse
-
-                    | sym::wrapping_add
-                    | sym::wrapping_sub
-                    | sym::wrapping_mul
-                    | sym::add_with_overflow
-                    | sym::sub_with_overflow
-                    | sym::mul_with_overflow
-
-                    | sym::saturating_add
-                    | sym::saturating_sub
-
-                    | sym::unchecked_shl
-                    | sym::unchecked_shr
-
-                    | sym::rotate_left
-                    | sym::rotate_right
-
-                    | sym::ptr_offset_from
-
-                    | sym::transmute
-
-                    | sym::simd_insert
-
-                    | sym::simd_extract
-
-                    => Some(true),
-
-                    _ => Some(false)
-                }
-            }
+            Abi::PlatformIntrinsic => Some(tcx.lookup_const_stability(def_id).is_some()),
             _ => None
         }
     }

src/test/ui/consts/const-eval/simd/insert_extract.rs

@@ -2,14 +2,18 @@
 #![feature(const_fn)]
 #![feature(repr_simd)]
 #![feature(platform_intrinsics)]
+#![feature(staged_api)]
+#![stable(feature = "foo", since = "1.33.7")]
 #![allow(non_camel_case_types)]
 
 #[repr(simd)] struct i8x1(i8);
 #[repr(simd)] struct u16x2(u16, u16);
 #[repr(simd)] struct f32x3(f32, f32, f32);
 
 extern "platform-intrinsic" {
+    #[rustc_const_stable(feature = "foo", since = "1.3.37")]
     fn simd_insert<T, U>(x: T, idx: u32, val: U) -> T;
+    #[rustc_const_stable(feature = "foo", since = "1.3.37")]
     fn simd_extract<T, U>(x: T, idx: u32) -> U;
 }
 

src/test/ui/consts/const-fn-type-name.rs

@@ -2,6 +2,7 @@
 
 #![feature(core_intrinsics)]
 #![feature(const_fn)]
+#![feature(const_type_name)]
 #![allow(dead_code)]
 
 const fn type_name_wrapper<T>(_: &T) -> &'static str {