Make const_eval_select a rustc_intrinsic

Author: oli-obk

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -578,7 +578,7 @@ pub fn check_intrinsic_type(
 
             sym::is_val_statically_known => (1, 1, vec![param(0)], tcx.types.bool),
 
-            sym::const_eval_select => (4, 0, vec![param(0), param(1), param(2)], param(3)),
+            sym::const_eval_select => (4, 1, vec![param(0), param(1), param(2)], param(3)),
 
             sym::vtable_size | sym::vtable_align => {
                 (0, 0, vec![Ty::new_imm_ptr(tcx, Ty::new_unit(tcx))], tcx.types.usize)

compiler/rustc_middle/src/hir/map/mod.rs

@@ -1323,7 +1323,9 @@ impl<'hir> Visitor<'hir> for ItemCollector<'hir> {
 
     fn visit_item(&mut self, item: &'hir Item<'hir>) {
         if associated_body(Node::Item(item)).is_some() {
-            self.body_owners.push(item.owner_id.def_id);
+            if !self.tcx.has_attr(item.owner_id.def_id, sym::rustc_intrinsic_must_be_overridden) {
+                self.body_owners.push(item.owner_id.def_id);
+            }
         }
 
         self.items.push(item.item_id());
@@ -1373,7 +1375,9 @@ impl<'hir> Visitor<'hir> for ItemCollector<'hir> {
 
     fn visit_impl_item(&mut self, item: &'hir ImplItem<'hir>) {
         if associated_body(Node::ImplItem(item)).is_some() {
-            self.body_owners.push(item.owner_id.def_id);
+            if !self.tcx.has_attr(item.owner_id.def_id, sym::rustc_intrinsic_must_be_overridden) {
+                self.body_owners.push(item.owner_id.def_id);
+            }
         }
 
         self.impl_items.push(item.impl_item_id());

library/core/src/intrinsics.rs

@@ -2508,59 +2508,7 @@ extern "rust-intrinsic" {
     #[rustc_nounwind]
     pub fn vtable_align(ptr: *const ()) -> usize;
 
-    /// Selects which function to call depending on the context.
-    ///
-    /// If this function is evaluated at compile-time, then a call to this
-    /// intrinsic will be replaced with a call to `called_in_const`. It gets
-    /// replaced with a call to `called_at_rt` otherwise.
-    ///
-    /// # Type Requirements
-    ///
-    /// The two functions must be both function items. They cannot be function
-    /// pointers or closures. The first function must be a `const fn`.
-    ///
-    /// `arg` will be the tupled arguments that will be passed to either one of
-    /// the two functions, therefore, both functions must accept the same type of
-    /// arguments. Both functions must return RET.
-    ///
-    /// # Safety
-    ///
-    /// The two functions must behave observably equivalent. Safe code in other
-    /// crates may assume that calling a `const fn` at compile-time and at run-time
-    /// produces the same result. A function that produces a different result when
-    /// evaluated at run-time, or has any other observable side-effects, is
-    /// *unsound*.
-    ///
-    /// Here is an example of how this could cause a problem:
-    /// ```no_run
-    /// #![feature(const_eval_select)]
-    /// #![feature(core_intrinsics)]
-    /// # #![allow(internal_features)]
-    /// use std::hint::unreachable_unchecked;
-    /// use std::intrinsics::const_eval_select;
-    ///
-    /// // Crate A
-    /// pub const fn inconsistent() -> i32 {
-    ///     fn runtime() -> i32 { 1 }
-    ///     const fn compiletime() -> i32 { 2 }
-    ///
-    ///     unsafe {
-    //          // ⚠ This code violates the required equivalence of `compiletime`
-    ///         // and `runtime`.
-    ///         const_eval_select((), compiletime, runtime)
-    ///     }
-    /// }
-    ///
-    /// // Crate B
-    /// const X: i32 = inconsistent();
-    /// let x = inconsistent();
-    /// if x != X { unsafe { unreachable_unchecked(); }}
-    /// ```
-    ///
-    /// This code causes Undefined Behavior when being run, since the
-    /// `unreachable_unchecked` is actually being reached. The bug is in *crate A*,
-    /// which violates the principle that a `const fn` must behave the same at
-    /// compile-time and at run-time. The unsafe code in crate B is fine.
+    #[cfg(bootstrap)]
     #[rustc_const_unstable(feature = "const_eval_select", issue = "none")]
     pub fn const_eval_select<ARG: Tuple, F, G, RET>(
         arg: ARG,
@@ -2572,6 +2520,76 @@ extern "rust-intrinsic" {
         F: FnOnce<ARG, Output = RET>;
 }
 
+/// Selects which function to call depending on the context.
+///
+/// If this function is evaluated at compile-time, then a call to this
+/// intrinsic will be replaced with a call to `called_in_const`. It gets
+/// replaced with a call to `called_at_rt` otherwise.
+///
+/// # Type Requirements
+///
+/// The two functions must be both function items. They cannot be function
+/// pointers or closures. The first function must be a `const fn`.
+///
+/// `arg` will be the tupled arguments that will be passed to either one of
+/// the two functions, therefore, both functions must accept the same type of
+/// arguments. Both functions must return RET.
+///
+/// # Safety
+///
+/// The two functions must behave observably equivalent. Safe code in other
+/// crates may assume that calling a `const fn` at compile-time and at run-time
+/// produces the same result. A function that produces a different result when
+/// evaluated at run-time, or has any other observable side-effects, is
+/// *unsound*.
+///
+/// Here is an example of how this could cause a problem:
+/// ```no_run
+/// #![feature(const_eval_select)]
+/// #![feature(core_intrinsics)]
+/// # #![allow(internal_features)]
+/// use std::hint::unreachable_unchecked;
+/// use std::intrinsics::const_eval_select;
+///
+/// // Crate A
+/// pub const fn inconsistent() -> i32 {
+///     fn runtime() -> i32 { 1 }
+///     const fn compiletime() -> i32 { 2 }
+///
+///     unsafe {
+//          // ⚠ This code violates the required equivalence of `compiletime`
+///         // and `runtime`.
+///         const_eval_select((), compiletime, runtime)
+///     }
+/// }
+///
+/// // Crate B
+/// const X: i32 = inconsistent();
+/// let x = inconsistent();
+/// if x != X { unsafe { unreachable_unchecked(); }}
+/// ```
+///
+/// This code causes Undefined Behavior when being run, since the
+/// `unreachable_unchecked` is actually being reached. The bug is in *crate A*,
+/// which violates the principle that a `const fn` must behave the same at
+/// compile-time and at run-time. The unsafe code in crate B is fine.
+#[rustc_const_unstable(feature = "const_eval_select", issue = "none")]
+#[unstable(feature = "core_intrinsics", issue = "none")]
+#[cfg(not(bootstrap))]
+#[rustc_intrinsic]
+#[rustc_intrinsic_must_be_overridden]
+pub const unsafe fn const_eval_select<ARG: Tuple, F, G, RET>(
+    _arg: ARG,
+    _called_in_const: F,
+    _called_at_rt: G,
+) -> RET
+where
+    G: FnOnce<ARG, Output = RET>,
+    F: FnOnce<ARG, Output = RET>,
+{
+    unreachable!()
+}
+
 /// Returns whether the argument's value is statically known at
 /// compile-time.
 ///

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/minicore.rs

@@ -509,16 +509,19 @@ trait StructuralPartialEq {}
 
 const fn drop<T: ~const Destruct>(_: T) {}
 
-extern "rust-intrinsic" {
-    #[rustc_const_stable(feature = "const_eval_select", since = "1.0.0")]
-    fn const_eval_select<ARG: Tuple, F, G, RET>(
-        arg: ARG,
-        called_in_const: F,
-        called_at_rt: G,
-    ) -> RET
-    where
-        F: const FnOnce<ARG, Output = RET>,
-        G: FnOnce<ARG, Output = RET>;
+#[rustc_const_stable(feature = "const_eval_select", since = "1.0.0")]
+#[rustc_intrinsic_must_be_overridden]
+#[rustc_intrinsic]
+const unsafe fn const_eval_select<ARG: Tuple, F, G, RET>(
+    arg: ARG,
+    called_in_const: F,
+    called_at_rt: G,
+) -> RET
+where
+    F: const FnOnce<ARG, Output = RET>,
+    G: FnOnce<ARG, Output = RET>,
+{
+    loop {}
 }
 
 fn test_const_eval_select() {