Add is_const_eval intrinsic

The `core::intrinsic::is_const_eval` returns `true` if it is
evaluated at compile-time and false otherwise.

Author: gnzlbg

src/libcore/intrinsics.rs

@@ -660,6 +660,26 @@ extern "rust-intrinsic" {
     /// Executes a breakpoint trap, for inspection by a debugger.
     pub fn breakpoint();
 
+    /// Returns `true` during constant evaluation and `false` otherwise.
+    ///
+    /// # Safety
+    ///
+    /// This intrinsic allows breaking [referential transparency] in `const fn`
+    /// and is therefore `unsafe`.
+    ///
+    /// Code that uses this intrinsic must be extremely careful to ensure that
+    /// `const fn`s remain referentially-transparent independently of when they
+    /// are evaluated.
+    ///
+    /// The Rust compiler assumes that it is sound to replace a call to a `const
+    /// fn` with the result produced by evaluating it at compile-time. If
+    /// evaluating the function at run-time were to produce a different result,
+    /// or have any other observable side-effects, the behavior is undefined.
+    ///
+    /// [referential transparency]: https://en.wikipedia.org/wiki/Referential_transparency
+    #[cfg(not(boostrap_stdarch_ignore_this))]
+    pub fn is_const_eval() -> bool;
+
     /// The size of a type in bytes.
     ///
     /// More specifically, this is the offset in bytes between successive

src/librustc_codegen_llvm/intrinsic.rs

@@ -518,6 +518,9 @@ impl IntrinsicCallMethods<'tcx> for Builder<'a, 'll, 'tcx> {
                 }
 
             },
+            "is_const_eval" => {
+                self.const_bool(false)
+            },
             "fadd_fast" | "fsub_fast" | "fmul_fast" | "fdiv_fast" | "frem_fast" => {
                 match float_type_width(arg_tys[0]) {
                     Some(_width) =>

src/librustc_mir/interpret/intrinsics.rs

@@ -46,8 +46,11 @@ crate fn eval_nullary_intrinsic<'tcx>(
     def_id: DefId,
     substs: SubstsRef<'tcx>,
 ) -> InterpResult<'tcx, &'tcx ty::Const<'tcx>> {
-    let tp_ty = substs.type_at(0);
     let name = &*tcx.item_name(def_id).as_str();
+    if name == "is_const_eval" {
+        return Ok(ty::Const::from_bool(tcx, true));
+    }
+    let tp_ty = substs.type_at(0);
     Ok(match name {
         "type_name" => {
             let alloc = type_name::alloc_type_name(tcx, tp_ty);
@@ -94,6 +97,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
         let intrinsic_name = &self.tcx.item_name(instance.def_id()).as_str()[..];
         match intrinsic_name {
+            "is_const_eval" |
             "min_align_of" |
             "pref_align_of" |
             "needs_drop" |

src/librustc_mir/transform/qualify_consts.rs

@@ -557,8 +557,8 @@ impl Qualif for IsNotPromotable {
                             | "saturating_add"
                             | "saturating_sub"
                             | "transmute"
+                            | "is_const_eval"
                             => return true,
-
                             _ => {}
                         }
                     }

src/librustc_typeck/check/intrinsic.rs

@@ -145,6 +145,7 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem) {
             "rustc_peek" => (1, vec![param(0)], param(0)),
             "panic_if_uninhabited" => (1, Vec::new(), tcx.mk_unit()),
             "init" => (1, Vec::new(), param(0)),
+            "is_const_eval" => (0, Vec::new(), tcx.types.bool),
             "uninit" => (1, Vec::new(), param(0)),
             "forget" => (1, vec![param(0)], tcx.mk_unit()),
             "transmute" => (2, vec![ param(0) ], param(1)),

src/test/codegen/intrinsics/is_const_eval.rs

@@ -0,0 +1,16 @@
+// compile-flags: -C no-prepopulate-passes
+
+#![crate_type = "lib"]
+#![feature(core_intrinsics)]
+
+use std::intrinsics::is_const_eval;
+
+// CHECK-LABEL: @is_const_eval_test
+#[no_mangle]
+pub unsafe fn is_const_eval_test() -> bool {
+    // CHECK: %0 = alloca i8, align 1
+    // CHECK: store i8 0, i8* %0, align 1
+    // CHECK: %2 = trunc i8 %1 to i1
+    // CHECK: ret i1 %2
+    is_const_eval()
+}

src/test/ui/intrinsics/is_const_eval.rs

@@ -0,0 +1,11 @@
+// run-pass
+
+#![feature(core_intrinsics)]
+use std::intrinsics::is_const_eval;
+
+fn main() {
+    const X: bool = unsafe { is_const_eval() };
+    let y = unsafe { is_const_eval() };
+    assert_eq!(X, true);
+    assert_eq!(y, false);
+}