Make likely and unlikely const, gated by feature const_unlikely

src/libcore/intrinsics.rs

@@ -952,6 +952,7 @@ extern "rust-intrinsic" {
     /// Any use other than with `if` statements will probably not have an effect.
     ///
     /// This intrinsic does not have a stable counterpart.
+    #[rustc_const_unstable(feature = "const_likely", issue = "none")]
     pub fn likely(b: bool) -> bool;
 
     /// Hints to the compiler that branch condition is likely to be false.
@@ -960,6 +961,7 @@ extern "rust-intrinsic" {
     /// Any use other than with `if` statements will probably not have an effect.
     ///
     /// This intrinsic does not have a stable counterpart.
+    #[rustc_const_unstable(feature = "const_likely", issue = "none")]
     pub fn unlikely(b: bool) -> bool;
 
     /// Executes a breakpoint trap, for inspection by a debugger.

src/libcore/lib.rs

@@ -92,6 +92,7 @@
 #![feature(const_slice_from_raw_parts)]
 #![feature(const_slice_ptr_len)]
 #![feature(const_type_name)]
+#![feature(const_likely)]
 #![feature(custom_inner_attributes)]
 #![feature(decl_macro)]
 #![feature(doc_cfg)]

src/librustc_mir/interpret/intrinsics.rs

@@ -399,6 +399,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 );
                 self.copy_op(self.operand_index(args[0], index)?, dest)?;
             }
+            sym::likely | sym::unlikely => {
+                // These just return their argument
+                let a = self.read_immediate(args[0])?;
+                self.write_immediate(*a, dest)?;
+            }
             // FIXME(#73156): Handle source code coverage in const eval
             sym::count_code_region => (),
             _ => return Ok(false),

src/librustc_mir/transform/check_consts/validation.rs

@@ -548,9 +548,12 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
                 if is_lang_panic_fn(self.tcx, def_id) {
                     self.check_op(ops::Panic);
                 } else if let Some(feature) = is_unstable_const_fn(self.tcx, def_id) {
-                    // Exempt unstable const fns inside of macros with
+                    // Exempt unstable const fns inside of macros or functions with
                     // `#[allow_internal_unstable]`.
-                    if !self.span.allows_unstable(feature) {
+                    use crate::transform::qualify_min_const_fn::lib_feature_allowed;
+                    if !self.span.allows_unstable(feature)
+                        && !lib_feature_allowed(self.tcx, self.def_id, feature)
+                    {
                         self.check_op(ops::FnCallUnstable(def_id, feature));
                     }
                 } else {

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -334,6 +334,26 @@ fn feature_allowed(tcx: TyCtxt<'tcx>, def_id: DefId, feature_gate: Symbol) -> bo
         .map_or(false, |mut features| features.any(|name| name == feature_gate))
 }
 
+/// Returns `true` if the given library feature gate is allowed within the function with the given `DefId`.
+pub fn lib_feature_allowed(tcx: TyCtxt<'tcx>, def_id: DefId, feature_gate: Symbol) -> bool {
+    // All features require that the corresponding gate be enabled,
+    // even if the function has `#[allow_internal_unstable(the_gate)]`.
+    if !tcx.features().declared_lib_features.iter().any(|&(sym, _)| sym == feature_gate) {
+        return false;
+    }
+
+    // If this crate is not using stability attributes, or this function is not claiming to be a
+    // stable `const fn`, that is all that is required.
+    if !tcx.features().staged_api || tcx.has_attr(def_id, sym::rustc_const_unstable) {
+        return true;
+    }
+
+    // However, we cannot allow stable `const fn`s to use unstable features without an explicit
+    // opt-in via `allow_internal_unstable`.
+    attr::allow_internal_unstable(&tcx.get_attrs(def_id), &tcx.sess.diagnostic())
+        .map_or(false, |mut features| features.any(|name| name == feature_gate))
+}
+
 fn check_terminator(
     tcx: TyCtxt<'tcx>,
     body: &'a Body<'tcx>,
@@ -377,8 +397,17 @@ fn check_terminator(
             fn_span: _,
         } => {
             let fn_ty = func.ty(body, tcx);
-            if let ty::FnDef(def_id, _) = fn_ty.kind {
-                if !crate::const_eval::is_min_const_fn(tcx, def_id) {
+            if let ty::FnDef(fn_def_id, _) = fn_ty.kind {
+                // Allow unstable const if we opt in by using #[allow_internal_unstable]
+                // on function or macro declaration.
+                if !crate::const_eval::is_min_const_fn(tcx, fn_def_id)
+                    && !crate::const_eval::is_unstable_const_fn(tcx, fn_def_id)
+                        .map(|feature| {
+                            span.allows_unstable(feature)
+                                || lib_feature_allowed(tcx, def_id, feature)
+                        })
+                        .unwrap_or(false)
+                {
                     return Err((
                         span,
                         format!(
@@ -390,10 +419,10 @@ fn check_terminator(
                     ));
                 }
 
-                check_operand(tcx, func, span, def_id, body)?;
+                check_operand(tcx, func, span, fn_def_id, body)?;
 
                 for arg in args {
-                    check_operand(tcx, arg, span, def_id, body)?;
+                    check_operand(tcx, arg, span, fn_def_id, body)?;
                 }
                 Ok(())
             } else {

src/librustc_span/symbol.rs

@@ -427,6 +427,7 @@ symbols! {
         lhs,
         lib,
         lifetime,
+        likely,
         line,
         link,
         linkage,
@@ -813,6 +814,7 @@ symbols! {
         underscore_lifetimes,
         uniform_paths,
         universal_impl_trait,
+        unlikely,
         unmarked_api,
         unreachable_code,
         unrestricted_attribute_tokens,