Add const_eval_select intrinsic

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, NeedsNonConstDrop};
 use super::resolver::FlowSensitiveAnalysis;
-use super::{is_lang_panic_fn, ConstCx, Qualif};
+use super::{is_lang_special_const_fn, 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
@@ -259,7 +259,9 @@ impl Checker<'mir, 'tcx> {
             self.check_local_or_return_ty(return_ty.skip_binder(), RETURN_PLACE);
         }
 
-        self.visit_body(&body);
+        if !tcx.has_attr(def_id.to_def_id(), sym::rustc_do_not_const_check) {
+            self.visit_body(&body);
+        }
 
         // Ensure that the end result is `Sync` in a non-thread local `static`.
         let should_check_for_sync = self.const_kind()
@@ -886,7 +888,7 @@ impl Visitor<'tcx> for Checker<'mir, 'tcx> {
                 }
 
                 // At this point, we are calling a function, `callee`, whose `DefId` is known...
-                if is_lang_panic_fn(tcx, callee) {
+                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.

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

@@ -74,9 +74,6 @@ 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 {
-    // We can allow calls to these functions because `hook_panic_fn` in
-    // `const_eval/machine.rs` ensures the calls are handled specially.
-    // Keep in sync with what that function handles!
     Some(def_id) == tcx.lang_items().panic_fn()
         || Some(def_id) == tcx.lang_items().panic_str()
         || Some(def_id) == tcx.lang_items().panic_display()
@@ -85,6 +82,15 @@ pub fn is_lang_panic_fn(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
         || Some(def_id) == tcx.lang_items().begin_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/check_consts/post_drop_elaboration.rs

@@ -1,7 +1,7 @@
 use rustc_middle::mir::visit::Visitor;
 use rustc_middle::mir::{self, BasicBlock, Location};
 use rustc_middle::ty::TyCtxt;
-use rustc_span::Span;
+use rustc_span::{symbol::sym, Span};
 
 use super::check::Qualifs;
 use super::ops::{self, NonConstOp};
@@ -30,6 +30,10 @@ pub fn check_live_drops(tcx: TyCtxt<'tcx>, body: &mir::Body<'tcx>) {
         return;
     }
 
+    if tcx.has_attr(def_id.to_def_id(), sym::rustc_do_not_const_check) {
+        return;
+    }
+
     let ccx = ConstCx { body, tcx, const_kind, param_env: tcx.param_env(def_id) };
     if !checking_enabled(&ccx) {
         return;