make it possible to enable const_precise_live_drops per-function

compiler/rustc_const_eval/src/check_consts/post_drop_elaboration.rs

@@ -9,13 +9,19 @@ use super::check::Qualifs;
 use super::ops::{self, NonConstOp};
 use super::qualifs::{NeedsNonConstDrop, Qualif};
 use super::ConstCx;
+use crate::check_consts::rustc_allow_const_fn_unstable;
 
 /// Returns `true` if we should use the more precise live drop checker that runs after drop
 /// elaboration.
 pub fn checking_enabled(ccx: &ConstCx<'_, '_>) -> bool {
-    // Const-stable functions must always use the stable live drop checker.
+    // Const-stable functions must always use the stable live drop checker...
     if ccx.is_const_stable_const_fn() {
-        return false;
+        // ...except if they have the feature flag set via `rustc_allow_const_fn_unstable`.
+        return rustc_allow_const_fn_unstable(
+            ccx.tcx,
+            ccx.body.source.def_id().expect_local(),
+            sym::const_precise_live_drops,
+        );
     }
 
     ccx.tcx.features().const_precise_live_drops

tests/ui/consts/precise-drop-allow-const-fn-unstable.not_allow.stderr

@@ -0,0 +1,12 @@
+error[E0493]: destructor of `Option<T>` cannot be evaluated at compile-time
+  --> $DIR/precise-drop-allow-const-fn-unstable.rs:11:24
+   |
+LL | pub const fn unwrap<T>(this: Option<T>) -> T {
+   |                        ^^^^ the destructor for this type cannot be evaluated in constant functions
+...
+LL | }
+   | - value is dropped here
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0493`.

tests/ui/consts/precise-drop-allow-const-fn-unstable.rs

@@ -0,0 +1,17 @@
+//@ revisions: allow not_allow
+//@ compile-flags: --crate-type=lib -Cinstrument-coverage  -Zno-profiler-runtime
+//@[allow] check-pass
+
+#![feature(staged_api, rustc_allow_const_fn_unstable)]
+#![stable(feature = "rust_test", since = "1.0.0")]
+
+#[stable(feature = "rust_test", since = "1.0.0")]
+#[rustc_const_stable(feature = "rust_test", since = "1.0.0")]
+#[cfg_attr(allow, rustc_allow_const_fn_unstable(const_precise_live_drops))]
+pub const fn unwrap<T>(this: Option<T>) -> T {
+//[not_allow]~^ ERROR: cannot be evaluated
+    match this {
+        Some(x) => x,
+        None => panic!(),
+    }
+}