make it possible to enable const_precise_live_drops per-function

Author: RalfJung

Diff for: compiler/rustc_const_eval/messages.ftl

@@ -419,7 +419,7 @@ const_eval_unstable_const_fn = `{$def_path}` is not yet stable as a const fn
 const_eval_unstable_in_stable =
     const-stable function cannot use `#[feature({$gate})]`
     .unstable_sugg = if it is not part of the public API, make this function unstably const
-    .bypass_sugg = otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
+    .bypass_sugg = otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (but requires team approval)
 
 const_eval_unterminated_c_string =
     reading a null-terminated string starting at {$pointer} with no null found before end of allocation

Diff for: 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

Diff for: tests/ui/consts/min_const_fn/allow_const_fn_ptr_run_pass.rs

@@ -1,6 +1,4 @@
 //@ run-pass
-#![feature(rustc_allow_const_fn_unstable)]
-
 #![feature(rustc_attrs, staged_api)]
 #![stable(feature = "rust1", since = "1.0.0")]
 

Diff for: tests/ui/consts/min_const_fn/min_const_fn_libstd_stability.stderr

@@ -25,7 +25,7 @@ help: if it is not part of the public API, make this function unstably const
 LL + #[rustc_const_unstable(feature = "...", issue = "...")]
 LL | const fn bar3() -> u32 { (5f32 + 6f32) as u32 }
    |
-help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
+help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (but requires team approval)
    |
 LL + #[rustc_allow_const_fn_unstable(const_fn_floating_point_arithmetic)]
 LL | const fn bar3() -> u32 { (5f32 + 6f32) as u32 }

Diff for: tests/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability.stderr

@@ -25,7 +25,7 @@ help: if it is not part of the public API, make this function unstably const
 LL + #[rustc_const_unstable(feature = "...", issue = "...")]
 LL | const unsafe fn bar3() -> u32 { (5f32 + 6f32) as u32 }
    |
-help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
+help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (but requires team approval)
    |
 LL + #[rustc_allow_const_fn_unstable(const_fn_floating_point_arithmetic)]
 LL | const unsafe fn bar3() -> u32 { (5f32 + 6f32) as u32 }

Diff for: 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`.

Diff for: 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!(),
+    }
+}

Diff for: tests/ui/internal/internal-unstable-const.stderr

@@ -9,7 +9,7 @@ help: if it is not part of the public API, make this function unstably const
 LL + #[rustc_const_unstable(feature = "...", issue = "...")]
 LL | pub const fn foo() -> f32 {
    |
-help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
+help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (but requires team approval)
    |
 LL + #[rustc_allow_const_fn_unstable(const_fn_floating_point_arithmetic)]
 LL | pub const fn foo() -> f32 {