make it possible to enable const_precise_live_drops per-function

Author: RalfJung

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

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/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")]
 

tests/ui/consts/min_const_fn/min_const_fn_libstd_stability.rs

@@ -28,9 +28,11 @@ const fn bar2() -> u32 { foo2() } //~ ERROR not yet stable as a const fn
 // conformity is required
 const fn bar3() -> u32 {
     let x = std::cell::Cell::new(0u32);
-    x.get()
+    x.get();
     //~^ ERROR const-stable function cannot use `#[feature(const_refs_to_cell)]`
     //~| ERROR cannot call non-const fn
+    foo()
+    //~^ ERROR is not yet stable as a const fn
 }
 
 // check whether this function cannot be called even with the feature gate active

tests/ui/consts/min_const_fn/min_const_fn_libstd_stability.stderr

@@ -17,15 +17,15 @@ LL | const fn bar2() -> u32 { foo2() }
 error: const-stable function cannot use `#[feature(const_refs_to_cell)]`
   --> $DIR/min_const_fn_libstd_stability.rs:31:5
    |
-LL |     x.get()
+LL |     x.get();
    |     ^
    |
 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 {
    |
-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_refs_to_cell)]
 LL | const fn bar3() -> u32 {
@@ -34,19 +34,27 @@ LL | const fn bar3() -> u32 {
 error[E0015]: cannot call non-const fn `Cell::<u32>::get` in constant functions
   --> $DIR/min_const_fn_libstd_stability.rs:31:7
    |
-LL |     x.get()
+LL |     x.get();
    |       ^^^^^
    |
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
+error: `foo` is not yet stable as a const fn
+  --> $DIR/min_const_fn_libstd_stability.rs:34:5
+   |
+LL |     foo()
+   |     ^^^^^
+   |
+   = help: const-stable functions can only call other const-stable functions
+
 error: `foo2_gated` is not yet stable as a const fn
-  --> $DIR/min_const_fn_libstd_stability.rs:43:32
+  --> $DIR/min_const_fn_libstd_stability.rs:45:32
    |
 LL | const fn bar2_gated() -> u32 { foo2_gated() }
    |                                ^^^^^^^^^^^^
    |
    = help: const-stable functions can only call other const-stable functions
 
-error: aborting due to 5 previous errors
+error: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0015`.

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!(),
+    }
+}