Re-do recursive const stability checks

Fundamentally, we have *three* disjoint categories of functions:
1. const-stable functions
2. private/unstable functions that are meant to be callable from const-stable functions
3. functions that can make use of unstable const features

This PR implements the following system:
- `#[rustc_const_stable]` puts functions in the first category. It may only be applied to `#[stable]` functions.
- `#[rustc_const_unstable]` by default puts functions in the third category. The new attribute `#[rustc_const_stable_indirect]` can be added to such a function to move it into the second category.
- `const fn` without a const stability marker are in the second category if they are still unstable. They automatically inherit the feature gate for regular calls, it can now also be used for const-calls.

Also, several holes in recursive const stability checking are being closed.
There's still one potential hole that is hard to avoid, which is when MIR
building automatically inserts calls to a particular function in stable
functions -- which happens in the panic machinery. Those need to *not* be
`rustc_const_unstable` (or manually get a `rustc_const_stable_indirect`) to be
sure they follow recursive const stability. But that's a fairly rare and special
case so IMO it's fine.

The net effect of this is that a `#[unstable]` or unmarked function can be
constified simply by marking it as `const fn`, and it will then be
const-callable from stable `const fn` and subject to recursive const stability
requirements. If it is publicly reachable (which implies it cannot be unmarked),
it will be const-unstable under the same feature gate. Only if the function ever
becomes `#[stable]` does it need a `#[rustc_const_unstable]` or
`#[rustc_const_stable]` marker to decide if this should also imply
const-stability.

Adding `#[rustc_const_unstable]` is only needed for (a) functions that need to
use unstable const lang features (including intrinsics), or (b) `#[stable]`
functions that are not yet intended to be const-stable. Adding
`#[rustc_const_stable]` is only needed for functions that are actually meant to
be directly callable from stable const code. `#[rustc_const_stable_indirect]` is
used to mark intrinsics as const-callable and for `#[rustc_const_unstable]`
functions that are actually called from other, exposed-on-stable `const fn`. No
other attributes are required.

2 files changed, 2 insertions, 19 deletions

diff --git a/compiler/rustc_const_eval/src/const_eval/fn_queries.rs b/compiler/rustc_const_eval/src/const_eval/fn_queries.rs
index ca0993f0580..037fdcbcf9b 100644
--- a/compiler/rustc_const_eval/src/const_eval/fn_queries.rs
+++ b/compiler/rustc_const_eval/src/const_eval/fn_queries.rs
@@ -1,25 +1,8 @@
+use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::TyCtxt;
-use rustc_span::symbol::Symbol;
-use {rustc_attr as attr, rustc_hir as hir};
-
-/// Whether the `def_id` is an unstable const fn and what feature gate(s) are necessary to enable
-/// it.
-pub fn is_unstable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> Option<(Symbol, Option<Symbol>)> {
-    if tcx.is_const_fn_raw(def_id) {
-        let const_stab = tcx.lookup_const_stability(def_id)?;
-        match const_stab.level {
-            attr::StabilityLevel::Unstable { implied_by, .. } => {
-                Some((const_stab.feature, implied_by))
-            }
-            attr::StabilityLevel::Stable { .. } => None,
-        }
-    } else {
-        None
-    }
-}
 
 pub fn is_parent_const_impl_raw(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
     let parent_id = tcx.local_parent(def_id);
diff --git a/compiler/rustc_const_eval/src/const_eval/machine.rs b/compiler/rustc_const_eval/src/const_eval/machine.rs
index 2db43a0f787..7793de9c933 100644
--- a/compiler/rustc_const_eval/src/const_eval/machine.rs
+++ b/compiler/rustc_const_eval/src/const_eval/machine.rs
@@ -219,7 +219,7 @@ impl<'tcx> CompileTimeInterpCx<'tcx> {
     }
 
     /// "Intercept" a function call, because we have something special to do for it.
-    /// All `#[rustc_do_not_const_check]` functions should be hooked here.
+    /// All `#[rustc_do_not_const_check]` functions MUST be hooked here.
     /// If this returns `Some` function, which may be `instance` or a different function with
     /// compatible arguments, then evaluation should continue with that function.
     /// If this returns `None`, the function call has been handled and the function has returned.