Auto merge of #80235 - RalfJung:validate-promoteds, r=oli-obk

validate promoteds

Turn on const-value validation for promoteds. This is made possible now that #67534 is resolved.

I don't think this is a breaking change. We don't promote any unsafe operation any more (since #77526 landed). We *do* promote `const fn` calls under some circumstances (in `const`/`static` initializers), but union field access and similar operations are not allowed in `const fn`. So now is a perfect time to add this check. :D

r? `@oli-obk`
Fixes #67465

Author: bors

Diff for: compiler/rustc_mir/src/const_eval/eval_queries.rs

@@ -383,25 +383,19 @@ pub fn eval_to_allocation_raw_provider<'tcx>(
         Ok(mplace) => {
             // Since evaluation had no errors, valiate the resulting constant:
             let validation = try {
-                // FIXME do not validate promoteds until a decision on
-                // https://github.com/rust-lang/rust/issues/67465 and
-                // https://github.com/rust-lang/rust/issues/67534 is made.
-                // Promoteds can contain unexpected `UnsafeCell` and reference `static`s, but their
-                // otherwise restricted form ensures that this is still sound. We just lose the
-                // extra safety net of some of the dynamic checks. They can also contain invalid
-                // values, but since we do not usually check intermediate results of a computation
-                // for validity, it might be surprising to do that here.
-                if cid.promoted.is_none() {
-                    let mut ref_tracking = RefTracking::new(mplace);
-                    let mut inner = false;
-                    while let Some((mplace, path)) = ref_tracking.todo.pop() {
-                        let mode = match tcx.static_mutability(cid.instance.def_id()) {
-                            Some(_) => CtfeValidationMode::Regular, // a `static`
-                            None => CtfeValidationMode::Const { inner },
-                        };
-                        ecx.const_validate_operand(mplace.into(), path, &mut ref_tracking, mode)?;
-                        inner = true;
-                    }
+                let mut ref_tracking = RefTracking::new(mplace);
+                let mut inner = false;
+                while let Some((mplace, path)) = ref_tracking.todo.pop() {
+                    let mode = match tcx.static_mutability(cid.instance.def_id()) {
+                        Some(_) if cid.promoted.is_some() => {
+                            // Promoteds in statics are allowed to point to statics.
+                            CtfeValidationMode::Const { inner, allow_static_ptrs: true }
+                        }
+                        Some(_) => CtfeValidationMode::Regular, // a `static`
+                        None => CtfeValidationMode::Const { inner, allow_static_ptrs: false },
+                    };
+                    ecx.const_validate_operand(mplace.into(), path, &mut ref_tracking, mode)?;
+                    inner = true;
                 }
             };
             if let Err(error) = validation {

Diff for: compiler/rustc_mir/src/interpret/validity.rs

@@ -117,11 +117,12 @@ pub enum PathElem {
 pub enum CtfeValidationMode {
     /// Regular validation, nothing special happening.
     Regular,
-    /// Validation of a `const`. `inner` says if this is an inner, indirect allocation (as opposed
-    /// to the top-level const allocation).
-    /// Being an inner allocation makes a difference because the top-level allocation of a `const`
-    /// is copied for each use, but the inner allocations are implicitly shared.
-    Const { inner: bool },
+    /// Validation of a `const`.
+    /// `inner` says if this is an inner, indirect allocation (as opposed to the top-level const
+    /// allocation). Being an inner allocation makes a difference because the top-level allocation
+    /// of a `const` is copied for each use, but the inner allocations are implicitly shared.
+    /// `allow_static_ptrs` says if pointers to statics are permitted (which is the case for promoteds in statics).
+    Const { inner: bool, allow_static_ptrs: bool },
 }
 
 /// State for tracking recursive validation of references
@@ -437,7 +438,10 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
                 if let Some(GlobalAlloc::Static(did)) = alloc_kind {
                     assert!(!self.ecx.tcx.is_thread_local_static(did));
                     assert!(self.ecx.tcx.is_static(did));
-                    if matches!(self.ctfe_mode, Some(CtfeValidationMode::Const { .. })) {
+                    if matches!(
+                        self.ctfe_mode,
+                        Some(CtfeValidationMode::Const { allow_static_ptrs: false, .. })
+                    ) {
                         // See const_eval::machine::MemoryExtra::can_access_statics for why
                         // this check is so important.
                         // This check is reachable when the const just referenced the static,
@@ -742,9 +746,9 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
         // Sanity check: `builtin_deref` does not know any pointers that are not primitive.
         assert!(op.layout.ty.builtin_deref(true).is_none());
 
-        // Special check preventing `UnsafeCell` in constants
+        // Special check preventing `UnsafeCell` in the inner part of constants
         if let Some(def) = op.layout.ty.ty_adt_def() {
-            if matches!(self.ctfe_mode, Some(CtfeValidationMode::Const { inner: true }))
+            if matches!(self.ctfe_mode, Some(CtfeValidationMode::Const { inner: true, .. }))
                 && Some(def.did) == self.ecx.tcx.lang_items().unsafe_cell_type()
             {
                 throw_validation_failure!(self.path, { "`UnsafeCell` in a `const`" });