Try to find cyclic data dependencies during const-checking

src/librustc_mir/transform/check_consts/validation.rs

@@ -3,7 +3,6 @@
 use rustc_errors::struct_span_err;
 use rustc_hir::lang_items;
 use rustc_hir::{def_id::DefId, HirId};
-use rustc_index::bit_set::BitSet;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
@@ -28,70 +27,100 @@ use crate::dataflow::{self, Analysis};
 // We are using `MaybeMutBorrowedLocals` as a proxy for whether an item may have been mutated
 // through a pointer prior to the given point. This is okay even though `MaybeMutBorrowedLocals`
 // kills locals upon `StorageDead` because a local will never be used after a `StorageDead`.
-pub type IndirectlyMutableResults<'mir, 'tcx> =
+type IndirectlyMutableResults<'mir, 'tcx> =
     dataflow::ResultsCursor<'mir, 'tcx, MaybeMutBorrowedLocals<'mir, 'tcx>>;
 
-struct QualifCursor<'a, 'mir, 'tcx, Q: Qualif> {
-    cursor: dataflow::ResultsCursor<'mir, 'tcx, FlowSensitiveAnalysis<'a, 'mir, 'tcx, Q>>,
-    in_any_value_of_ty: BitSet<Local>,
-}
-
-impl<Q: Qualif> QualifCursor<'a, 'mir, 'tcx, Q> {
-    pub fn new(q: Q, ccx: &'a ConstCx<'mir, 'tcx>) -> Self {
-        let cursor = FlowSensitiveAnalysis::new(q, ccx)
-            .into_engine(ccx.tcx, ccx.body, ccx.def_id)
-            .iterate_to_fixpoint()
-            .into_results_cursor(ccx.body);
-
-        let mut in_any_value_of_ty = BitSet::new_empty(ccx.body.local_decls.len());
-        for (local, decl) in ccx.body.local_decls.iter_enumerated() {
-            if Q::in_any_value_of_ty(ccx, decl.ty) {
-                in_any_value_of_ty.insert(local);
-            }
-        }
-
-        QualifCursor { cursor, in_any_value_of_ty }
-    }
-}
+type QualifResults<'mir, 'tcx, Q> =
+    dataflow::ResultsCursor<'mir, 'tcx, FlowSensitiveAnalysis<'mir, 'mir, 'tcx, Q>>;
 
-pub struct Qualifs<'a, 'mir, 'tcx> {
-    has_mut_interior: QualifCursor<'a, 'mir, 'tcx, HasMutInterior>,
-    needs_drop: QualifCursor<'a, 'mir, 'tcx, NeedsDrop>,
-    indirectly_mutable: IndirectlyMutableResults<'mir, 'tcx>,
+#[derive(Default)]
+pub struct Qualifs<'mir, 'tcx> {
+    has_mut_interior: Option<QualifResults<'mir, 'tcx, HasMutInterior>>,
+    needs_drop: Option<QualifResults<'mir, 'tcx, NeedsDrop>>,
+    indirectly_mutable: Option<IndirectlyMutableResults<'mir, 'tcx>>,
 }
 
-impl Qualifs<'a, 'mir, 'tcx> {
-    fn indirectly_mutable(&mut self, local: Local, location: Location) -> bool {
-        self.indirectly_mutable.seek_before(location);
-        self.indirectly_mutable.get().contains(local)
+impl Qualifs<'mir, 'tcx> {
+    fn indirectly_mutable(
+        &mut self,
+        ccx: &'mir ConstCx<'mir, 'tcx>,
+        local: Local,
+        location: Location,
+    ) -> bool {
+        let indirectly_mutable = self.indirectly_mutable.get_or_insert_with(|| {
+            let ConstCx { tcx, body, def_id, param_env, .. } = *ccx;
+
+            // We can use `unsound_ignore_borrow_on_drop` here because custom drop impls are not
+            // allowed in a const.
+            //
+            // FIXME(ecstaticmorse): Someday we want to allow custom drop impls. How do we do this
+            // without breaking stable code?
+            MaybeMutBorrowedLocals::mut_borrows_only(tcx, &body, param_env)
+                .unsound_ignore_borrow_on_drop()
+                .into_engine(tcx, &body, def_id)
+                .iterate_to_fixpoint()
+                .into_results_cursor(&body)
+        });
+
+        indirectly_mutable.seek_before(location);
+        indirectly_mutable.get().contains(local)
     }
 
     /// Returns `true` if `local` is `NeedsDrop` at the given `Location`.
     ///
     /// Only updates the cursor if absolutely necessary
-    fn needs_drop(&mut self, local: Local, location: Location) -> bool {
-        if !self.needs_drop.in_any_value_of_ty.contains(local) {
+    fn needs_drop(
+        &mut self,
+        ccx: &'mir ConstCx<'mir, 'tcx>,
+        local: Local,
+        location: Location,
+    ) -> bool {
+        let ty = ccx.body.local_decls[local].ty;
+        if !NeedsDrop::in_any_value_of_ty(ccx, ty) {
             return false;
         }
 
-        self.needs_drop.cursor.seek_before(location);
-        self.needs_drop.cursor.get().contains(local) || self.indirectly_mutable(local, location)
+        let needs_drop = self.needs_drop.get_or_insert_with(|| {
+            let ConstCx { tcx, body, def_id, .. } = *ccx;
+
+            FlowSensitiveAnalysis::new(NeedsDrop, ccx)
+                .into_engine(tcx, &body, def_id)
+                .iterate_to_fixpoint()
+                .into_results_cursor(&body)
+        });
+
+        needs_drop.seek_before(location);
+        needs_drop.get().contains(local) || self.indirectly_mutable(ccx, local, location)
     }
 
     /// Returns `true` if `local` is `HasMutInterior` at the given `Location`.
     ///
     /// Only updates the cursor if absolutely necessary.
-    fn has_mut_interior(&mut self, local: Local, location: Location) -> bool {
-        if !self.has_mut_interior.in_any_value_of_ty.contains(local) {
+    fn has_mut_interior(
+        &mut self,
+        ccx: &'mir ConstCx<'mir, 'tcx>,
+        local: Local,
+        location: Location,
+    ) -> bool {
+        let ty = ccx.body.local_decls[local].ty;
+        if !HasMutInterior::in_any_value_of_ty(ccx, ty) {
             return false;
         }
 
-        self.has_mut_interior.cursor.seek_before(location);
-        self.has_mut_interior.cursor.get().contains(local)
-            || self.indirectly_mutable(local, location)
+        let has_mut_interior = self.has_mut_interior.get_or_insert_with(|| {
+            let ConstCx { tcx, body, def_id, .. } = *ccx;
+
+            FlowSensitiveAnalysis::new(HasMutInterior, ccx)
+                .into_engine(tcx, &body, def_id)
+                .iterate_to_fixpoint()
+                .into_results_cursor(&body)
+        });
+
+        has_mut_interior.seek_before(location);
+        has_mut_interior.get().contains(local) || self.indirectly_mutable(ccx, local, location)
     }
 
-    fn in_return_place(&mut self, ccx: &ConstCx<'_, 'tcx>) -> ConstQualifs {
+    fn in_return_place(&mut self, ccx: &'mir ConstCx<'mir, 'tcx>) -> ConstQualifs {
         // Find the `Return` terminator if one exists.
         //
         // If no `Return` terminator exists, this MIR is divergent. Just return the conservative
@@ -114,49 +143,31 @@ impl Qualifs<'a, 'mir, 'tcx> {
         let return_loc = ccx.body.terminator_loc(return_block);
 
         ConstQualifs {
-            needs_drop: self.needs_drop(RETURN_PLACE, return_loc),
-            has_mut_interior: self.has_mut_interior(RETURN_PLACE, return_loc),
+            needs_drop: self.needs_drop(ccx, RETURN_PLACE, return_loc),
+            has_mut_interior: self.has_mut_interior(ccx, RETURN_PLACE, return_loc),
         }
     }
 }
 
-pub struct Validator<'a, 'mir, 'tcx> {
-    ccx: &'a ConstCx<'mir, 'tcx>,
-    qualifs: Qualifs<'a, 'mir, 'tcx>,
+pub struct Validator<'mir, 'tcx> {
+    ccx: &'mir ConstCx<'mir, 'tcx>,
+    qualifs: Qualifs<'mir, 'tcx>,
 
     /// The span of the current statement.
     span: Span,
 }
 
-impl Deref for Validator<'_, 'mir, 'tcx> {
+impl Deref for Validator<'mir, 'tcx> {
     type Target = ConstCx<'mir, 'tcx>;
 
     fn deref(&self) -> &Self::Target {
         &self.ccx
     }
 }
 
-impl Validator<'a, 'mir, 'tcx> {
-    pub fn new(ccx: &'a ConstCx<'mir, 'tcx>) -> Self {
-        let ConstCx { tcx, body, def_id, param_env, .. } = *ccx;
-
-        let needs_drop = QualifCursor::new(NeedsDrop, ccx);
-        let has_mut_interior = QualifCursor::new(HasMutInterior, ccx);
-
-        // We can use `unsound_ignore_borrow_on_drop` here because custom drop impls are not
-        // allowed in a const.
-        //
-        // FIXME(ecstaticmorse): Someday we want to allow custom drop impls. How do we do this
-        // without breaking stable code?
-        let indirectly_mutable = MaybeMutBorrowedLocals::mut_borrows_only(tcx, body, param_env)
-            .unsound_ignore_borrow_on_drop()
-            .into_engine(tcx, body, def_id)
-            .iterate_to_fixpoint()
-            .into_results_cursor(body);
-
-        let qualifs = Qualifs { needs_drop, has_mut_interior, indirectly_mutable };
-
-        Validator { span: ccx.body.span, ccx, qualifs }
+impl Validator<'mir, 'tcx> {
+    pub fn new(ccx: &'mir ConstCx<'mir, 'tcx>) -> Self {
+        Validator { span: ccx.body.span, ccx, qualifs: Default::default() }
     }
 
     pub fn check_body(&mut self) {
@@ -239,7 +250,7 @@ impl Validator<'a, 'mir, 'tcx> {
     }
 }
 
-impl Visitor<'tcx> for Validator<'_, 'mir, 'tcx> {
+impl Visitor<'tcx> for Validator<'mir, 'tcx> {
     fn visit_basic_block_data(&mut self, bb: BasicBlock, block: &BasicBlockData<'tcx>) {
         trace!("visit_basic_block_data: bb={:?} is_cleanup={:?}", bb, block.is_cleanup);
 
@@ -345,7 +356,7 @@ impl Visitor<'tcx> for Validator<'_, 'mir, 'tcx> {
             | Rvalue::AddressOf(Mutability::Not, ref place) => {
                 let borrowed_place_has_mut_interior = qualifs::in_place::<HasMutInterior, _>(
                     &self.ccx,
-                    &mut |local| self.qualifs.has_mut_interior(local, location),
+                    &mut |local| self.qualifs.has_mut_interior(self.ccx, local, location),
                     place.as_ref(),
                 );
 
@@ -395,14 +406,31 @@ impl Visitor<'tcx> for Validator<'_, 'mir, 'tcx> {
         );
     }
 
-    fn visit_operand(&mut self, op: &Operand<'tcx>, location: Location) {
-        self.super_operand(op, location);
-        if let Operand::Constant(c) = op {
-            if let Some(def_id) = c.check_static_ptr(self.tcx) {
-                self.check_static(def_id, self.span);
+    fn visit_constant(&mut self, constant: &Constant<'tcx>, location: Location) {
+        self.super_constant(constant, location);
+
+        if let ty::ConstKind::Unevaluated(def_id, _, promoted) = constant.literal.val {
+            assert!(promoted.is_none(), "Const-checking should run before promotion");
+
+            // If a cyclic data dependency exists within a const initializer, try to find
+            // it during const-checking. This is important because MIR optimizations could
+            // eliminate a cycle before const-eval runs. See #71078 for an example of this.
+            //
+            // FIXME: This means we don't look for cycles involving associated constants, but we
+            // should handle fully monomorphized ones here at least.
+            if self.tcx.trait_of_item(def_id).is_none() {
+                let _ = self.tcx.at(self.span).mir_const_qualif(def_id);
             }
         }
+
+        if let Some(def_id) = constant.check_static_ptr(self.tcx) {
+            self.check_static(def_id, self.span);
+
+            // NOTE: Because we are allowed to refer to the address of a static within its
+            // initializer, we don't try to trigger cycle errors every time we see a static.
+        }
     }
+
     fn visit_projection_elem(
         &mut self,
         place_local: Local,
@@ -571,7 +599,7 @@ impl Visitor<'tcx> for Validator<'_, 'mir, 'tcx> {
                 let needs_drop = if let Some(local) = dropped_place.as_local() {
                     // Use the span where the local was declared as the span of the drop error.
                     err_span = self.body.local_decls[local].source_info.span;
-                    self.qualifs.needs_drop(local, location)
+                    self.qualifs.needs_drop(self.ccx, local, location)
                 } else {
                     true
                 };

src/test/ui/issues/issue-17252.stderr

@@ -1,11 +1,22 @@
-error[E0391]: cycle detected when const checking `FOO`
-  --> $DIR/issue-17252.rs:1:20
+error[E0391]: cycle detected when normalizing `FOO`
+   |
+note: ...which requires const-evaluating + checking `FOO`...
+  --> $DIR/issue-17252.rs:1:1
+   |
+LL | const FOO: usize = FOO;
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires const-evaluating + checking `FOO`...
+  --> $DIR/issue-17252.rs:1:1
    |
 LL | const FOO: usize = FOO;
-   |                    ^^^
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires const-evaluating `FOO`...
+  --> $DIR/issue-17252.rs:1:1
    |
-   = note: ...which again requires const checking `FOO`, completing the cycle
-note: cycle used when const checking `main::{{constant}}#0`
+LL | const FOO: usize = FOO;
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+   = note: ...which again requires normalizing `FOO`, completing the cycle
+note: cycle used when const-evaluating `main::{{constant}}#0`
   --> $DIR/issue-17252.rs:4:18
    |
 LL |     let _x: [u8; FOO]; // caused stack overflow prior to fix

src/test/ui/issues/issue-23302-1.stderr

@@ -1,15 +1,26 @@
-error[E0391]: cycle detected when const checking `X::A::{{constant}}#0`
+error[E0391]: cycle detected when const-evaluating + checking `X::A::{{constant}}#0`
   --> $DIR/issue-23302-1.rs:4:9
    |
 LL |     A = X::A as isize,
    |         ^^^^^^^^^^^^^
    |
-   = note: ...which again requires const checking `X::A::{{constant}}#0`, completing the cycle
-note: cycle used when processing `X::A::{{constant}}#0`
+note: ...which requires const-evaluating + checking `X::A::{{constant}}#0`...
   --> $DIR/issue-23302-1.rs:4:9
    |
 LL |     A = X::A as isize,
    |         ^^^^^^^^^^^^^
+note: ...which requires const-evaluating `X::A::{{constant}}#0`...
+  --> $DIR/issue-23302-1.rs:4:9
+   |
+LL |     A = X::A as isize,
+   |         ^^^^^^^^^^^^^
+   = note: ...which requires normalizing `X::A as isize`...
+   = note: ...which again requires const-evaluating + checking `X::A::{{constant}}#0`, completing the cycle
+note: cycle used when collecting item types in top-level module
+  --> $DIR/issue-23302-1.rs:3:1
+   |
+LL | enum X {
+   | ^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-23302-2.stderr

@@ -1,15 +1,26 @@
-error[E0391]: cycle detected when const checking `Y::A::{{constant}}#0`
+error[E0391]: cycle detected when const-evaluating + checking `Y::A::{{constant}}#0`
   --> $DIR/issue-23302-2.rs:4:9
    |
 LL |     A = Y::B as isize,
    |         ^^^^^^^^^^^^^
    |
-   = note: ...which again requires const checking `Y::A::{{constant}}#0`, completing the cycle
-note: cycle used when processing `Y::A::{{constant}}#0`
+note: ...which requires const-evaluating + checking `Y::A::{{constant}}#0`...
   --> $DIR/issue-23302-2.rs:4:9
    |
 LL |     A = Y::B as isize,
    |         ^^^^^^^^^^^^^
+note: ...which requires const-evaluating `Y::A::{{constant}}#0`...
+  --> $DIR/issue-23302-2.rs:4:9
+   |
+LL |     A = Y::B as isize,
+   |         ^^^^^^^^^^^^^
+   = note: ...which requires normalizing `Y::B as isize`...
+   = note: ...which again requires const-evaluating + checking `Y::A::{{constant}}#0`, completing the cycle
+note: cycle used when collecting item types in top-level module
+  --> $DIR/issue-23302-2.rs:3:1
+   |
+LL | enum Y {
+   | ^^^^^^
 
 error: aborting due to previous error