Allow references to interior mutable data behind a feature gate

Author: oli-obk

compiler/rustc_error_codes/src/error_codes/E0492.md

@@ -3,10 +3,11 @@ A borrow of a constant containing interior mutability was attempted.
 Erroneous code example:
 
 ```compile_fail,E0492
+#![feature(const_refs_to_cell)]
 use std::sync::atomic::AtomicUsize;
 
 const A: AtomicUsize = AtomicUsize::new(0);
-static B: &'static AtomicUsize = &A;
+const B: &'static AtomicUsize = &A;
 // error: cannot borrow a constant which may contain interior mutability,
 //        create a static instead
 ```
@@ -18,7 +19,7 @@ can't be changed via a shared `&` pointer, but interior mutability would allow
 it. That is, a constant value could be mutated. On the other hand, a `static` is
 explicitly a single memory location, which can be mutated at will.
 
-So, in order to solve this error, either use statics which are `Sync`:
+So, in order to solve this error, use statics which are `Sync`:
 
 ```
 use std::sync::atomic::AtomicUsize;
@@ -30,6 +31,7 @@ static B: &'static AtomicUsize = &A; // ok!
 You can also have this error while using a cell type:
 
 ```compile_fail,E0492
+#![feature(const_refs_to_cell)]
 use std::cell::Cell;
 
 const A: Cell<usize> = Cell::new(1);

compiler/rustc_feature/src/active.rs

@@ -626,6 +626,9 @@ declare_features! (
     /// Allows const generics to have default values (e.g. `struct Foo<const N: usize = 3>(...);`).
     (active, const_generics_defaults, "1.51.0", Some(44580), None),
 
+    /// Allows references to types with interior mutability within constants
+    (active, const_refs_to_cell, "1.51.0", Some(80384), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_mir/src/transform/check_consts/ops.rs

@@ -208,9 +208,29 @@ impl NonConstOp for LiveDrop {
     }
 }
 
+#[derive(Debug)]
+pub struct CellBorrowBehindRef;
+impl NonConstOp for CellBorrowBehindRef {
+    fn status_in_item(&self, _: &ConstCx<'_, '_>) -> Status {
+        Status::Unstable(sym::const_refs_to_cell)
+    }
+    fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
+        feature_err(
+            &ccx.tcx.sess.parse_sess,
+            sym::const_refs_to_cell,
+            span,
+            "cannot borrow here, since the borrowed element may contain interior mutability",
+        )
+    }
+}
+
 #[derive(Debug)]
 pub struct CellBorrow;
 impl NonConstOp for CellBorrow {
+    fn importance(&self) -> DiagnosticImportance {
+        // The problematic cases will already emit a `CellBorrowBehindRef`
+        DiagnosticImportance::Secondary
+    }
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
         struct_span_err!(
             ccx.tcx.sess,

compiler/rustc_mir/src/transform/check_consts/validation.rs

@@ -3,6 +3,7 @@
 use rustc_errors::{struct_span_err, Applicability, Diagnostic, ErrorReported};
 use rustc_hir::def_id::DefId;
 use rustc_hir::{self as hir, HirId, LangItem};
+use rustc_index::bit_set::BitSet;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_infer::traits::{ImplSource, Obligation, ObligationCause};
 use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
@@ -188,6 +189,9 @@ pub struct Validator<'mir, 'tcx> {
     /// The span of the current statement.
     span: Span,
 
+    /// A set that stores for each local whether it has a `StorageDead` for it somewhere.
+    local_has_storage_dead: Option<BitSet<Local>>,
+
     error_emitted: Option<ErrorReported>,
     secondary_errors: Vec<Diagnostic>,
 }
@@ -206,6 +210,7 @@ impl Validator<'mir, 'tcx> {
             span: ccx.body.span,
             ccx,
             qualifs: Default::default(),
+            local_has_storage_dead: None,
             error_emitted: None,
             secondary_errors: Vec::new(),
         }
@@ -282,6 +287,27 @@ impl Validator<'mir, 'tcx> {
         }
     }
 
+    fn local_has_storage_dead(&mut self, local: Local) -> bool {
+        let ccx = self.ccx;
+        self.local_has_storage_dead
+            .get_or_insert_with(|| {
+                struct StorageDeads {
+                    locals: BitSet<Local>,
+                }
+                impl Visitor<'tcx> for StorageDeads {
+                    fn visit_statement(&mut self, stmt: &Statement<'tcx>, _: Location) {
+                        if let StatementKind::StorageDead(l) = stmt.kind {
+                            self.locals.insert(l);
+                        }
+                    }
+                }
+                let mut v = StorageDeads { locals: BitSet::new_empty(ccx.body.local_decls.len()) };
+                v.visit_body(ccx.body);
+                v.locals
+            })
+            .contains(local)
+    }
+
     pub fn qualifs_in_return_place(&mut self) -> ConstQualifs {
         self.qualifs.in_return_place(self.ccx, self.error_emitted)
     }
@@ -556,7 +582,13 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
                 );
 
                 if borrowed_place_has_mut_interior {
-                    self.check_op(ops::CellBorrow);
+                    // Locals without StorageDead follow the "trailing expression" rule, meaning
+                    // they are essentially anonymous static items themselves.
+                    if self.local_has_storage_dead(place.local) {
+                        self.check_op(ops::CellBorrowBehindRef);
+                    } else {
+                        self.check_op(ops::CellBorrow);
+                    }
                 }
             }
 

compiler/rustc_span/src/symbol.rs

@@ -381,6 +381,7 @@ symbols! {
         const_ptr,
         const_raw_ptr_deref,
         const_raw_ptr_to_usize_cast,
+        const_refs_to_cell,
         const_slice_ptr,
         const_trait_bound_opt_out,
         const_trait_impl,

src/test/ui/consts/const-address-of-interior-mut.stderr

@@ -1,27 +1,39 @@
-error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+error[E0658]: cannot borrow here, since the borrowed element may contain interior mutability
   --> $DIR/const-address-of-interior-mut.rs:5:39
    |
 LL | const A: () = { let x = Cell::new(2); &raw const x; };
    |                                       ^^^^^^^^^^^^
+   |
+   = note: see issue #80384 <https://github.com/rust-lang/rust/issues/80384> for more information
+   = help: add `#![feature(const_refs_to_cell)]` to the crate attributes to enable
 
-error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+error[E0658]: cannot borrow here, since the borrowed element may contain interior mutability
   --> $DIR/const-address-of-interior-mut.rs:7:40
    |
 LL | static B: () = { let x = Cell::new(2); &raw const x; };
    |                                        ^^^^^^^^^^^^
+   |
+   = note: see issue #80384 <https://github.com/rust-lang/rust/issues/80384> for more information
+   = help: add `#![feature(const_refs_to_cell)]` to the crate attributes to enable
 
-error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+error[E0658]: cannot borrow here, since the borrowed element may contain interior mutability
   --> $DIR/const-address-of-interior-mut.rs:9:44
    |
 LL | static mut C: () = { let x = Cell::new(2); &raw const x; };
    |                                            ^^^^^^^^^^^^
+   |
+   = note: see issue #80384 <https://github.com/rust-lang/rust/issues/80384> for more information
+   = help: add `#![feature(const_refs_to_cell)]` to the crate attributes to enable
 
-error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+error[E0658]: cannot borrow here, since the borrowed element may contain interior mutability
   --> $DIR/const-address-of-interior-mut.rs:13:13
    |
 LL |     let y = &raw const x;
    |             ^^^^^^^^^^^^
+   |
+   = note: see issue #80384 <https://github.com/rust-lang/rust/issues/80384> for more information
+   = help: add `#![feature(const_refs_to_cell)]` to the crate attributes to enable
 
 error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0492`.
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/consts/const-multi-ref.rs

@@ -13,7 +13,7 @@ const _: i32 = {
 
 const _: std::cell::Cell<i32> = {
     let mut a = std::cell::Cell::new(5);
-    let p = &a; //~ ERROR cannot borrow a constant which may contain interior mutability
+    let p = &a; //~ ERROR borrowed element may contain interior mutability
 
     let reborrow = {p};
     let pp = &reborrow;

src/test/ui/consts/const-multi-ref.stderr

@@ -4,13 +4,16 @@ error[E0764]: mutable references are not allowed in constants
 LL |     let p = &mut a;
    |             ^^^^^^ `&mut` is only allowed in `const fn`
 
-error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+error[E0658]: cannot borrow here, since the borrowed element may contain interior mutability
   --> $DIR/const-multi-ref.rs:16:13
    |
 LL |     let p = &a;
    |             ^^
+   |
+   = note: see issue #80384 <https://github.com/rust-lang/rust/issues/80384> for more information
+   = help: add `#![feature(const_refs_to_cell)]` to the crate attributes to enable
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0492, E0764.
-For more information about an error, try `rustc --explain E0492`.
+Some errors have detailed explanations: E0658, E0764.
+For more information about an error, try `rustc --explain E0658`.

src/test/ui/consts/partial_qualif.rs

@@ -1,3 +1,5 @@
+#![feature(const_refs_to_cell)]
+
 use std::cell::Cell;
 
 const FOO: &(Cell<usize>, bool) = {

src/test/ui/consts/partial_qualif.stderr

@@ -1,5 +1,5 @@
 error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
-  --> $DIR/partial_qualif.rs:6:5
+  --> $DIR/partial_qualif.rs:8:5
    |
 LL |     &{a}
    |     ^^^^