Allow references to interior mutable data behind a feature gate

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

@@ -238,13 +238,32 @@ impl NonConstOp for TransientCellBorrow {
 pub struct CellBorrow;
 impl NonConstOp for CellBorrow {
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
-        struct_span_err!(
+        let mut err = struct_span_err!(
             ccx.tcx.sess,
             span,
             E0492,
-            "this borrow to an interior mutable value may end up in the final value of this {}",
+            "{}s cannot refer to interior mutable data",
             ccx.const_kind(),
-        )
+        );
+        err.span_label(
+            span,
+            format!("this borrow of an interior mutable value may end up in the final value"),
+        );
+        if let hir::ConstContext::Static(_) = ccx.const_kind() {
+            err.help(
+                "To fix this, the value can be extracted to separate \
+                `static` and then referenced.",
+            );
+        }
+        if ccx.tcx.sess.teach(&err.get_code().unwrap()) {
+            err.note(
+                "A constant containing interior mutable data behind a reference can allow you
+                 to modify that data. This would make multiple uses of a constant to be able to
+                 see different values and allow one to escape the `Send` and `Sync` requirements
+                 for shared mutable data, which is unsound.",
+            );
+        }
+        err
     }
 }
 

src/test/ui/consts/partial_qualif.rs

@@ -3,7 +3,7 @@ use std::cell::Cell;
 const FOO: &(Cell<usize>, bool) = {
     let mut a = (Cell::new(0), false);
     a.1 = true; // sets `qualif(a)` to `qualif(a) | qualif(true)`
-    &{a} //~ ERROR borrow to an interior mutable value may end up in the final value
+    &{a} //~ ERROR cannot refer to interior mutable
 };
 
 fn main() {}

src/test/ui/consts/partial_qualif.stderr

@@ -1,8 +1,8 @@
-error[E0492]: this borrow to an interior mutable value may end up in the final value of this constant
+error[E0492]: constants cannot refer to interior mutable data
   --> $DIR/partial_qualif.rs:6:5
    |
 LL |     &{a}
-   |     ^^^^
+   |     ^^^^ this borrow of an interior mutable value may end up in the final value
 
 error: aborting due to previous error
 

src/test/ui/consts/qualif_overwrite.rs

@@ -7,7 +7,7 @@ use std::cell::Cell;
 const FOO: &Option<Cell<usize>> = {
     let mut a = Some(Cell::new(0));
     a = None; // sets `qualif(a)` to `qualif(a) | qualif(None)`
-    &{a} //~ ERROR borrow to an interior mutable value may end up in the final value
+    &{a} //~ ERROR cannot refer to interior mutable
 };
 
 fn main() {}

src/test/ui/consts/qualif_overwrite.stderr

@@ -1,8 +1,8 @@
-error[E0492]: this borrow to an interior mutable value may end up in the final value of this constant
+error[E0492]: constants cannot refer to interior mutable data
   --> $DIR/qualif_overwrite.rs:10:5
    |
 LL |     &{a}
-   |     ^^^^
+   |     ^^^^ this borrow of an interior mutable value may end up in the final value
 
 error: aborting due to previous error
 

src/test/ui/consts/qualif_overwrite_2.rs

@@ -5,7 +5,7 @@ use std::cell::Cell;
 const FOO: &Option<Cell<usize>> = {
     let mut a = (Some(Cell::new(0)),);
     a.0 = None; // sets `qualif(a)` to `qualif(a) | qualif(None)`
-    &{a.0} //~ ERROR borrow to an interior mutable value may end up in the final value
+    &{a.0} //~ ERROR cannot refer to interior mutable
 };
 
 fn main() {}

src/test/ui/consts/qualif_overwrite_2.stderr

@@ -1,8 +1,8 @@
-error[E0492]: this borrow to an interior mutable value may end up in the final value of this constant
+error[E0492]: constants cannot refer to interior mutable data
   --> $DIR/qualif_overwrite_2.rs:8:5
    |
 LL |     &{a.0}
-   |     ^^^^^^
+   |     ^^^^^^ this borrow of an interior mutable value may end up in the final value
 
 error: aborting due to previous error
 

src/test/ui/error-codes/E0492.stderr

@@ -1,14 +1,16 @@
-error[E0492]: this borrow to an interior mutable value may end up in the final value of this constant
+error[E0492]: constants cannot refer to interior mutable data
   --> $DIR/E0492.rs:4:33
    |
 LL | const B: &'static AtomicUsize = &A;
-   |                                 ^^
+   |                                 ^^ this borrow of an interior mutable value may end up in the final value
 
-error[E0492]: this borrow to an interior mutable value may end up in the final value of this static
+error[E0492]: statics cannot refer to interior mutable data
   --> $DIR/E0492.rs:5:34
    |
 LL | static C: &'static AtomicUsize = &A;
-   |                                  ^^
+   |                                  ^^ this borrow of an interior mutable value may end up in the final value
+   |
+   = help: To fix this, the value can be extracted to separate `static` and then referenced.
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-17718-const-borrow.rs

@@ -2,13 +2,13 @@ use std::cell::UnsafeCell;
 
 const A: UnsafeCell<usize> = UnsafeCell::new(1);
 const B: &'static UnsafeCell<usize> = &A;
-//~^ ERROR: borrow to an interior mutable value
+//~^ ERROR: cannot refer to interior mutable
 
 struct C { a: UnsafeCell<usize> }
 const D: C = C { a: UnsafeCell::new(1) };
 const E: &'static UnsafeCell<usize> = &D.a;
-//~^ ERROR: borrow to an interior mutable value
+//~^ ERROR: cannot refer to interior mutable
 const F: &'static C = &D;
-//~^ ERROR: borrow to an interior mutable value
+//~^ ERROR: cannot refer to interior mutable
 
 fn main() {}

src/test/ui/issues/issue-17718-const-borrow.stderr

@@ -1,20 +1,20 @@
-error[E0492]: this borrow to an interior mutable value may end up in the final value of this constant
+error[E0492]: constants cannot refer to interior mutable data
   --> $DIR/issue-17718-const-borrow.rs:4:39
    |
 LL | const B: &'static UnsafeCell<usize> = &A;
-   |                                       ^^
+   |                                       ^^ this borrow of an interior mutable value may end up in the final value
 
-error[E0492]: this borrow to an interior mutable value may end up in the final value of this constant
+error[E0492]: constants cannot refer to interior mutable data
   --> $DIR/issue-17718-const-borrow.rs:9:39
    |
 LL | const E: &'static UnsafeCell<usize> = &D.a;
-   |                                       ^^^^
+   |                                       ^^^^ this borrow of an interior mutable value may end up in the final value
 
-error[E0492]: this borrow to an interior mutable value may end up in the final value of this constant
+error[E0492]: constants cannot refer to interior mutable data
   --> $DIR/issue-17718-const-borrow.rs:11:23
    |
 LL | const F: &'static C = &D;
-   |                       ^^
+   |                       ^^ this borrow of an interior mutable value may end up in the final value
 
 error: aborting due to 3 previous errors