Update now-more-precise operation with a preciser message

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

@@ -242,8 +242,8 @@ fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<
             ccx.tcx.sess,
             span,
             E0492,
-            "cannot borrow a constant which may contain \
-            interior mutability, create a static instead"
+            "this borrow to an interior mutable value may end up in the final value of this {}",
+            ccx.const_kind(),
         )
     }
 }

src/test/ui/consts/partial_qualif.rs

@@ -3,7 +3,7 @@
 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 cannot borrow a constant which may contain interior mutability
+    &{a} //~ ERROR borrow to an interior mutable value may end up in the final value
 };
 
 fn main() {}

src/test/ui/consts/partial_qualif.stderr

@@ -1,4 +1,4 @@
-error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+error[E0492]: this borrow to an interior mutable value may end up in the final value of this constant
   --> $DIR/partial_qualif.rs:6:5
    |
 LL |     &{a}

src/test/ui/consts/qualif_overwrite.rs

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

src/test/ui/consts/qualif_overwrite.stderr

@@ -1,4 +1,4 @@
-error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+error[E0492]: this borrow to an interior mutable value may end up in the final value of this constant
   --> $DIR/qualif_overwrite.rs:10:5
    |
 LL |     &{a}

src/test/ui/consts/qualif_overwrite_2.rs

@@ -5,7 +5,7 @@
 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 cannot borrow a constant which may contain interior mutability
+    &{a.0} //~ ERROR borrow to an interior mutable value may end up in the final value
 };
 
 fn main() {}

src/test/ui/consts/qualif_overwrite_2.stderr

@@ -1,4 +1,4 @@
-error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+error[E0492]: this borrow to an interior mutable value may end up in the final value of this constant
   --> $DIR/qualif_overwrite_2.rs:8:5
    |
 LL |     &{a.0}

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

@@ -1,10 +1,10 @@
-error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+error[E0492]: this borrow to an interior mutable value may end up in the final value of this constant
   --> $DIR/E0492.rs:4:33
    |
 LL | const B: &'static AtomicUsize = &A;
    |                                 ^^
 
-error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+error[E0492]: this borrow to an interior mutable value may end up in the final value of this static
   --> $DIR/E0492.rs:5:34
    |
 LL | static C: &'static AtomicUsize = &A;

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

@@ -2,13 +2,13 @@
 
 const A: UnsafeCell<usize> = UnsafeCell::new(1);
 const B: &'static UnsafeCell<usize> = &A;
-//~^ ERROR: may contain interior mutability
+//~^ ERROR: borrow to an interior mutable value
 
 struct C { a: UnsafeCell<usize> }
 const D: C = C { a: UnsafeCell::new(1) };
 const E: &'static UnsafeCell<usize> = &D.a;
-//~^ ERROR: may contain interior mutability
+//~^ ERROR: borrow to an interior mutable value
 const F: &'static C = &D;
-//~^ ERROR: may contain interior mutability
+//~^ ERROR: borrow to an interior mutable value
 
 fn main() {}

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

@@ -1,16 +1,16 @@
-error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+error[E0492]: this borrow to an interior mutable value may end up in the final value of this constant
   --> $DIR/issue-17718-const-borrow.rs:4:39
    |
 LL | const B: &'static UnsafeCell<usize> = &A;
    |                                       ^^
 
-error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+error[E0492]: this borrow to an interior mutable value may end up in the final value of this constant
   --> $DIR/issue-17718-const-borrow.rs:9:39
    |
 LL | const E: &'static UnsafeCell<usize> = &D.a;
    |                                       ^^^^
 
-error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+error[E0492]: this borrow to an interior mutable value may end up in the final value of this constant
   --> $DIR/issue-17718-const-borrow.rs:11:23
    |
 LL | const F: &'static C = &D;