Rollup merge of #42490 - gaurikholkar:master, r=eddyb

Changing error message from `contains interior mutability` to `may contain interior mutability`

Fixes #40313 . I have changed the message from `contains interior mutability` to `may contain interior mutability` for the following example
```
use std::cell::Cell;
use std::panic::catch_unwind;
fn main() {
    let mut x = Cell::new(22);
    catch_unwind(|| { x.set(23); });
}
```
which has been added as a ui test.

Also, the message [here](https://github.com/gaurikholkar/rust/blob/master/src/librustc_mir/transform/qualify_consts.rs#L666) and it's respective `compile-fail` test have been modified.

cc @nikomatsakis  @Mark-Simulacrum  @eddyb

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libstd/panic.rs b/src/libstd/panic.rs
index f99634ecac2..58356bc43ee 100644
--- a/src/libstd/panic.rs
+++ b/src/libstd/panic.rs
@@ -112,7 +112,7 @@ pub trait UnwindSafe {}
 /// This is a "helper marker trait" used to provide impl blocks for the
 /// `UnwindSafe` trait, for more information see that documentation.
 #[stable(feature = "catch_unwind", since = "1.9.0")]
-#[rustc_on_unimplemented = "the type {Self} contains interior mutability \
+#[rustc_on_unimplemented = "the type {Self} may contain interior mutability \
                             and a reference may not be safely transferrable \
                             across a catch_unwind boundary"]
 pub trait RefUnwindSafe {}