error[E0658]: mutable references are not allowed in constant functions
  --> $DIR/mutable_borrow.rs:3:13
   |
LL |     let b = &mut a;
   |             ^^^^^^
   |
   = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error[E0658]: mutable references are not allowed in constant functions
  --> $DIR/mutable_borrow.rs:12:17
   |
LL |         let b = &mut a;
   |                 ^^^^^^
   |
   = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0658`.