Warning when compiling with rustc 1.61 (or later, probably)

[itamarst]

warning: cannot borrow `*self` as mutable because it is also borrowed as immutable
    --> src/validator/json.rs:1989:19
     |
1900 |         Value::Object(o) => match &self.occurrence {
     |                                   ---------------- immutable borrow occurs here
...
1989 | /                   self.add_error(format!(
1990 | |                     "object must contain no more than {} entries of key of type {}",
1991 | |                     upper, ident,
     | |                     ----- immutable borrow later used here
1992 | |                   ));
     | |____________________^ mutable borrow occurs here
     |
     = warning: this borrowing pattern was not meant to be accepted, and may become a hard error in the future
     = note: for more information, see issue #59159 <https://github.com/rust-lang/rust/issues/59159>

The same warning occurs multiple times for the same code pattern used elsewhere.

[anweiss]

See rust-lang/rust#59159. I believe this warning went away with 1.62.0