cannot borrow `stream` as mutable, as it is not declared as mutable

double-mut-borrow.rs

fn outer(data: &mut Vec<u32>) { // unexpected error on this line
    inner(&mut data);
    inner(&mut data);
}

fn inner(data: &mut Vec<u32>) {}

// error[E0596]: cannot borrow `data` as mutable, as it is not declared as mutable
//  --> src/example.rs:1:10
//   |
// 1 | fn outer(data: &mut Vec<u32>) {
//   |          ^^^^ not mutable
// 2 |     inner(&mut data);
//   |           --------- cannot borrow as mutable
// 3 |     inner(&mut data);
//   |           --------- cannot borrow as mutable
//   |
// note: the binding is already a mutable borrow
//  --> src/example.rs:1:16
//   |
// 1 | fn outer(data: &mut Vec<u32>) {
//   |                ^^^^^^^^^^^^^
// help: try removing `&mut` here
//   |
// 2 -     inner(&mut data);
// 2 +     inner(data);
//   |
// help: try removing `&mut` here
//   |
// 3 -     inner(&mut data);
// 3 +     inner(data);
//   |

// ======================================================================

// However, changing either `inner(&mut data);` to `inner(data);` clears the error on the method's parameter:

fn outer(data: &mut Vec<u32>) { // no longer an error here
    inner(data); // line changed
    inner(&mut data);
}

fn inner(data: &mut Vec<u32>) {}

// error[E0596]: cannot borrow `data` as mutable, as it is not declared as mutable
//  --> src/example.rs:3:11
//   |
// 3 |     inner(&mut data);
//   |           ^^^^^^^^^ cannot borrow as mutable
//   |
// note: the binding is already a mutable borrow
//  --> src/example.rs:1:16
//   |
// 1 | fn outer(data: &mut Vec<u32>) {
//   |                ^^^^^^^^^^^^^
// help: try removing `&mut` here
//   |
// 3 -     inner(&mut data);
// 3 +     inner(data);
//   |