Diagnostics should not assume omission of impl DerefMut is unintended

Code

struct A;

impl A {
    fn edit(&mut self) {}
}

struct B(A);

impl std::ops::Deref for B {
    type Target = A;
    fn deref(&self) -> &A { &self.0 }
}

fn main() {
    let b = B(A);
    b.edit();
}

Current output

error[E0596]: cannot borrow data in dereference of `B` as mutable
  --> src/main.rs:16:5
   |
16 |     b.edit();
   |     ^ cannot borrow as mutable
   |
   = help: trait `DerefMut` is required to modify through a dereference, but it is not implemented for `B`

For more information about this error, try `rustc --explain E0596`.
error: could not compile `playground` (bin "playground") due to 1 previous error

Desired output

error[E0599]: no method named `edit` found for struct `B` in the current scope
  --> src/main.rs:11:7
   |
 7 | struct B(A);
   | -------- method `edit` not found for this struct
...
11 |     b.edit();
   |       ^^^^ method not found in `B`
   |
help: one of the expressions' fields has a method of the same name
   |
11 |     b.0.edit();
   |       ++

The above is the output shown when impl Deref is removed. It could be amended to mention the lack of a DerefMut impl but should also read well when the latter trait is intentionally not implemented.

Rationale and extra context

Motivation: it may be that the derived type B can safely implement read support for its component A via Deref, but that other actions must be performed on edits and thus that DerefMut cannot be implemented.

Other cases

Note also: if B does implement fn edit(&mut self) {} but this is inaccessible due to privacy rules, the existing diagnostic does not mention fn B::edit at all.

Rust Version

Tested against both stable (1.94.0) and nightly (2026-03-12) on play.rust-lang.org

Start by compiling the supplied Rust example on stable and nightly, then compare the E0596 output with the E0599 output after removing Deref. Trace the compiler diagnostics for method lookup and mutable dereference, including the privacy case. Done means the diagnostic no longer assumes missing DerefMut is unintended and remains useful for inaccessible methods.