`Mutex` and `RwLock` borrow suggestions could suggest `get_mut` if applicable

Code

use std::sync::{Mutex, RwLock};

fn f(t: &mut (Mutex<Vec<()>>, RwLock<Vec<()>>))
{
    t.0.len();
    t.1.len();
}

Current output

error[E0599]: no method named `len` found for struct `Mutex` in the current scope
    --> src/lib.rs:5:9
     |
5    |     t.0.len();
     |         ^^^ method not found in `Mutex<Vec<()>>`
     |
note: the method `len` exists on the type `Vec<()>`
    --> /playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:2705:5
     |
2705 |     pub const fn len(&self) -> usize {
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: use `.lock().unwrap()` to borrow the `Vec<()>`, blocking the current thread until it can be acquired
     |
5    |     t.0.lock().unwrap().len();
     |        ++++++++++++++++

error[E0599]: no method named `len` found for struct `RwLock` in the current scope
    --> src/lib.rs:6:9
     |
6    |     t.1.len();
     |         ^^^ method not found in `RwLock<Vec<()>>`
     |
note: the method `len` exists on the type `Vec<()>`
    --> /playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:2705:5
     |
2705 |     pub const fn len(&self) -> usize {
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: use `.read().unwrap()` to borrow the `Vec<()>`, blocking the current thread until it can be acquired
     |
6    |     t.1.read().unwrap().len();
     |        ++++++++++++++++

For more information about this error, try `rustc --explain E0599`.

Desired output

error[E0599]: no method named `len` found for struct `Mutex` in the current scope
    --> src/lib.rs:5:9
     |
5    |     t.0.len();
     |         ^^^ method not found in `Mutex<Vec<()>>`
     |
note: the method `len` exists on the type `Vec<()>`
    --> /playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:2705:5
     |
2705 |     pub const fn len(&self) -> usize {
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: use `.get_mut().unwrap()` to borrow the `Vec<()>`
     |
5    |     t.0.get_mut().unwrap().len();
     |        ++++++++++++++++

error[E0599]: no method named `len` found for struct `RwLock` in the current scope
    --> src/lib.rs:6:9
     |
6    |     t.1.len();
     |         ^^^ method not found in `RwLock<Vec<()>>`
     |
note: the method `len` exists on the type `Vec<()>`
    --> /playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:2705:5
     |
2705 |     pub const fn len(&self) -> usize {
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: use `.get_mut().unwrap()` to borrow the `Vec<()>`
     |
6    |     t.1.get_mut().unwrap().len();
     |        ++++++++++++++++

For more information about this error, try `rustc --explain E0599`.

Rationale and extra context

If it is possible to call get_mut, there is no reason to call lock or read instead, as they just do the same thing* less efficiently in this context.

* Except if a guard is mem::forgetten, this will succeed instead of block indefinitely, but whatever.

Other cases

Rust Version

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: x86_64-unknown-linux-gnu
release: 1.86.0
LLVM version: 19.1.7
Compiler returned: 0

Anything else?

No response

Start by reproducing the diagnostic with the Rust example in the issue and compare the current and desired suggestions for Mutex and RwLock. Trace the compiler diagnostic code that generates borrow suggestions, then add coverage showing that &mut access selects get_mut().unwrap() and that the resulting help text matches the desired output.