Bogus "consider specifying this binding's type" when reference differs in mutability

[lukas-code]

Code

fn get() -> &'static Vec<i32> {
    todo!()
}

fn main() {
    let x = get();
    x.push(1);
}

Current output

error[E0596]: cannot borrow `*x` as mutable, as it is behind a `&` reference
 --> src/main.rs:7:5
  |
7 |     x.push(1);
  |     ^^^^^^^^^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
  |
help: consider specifying this binding's type
  |
6 |     let x: &mut Vec<i32> = get();
  |          +++++++++++++++

Desired output

error[E0596]: cannot borrow `*x` as mutable, as it is behind a `&` reference
 --> src/main.rs:7:5
  |
7 |     x.push(1);
  |     ^^^^^^^^^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
  |

Rationale and extra context

The type is defined as immutable reference by the return type of the function and specifying it as a mutable reference in the let binding won't help.

Other cases

This seems to happen only if

• the immutable reference doesn't come from a local borrow (eg. let x = &vec![];)
• the variable x goes through auto(de)ref with a method call or field access (eg. x.field = 1)

Anything else?

@rustbot label A-suggestion-diagnostics D-incorrect

[jmintb]

Can I try to fix this? :)

[awaitlink]

It seems this can happen in function arguments too, where the suggestion also results in invalid syntax.

If the &mut in this function...

pub async fn json_from_response<T: DeserializeOwned>(response: &mut Response) -> anyhow::Result<T> {
    response
        .json()
        .await
        .map_err(|e| anyhow!(e.to_string()))
        .context("could not get JSON")
}

...is replaced by & (e.g. per the invalid clippy suggestion rust-lang/rust-clippy#11199, which is a separate issue)...

Invalid clippy suggestion for completeness

warning: this argument is a mutable reference, but not used mutably
  --> src/network.rs:46:64
   |
46 | pub async fn json_from_response<T: DeserializeOwned>(response: &mut Response) -> anyhow::Result<T> {
   |                                                                ^^^^^^^^^^^^^ help: consider changing to: `&Response`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut
   = note: `#[warn(clippy::needless_pass_by_ref_mut)]` on by default

warning: `signalupdates-bot` (lib) generated 1 warning

...that results in:

error[E0596]: cannot borrow `*response` as mutable, as it is behind a `&` reference
  --> src/network.rs:47:5
   |
47 |     response
   |     ^^^^^^^^ `response` is a `&` reference, so the data it refers to cannot be borrowed as mutable
   |
help: consider specifying this binding's type
   |
46 | pub async fn json_from_response<T: DeserializeOwned>(response: &mut worker::Response: &Response) -> anyhow::Result<T> {
   |                                                              +++++++++++++++++++++++

which suggests adding a second type to the function argument.

Sorry for non-minimal example / if it's a different issue.

[lukas-code]

It seems this can happen in function arguments too, where the suggestion also results in invalid syntax.

It looks like this happens only for async fns:

async fn foo(x: &Vec<i32>) {
    x.push(42);
}

error[E0596]: cannot borrow `*x` as mutable, as it is behind a `&` reference
 --> src/lib.rs:2:5
  |
2 |     x.push(42);
  |     ^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
  |
help: consider specifying this binding's type
  |
1 | async fn foo(x: &mut Vec<i32>: &Vec<i32>) {
  |               +++++++++++++++

This makes sense, because the async fn gets desugared to this:

fn foo(x: &Vec<i32>) -> impl core::future::Future<Output = ()> + '_ {
    async move {
        let x = x; // <-- notice let binding here
        x.push(42);
    }
}

... and the compiler then tries to help by suggesting let x: &mut Vec<i32> = x;, similar to the original issue.

---

Can I try to fix this? :)

Yes, absolutely, feel free to work on this. Be sure to check out the dev guide and don't hesitate to ask in the help channel on zulip if you have any questions.

[jmintb]

Awesome! thanks 🦀

[jmintb]

@rustbot claim