needless_borrow false positive, with broken suggestion

[ijackson]

To reproduce

Run cargo clippy with this in src/main.rs:

struct A { }

#[derive(Debug)]
struct B { }

impl From<&A> for B {
    fn from(_a: &A) -> B { B { } }
}

fn main() {
    let a = A { };
    
    // warning: this expression borrows a value the compiler would automatically borrow
    // 12 | let b: B = (&a).into();
    //                 ^^^^ help: change this to: `a`
    let b: B = (&a).into();
    
    // error[E0277]: the trait bound `B: std::convert::From<A>` is not satisfied
    // let b: B = a.into();
    
    println!("{:?}", &b)
}

Output with current nightly (bad)

    Checking foo v0.1.0 (/home/rustcargo/d/foo)
warning: this expression borrows a value the compiler would automatically borrow
  --> src/main.rs:16:16
   |
16 |     let b: B = (&a).into();
   |                ^^^^ help: change this to: `a`
   |
   = note: `#[warn(clippy::needless_borrow)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

warning: `foo` (bin "foo") generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.18s

Discussion

The disagnostic is not correct. Changing the program as clippy suggests does not work (see comment in my example). Both nightly and stable reject the modified program.

Stable clippy IMO-rightly accepts the original program without complaint.

Meta

rustcargo@zealot:~/d/foo$ rustc +stable --version
rustc 1.58.1 (db9d1b20b 2022-01-20)
rustcargo@zealot:~/d/foo$ rustc +nightly --version
rustc 1.60.0-nightly (6abb6385b 2022-01-26)
rustcargo@zealot:~/d/foo$ 

[ijackson]

@rustbot modify labels +A-clippy +regression-from-stable-to-nightly

[ijackson]

FTR, my test case was with a fresh project made with cargo new foo, so here is the Cargo.toml:

[package]
name = "foo"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]

[ijackson]

For my reference, this affects arti: https://gitlab.torproject.org/tpo/core/arti/-/issues/310

[camelid]

@rustbot label: -I-prioritize

[Jarcho]

Duplicate of #8367 with a fix in #8355.