almost_swapped false positive a=a; a=a (in clap::Parser derive)

[MingweiSamuel]

Summary

#10177

• clippy::almost_swapped's error message is wrong when triggered in a derive macro. Not sure about other macros.

error: this looks like you are trying to swap __clap_subcommand and Parser

There is no variable called Parser, that is the name of the derive macro. Should be:

error: this looks like you are trying to swap __clap_subcommand and __clap_subcommand

• clippy::almost_swapped triggers when you would be "swapping" the same variable with itself. I.e. the following triggers it:

let a = a;
let a = a;

First impression wouldn't be that we're trying to swap a with a since they are the same name. And the generated suggestion std::mem::swap(&mut a, &mut a) is silly; breaks aliasing rules.

• clippy::almost_swapped triggers within macros in the first place. Should it?

Obviously that code above is a little silly, but can be generated in macros, e.g. in clap::Parser's derive. Not sure if clippy is supposed to trigger within macros in general--I.e. #10318 ?

Lint Name

almost_swapped

Reproducer

I tried this code:

fn main() {
    let a = 0;
    let a = a;
    let a = a;
}

I saw this happen:

error: this looks like you are trying to swap `a` and `a`
 --> src/main.rs:3:5
  |
3 | /     let a = a;
4 | |     let a = a;
  | |_____________^ help: try: `std::mem::swap(&mut a, &mut a)`
  |
  = note: or maybe you should use `std::mem::replace`?
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#almost_swapped
  = note: `#[deny(clippy::almost_swapped)]` on by default

I expected to see this happen:

No error

---

Clap macro

I tried this code:

use clap::Parser;

#[derive(Parser)]
enum MyEnum {
    MyVariant
}

Which expands to:

// ...
#[allow(dead_code, unreachable_code, unused_variables, unused_braces)]
#[allow(clippy :: style, clippy :: complexity, clippy :: pedantic, clippy ::
restriction, clippy :: perf, clippy :: deprecated, clippy :: nursery, clippy
:: cargo, clippy :: suspicious_else_formatting,)]
#[deny(clippy :: correctness)]
impl clap::Subcommand for MyEnum {
    fn augment_subcommands<'b>(__clap_app: clap::Command) -> clap::Command {
        ;
        let __clap_app = __clap_app;
        let __clap_app =
            __clap_app.subcommand({
                    let __clap_subcommand = clap::Command::new("my-variant");
                    let __clap_subcommand = __clap_subcommand;
                    let __clap_subcommand = __clap_subcommand;
                    __clap_subcommand
                });
        ;
        __clap_app
    }
    fn augment_subcommands_for_update<'b>(__clap_app: clap::Command)
        -> clap::Command {
        ;
        let __clap_app = __clap_app;
        let __clap_app =
            __clap_app.subcommand({
                    let __clap_subcommand = clap::Command::new("my-variant");
                    let __clap_subcommand = __clap_subcommand;
                    let __clap_subcommand = __clap_subcommand;
                    __clap_subcommand
                });
        ;
        __clap_app
    }
    fn has_subcommand(__clap_name: &str) -> bool {
        if "my-variant" == __clap_name { return true }
        false
    }
}

I saw this happen:

error: this looks like you are trying to swap `__clap_subcommand` and `Parser`
 --> src/lib.rs:3:10
  |
3 | #[derive(Parser)]
  |          ^^^^^^
  |
  = note: or maybe you should use `std::mem::replace`?
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#almost_swapped
note: the lint level is defined here
 --> src/lib.rs:3:10
  |
3 | #[derive(Parser)]
  |          ^^^^^^
  = note: `#[deny(clippy::almost_swapped)]` implied by `#[deny(clippy::correctness)]`
  = note: this error originates in the derive macro `Parser` (in Nightly builds, run with -Z macro-backtrace for more info)

error: could not compile `playground` due to previous error

I expected to see this happen:

No error

Or

error: this looks like you are trying to swap `__clap_subcommand` and `__clap_subcommand`

Version

Playground:
Build using the Nightly version: 1.69.0-nightly

(2023-02-27 7281249a19a9755e9d88)

Local windows:
$ rustc +nightly -Vv
rustc 1.69.0-nightly (d962ea578 2023-02-26)
binary: rustc
commit-hash: d962ea57899d64dc8a57040142c6b498a57c8064
commit-date: 2023-02-26
host: x86_64-pc-windows-msvc
release: 1.69.0-nightly
LLVM version: 15.0.7

Additional Labels

No response

[MingweiSamuel]

clap-rs/clap#4733
clap-rs/clap#4735
(fix for the duplicate lets: clap-rs/clap@c1caf46 )

[glandium]

Also, neither #![allow(clippy::correctness)] nor #![allow(clippy::almost_swapped)] silence the warning.

[MingweiSamuel]

@glandium

Also, neither #![allow(clippy::correctness)] nor #![allow(clippy::almost_swapped)] silence the warning.

Only in the clap derive since it had its own #[deny(clippy::correctness)] attribute. Removed in clap-rs/clap#4739

[blyxyas]

@rustbot claim

[blyxyas]

Task 1 and 3 should be marked as completed to indicate that they are also completed.

[MingweiSamuel]

Marked as done! 🎉