rust-lang / rust-lang/rust-clippy

FP derive_partial_eq_without_eq, breaks code

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Description

Summary

.

Lint Name

derive_partial_eq_without_eq

Reproducer

I tried this code:
Unfortunately I have no idea what is happening here :/

#![feature(decl_macro)]

#[derive(Copy, Clone, PartialEq, Debug)]
pub enum Field {
    RootCtxt,
    MacroCtxt,
}

#[rustfmt::skip]
macro x(
    $macro_name:ident,
    $macro2_name:ident,
    $type_name:ident,
    $field_name:ident,
    $const_name:ident
) {
    #[derive(Copy, Clone)]
    pub struct $type_name {
        pub field: Field,
        pub $field_name: Field,
    }

    pub const $const_name: $type_name =
        $type_name { field: Field::MacroCtxt, $field_name: Field::RootCtxt };

    #[macro_export]
    macro_rules! $macro_name {
        (check_fields_of $e:expr) => {{
            let e = $e;
            assert_eq!(e.field, Field::MacroCtxt);
            assert_eq!(e.$field_name, Field::RootCtxt);
        }};
        (check_fields) => {{
            assert_eq!($const_name.field, Field::MacroCtxt);
            assert_eq!($const_name.$field_name, Field::RootCtxt);
        }};
        (construct) => {
            $type_name { field: Field::MacroCtxt, $field_name: Field::RootCtxt }
        };
    }

    pub macro $macro2_name {
        (check_fields_of $e:expr) => {{
            let e = $e;
            assert_eq!(e.field, Field::MacroCtxt);
            assert_eq!(e.$field_name, Field::RootCtxt);
        }},
        (check_fields) => {{
            assert_eq!($const_name.field, Field::MacroCtxt);
            assert_eq!($const_name.$field_name, Field::RootCtxt);
        }},
        (construct) => {
            $type_name { field: Field::MacroCtxt, $field_name: Field::RootCtxt }
        }
    }
}

x!(test_fields, test_fields2, MyStruct, field, MY_CONST);

pub fn check_fields(s: MyStruct) {
    test_fields!(check_fields_of s);
}

pub fn main() {}

I saw this happen:
cargo clippy --fix -- -Aclippy::all -Wclippy::derive_partial_eq_without_eq

    Checking clpy v0.1.0 (/tmp/clpy)
warning: failed to automatically apply fixes suggested by rustc to crate `clpy`

after fixes were automatically applied the compiler reported errors within these files:

  * src/main.rs

This likely indicates a bug in either rustc or cargo itself,
and we would appreciate a bug report! You're likely to see
a number of compiler warnings after this message which cargo
attempted to fix but failed. If you could open an issue at
https://github.com/rust-lang/rust/issues
quoting the full output of this command we'd be very appreciative!
Note that you may be able to make some more progress in the near-term
fixing code with the `--broken-code` flag

The following errors were reported:
error[E0609]: no field `field` on type `MyStruct`
  --> src/main.rs:30:26
   |
30 |             assert_eq!(e.field, Field::MacroCtxt);
   |                          ^^^^^ help: a field with a similar name exists: `field`
...
61 |     test_fields!(check_fields_of s);
   |     ------------------------------- in this macro invocation
   |
   = note: this error originates in the macro `test_fields` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

For more information about this error, try `rustc --explain E0609`.
Original diagnostics will follow.

warning: you are deriving `PartialEq` and can implement `Eq`
 --> src/main.rs:3:23
  |
3 | #[derive(Copy, Clone, PartialEq, Debug)]
  |                       ^^^^^^^^^ help: consider deriving `Eq` as well: `PartialEq, Eq`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#derive_partial_eq_without_eq
  = note: requested on the command line with `-W clippy::derive-partial-eq-without-eq`

warning: `clpy` (bin "clpy") generated 1 warning (run `cargo fix --bin "clpy"` to apply 1 suggestion)
warning: `clpy` (bin "clpy" test) generated 1 warning 
Version
rustc 1.67.0-nightly (c5d82ed7a 2022-11-19)
binary: rustc
commit-hash: c5d82ed7a4ad94a538bb87e5016e7d5ce0bd434b
commit-date: 2022-11-19
host: x86_64-unknown-linux-gnu
release: 1.67.0-nightly
LLVM version: 15.0.4
Additional Labels

No response

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the derive_partial_eq_without_eq lint and the reproducer in src/main.rs. Run cargo clippy --fix with the lint enabled, then inspect the generated changes and compiler errors in src/main.rs. Done means the suggested fix applies without breaking the macro-based code and the lint still suggests deriving Eq.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
tooling
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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