derivable_impls wants derive(Default) on enum

[Arnavion]

Lint name: derivable_impls

I tried this code:

pub enum IntOrString {
    Int(i32),
    String(String),
}

impl Default for IntOrString {
    fn default() -> Self {
        IntOrString::Int(0)
    }
}

Clippy says:

warning: this `impl` can be derived
  --> src/lib.rs:6:1
   |
6  | / impl Default for IntOrString {
7  | |     fn default() -> Self {
8  | |         IntOrString::Int(0)
9  | |     }
10 | | }
   | |_^
   |
   = note: `#[warn(clippy::derivable_impls)]` on by default
   = help: try annotating `IntOrString` with `#[derive(Default)]`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#derivable_impls

This is incorrect because rustc cannot derive Default on enums (until feature(derive_enum_default) is stabilized.)

Meta

Rust version (rustc -Vv):

rustc 1.57.0-nightly (2b862bed9 2021-09-23)
binary: rustc
commit-hash: 2b862bed9889808b69629fd7246317189b9517a5
commit-date: 2021-09-23
host: x86_64-unknown-linux-gnu
release: 1.57.0-nightly
LLVM version: 13.0.0

The playground repro uses 2021-10-02 f03eb6bef8ced8a24385

---

cc @HKalbasi

@rustbot label +I-suggestion-causes-error

[HKalbasi]

🤦‍♂️

@rustbot claim

[AlexApps99]

I found a similar bug with unions, I assume this may fix that too.
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=617310465c57a719c089bb30374fb0f4

[AlexApps99]

Here's another one I found:
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=55789d00abff60aeba1fa92ff45de978
Edit: duplicate of #7683