Running clippy pedantic on a client crate fails on stable

[jmg-duarte]

The example is as follows:

use darling::FromMeta;

fn main() {
    println!("Hello, world!");
}

#[derive(FromMeta)]
struct Test {
    #[darling(default)]
    _argument: TOption<String>,
}

enum TOption<T> {
    Some(T),
    Default,
    None,
}

impl<T> Default for TOption<T> {
    fn default() -> Self {
        Self::None
    }
}

impl FromMeta for TOption<String> {
    /// If the input string is empty it returns `Ok(Default)`, otherwise it returns `Ok(Some(value))`.
    fn from_string(value: &str) -> darling::Result<Self> {
        if value.is_empty() {
            // arg = ""
            return Ok(Self::Default);
        }
        // arg = "..."
        Ok(Self::Some(value.to_string()))
    }

    /// Returns `Ok(Default)`.
    fn from_word() -> darling::Result<Self> {
        Ok(Self::Default)
    }
}

This is a minimized from my typestate crate. For context, TOption distinguishes a default value from "nulls" and other values.

If I run cargo clippy -- -Dclippy::pedantic on stable, it yields the following error:

error: calling `TOption::default()` is more clear than this expression
 --> src/main.rs:7:10
  |
7 | #[derive(FromMeta)]
  |          ^^^^^^^^
  |
  = note: `-D clippy::default-trait-access` implied by `-D clippy::pedantic`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#default_trait_access
  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

If I run cargo +nightly clippy -- -Dclippy::pedantic, forcing nightly, no error appears.

From running cargo expand over the example we get:

// cut for brevity sake
impl ::darling::FromMeta for Test {
    fn from_list(__items: &[::syn::NestedMeta]) -> ::darling::Result<Self> {
        // cut ...
        ::darling::export::Ok(Self {
            _argument: match _argument.1 {
                ::darling::export::Some(__val) => __val,
                ::darling::export::None => ::darling::export::Default::default(),
            },
        })
    }
}
// cut ...

From clippy's link, I believe the culprit to be:

::darling::export::None => ::darling::export::Default::default(),

Which should instead be TOption::default().

I've been looking around the code but I don't really know how/where to fix this.

[TedDriggs]

Which should instead be TOption::default().

The code uses Default::default to avoid needing to preserve and pass around type paths when it doesn't need them, so I wouldn't fix this particular issue. However, there may still be a way to make clippy happy.

There's an attribute - I believe it's #[automatically_derived] - that some other proc-macro crates started using after darling first released. If you add that to the generated trait impl here, does that make these lint issues go away?

[jmg-duarte]

I'll try and report back!

[TedDriggs]

Closing due to inactivity.