Invalid code suggested when encountering unsatisfied trait bounds in derive macro code

[jonasbb]

Given the following code: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=0345475f1b234a0cf26f97bfb712cceb

use std::collections::BinaryHeap;

#[derive(PartialEq, Eq, PartialOrd, Ord, serde::Serialize)]
struct PriorityQueueEntry<T> {
    value: T,
}

#[derive(serde::Serialize)]
struct PriorityQueue<T>(BinaryHeap<PriorityQueueEntry<T>>);

The current output is:

Compiling playground v0.0.1 (/playground)
error[[E0277]](https://doc.rust-lang.org/nightly/error-index.html#E0277): the trait bound `T: Ord` is not satisfied
   --> src/lib.rs:8:10
    |
8   | #[derive(serde::Serialize)]
    |          ^^^^^^^^^^^^^^^^ the trait `Ord` is not implemented for `T`
9   | struct PriorityQueue<T>(BinaryHeap<PriorityQueueEntry<T>>);
    |                         --------------------------------- required by a bound introduced by this call
    |
note: required for `PriorityQueueEntry<T>` to implement `Ord`
   --> src/lib.rs:3:37
    |
3   | #[derive(PartialEq, Eq, PartialOrd, Ord, serde::Serialize)]
    |                                     ^^^
    = note: required for `BinaryHeap<PriorityQueueEntry<T>>` to implement `Serialize`
note: required by a bound in `serialize_newtype_struct`
   --> /playground/.cargo/registry/src/github.com-1ecc6299db9ec823/serde-1.0.147/src/ser/mod.rs:904:12
    |
904 |         T: Serialize;
    |            ^^^^^^^^^ required by this bound in `serialize_newtype_struct`
    = note: this error originates in the derive macro `Ord` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider further restricting type parameter `T`
    |
8   | #[derive(serde::Serialize, T: std::cmp::Ord)]
    |                          ++++++++++++++++++

The last part of the suggestion #[derive(serde::Serialize, T: std::cmp::Ord)] is not valid Rust code and therefore should not be suggested like this.

This problem occurs on the Playground on both stable and nightly with basically identical error messages. serde v1.0.147 requires T: Ord such that BinaryHeap will be Serialize. Otherwise the problem does not seem serde specific.

[chenyukang]

@rustbot claim

[chenyukang]

@jonasbb do you know how to create a minimal case without serde?
so we may add it to unit testing.

I give a fix and verified it's OK with my local testing, but I'm struggle to construct a minimal case without serde to reproduce it.

[jonasbb]

@chenyukang I assume this reproduces when using the same setup of derive macro implementing a trait while the implementation is missing bounds. I don't know if it is relevant that the derive macro produces a trait.

Something along these lines, with the proc macro crate omitted.

// main crate
trait FooBar {
    fn dostuff(self);
}

#[derive(...)]
struct Struct<T>(T);

// derive macro expanded code
impl<T> FooBar for Struct<T> {
    fn dostuff(self) {
		use std::collections::BinaryHeap;
		let mut bh = BinaryHeap::new();
		bh.push(self.0);
    }
}

[chenyukang]

yes, it's caused by derive macro.

[chenyukang]

emm，met this error 😒

error: can't use a procedural macro from the same crate that defines it