Self: 'static bound gets mistakenly applied to references

[mauricelam]

In the following code, the Self: 'static bound gets copied to the derived impl TryFrom<&mut Foo<V>> for (&mut Wrapper<V>) where Self: 'static, which changes the bound from Foo<V>: 'static to &mut Foo<V>: 'static.

use derive_more;

#[derive(Debug)]
pub struct Wrapper<V>(V);

#[derive(derive_more::TryInto)]
#[try_into(ref, ref_mut)]
pub enum Foo<V> where Self: 'static {  // This line fails
// pub enum Foo<V> where Foo<V>: 'static {  // This line succeeds, after explicitly changing `Self` to `Foo<V>`
    V(Wrapper<V>),
}

fn main() {
    let x = Foo::<u8>::V(Wrapper(2));
    let u8ref: &Wrapper<u8> = (&x).try_into().unwrap();
    println!("{u8ref:?}");
}

And the resulting build fails with

error[E0597]: `x` does not live long enough
  --> src/main.rs:20:31
   |
19 |     let x = Foo::<u8>::V(Wrapper(2));
   |         - binding `x` declared here
20 |     let u8ref: &Wrapper<u8> = (&x).try_into().unwrap();
   |                               ^^^^-----------
   |                               |
   |                               borrowed value does not live long enough
   |                               argument requires that `x` is borrowed for `'static`
21 |     println!("{u8ref:?}");
22 | }
   | - `x` dropped here while still borrowed

https://www.rustexplorer.com/b#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

[JelteF]

Hmm, that does seem like an annoying bug. Just double checking: is it also happening when using the 1.0.0-beta.6 release of derive_more?

[mauricelam]

Trying on 1.0.0-beta.6, it's failing with a different issue:

error[E0107]: missing generics for enum `Foo`
 --> src/main.rs:8:10
  |
8 | pub enum Foo<V> where Self: 'static {  // This line fails
  |          ^^^ expected 1 generic argument
  |
note: enum defined here, with 1 generic parameter: `V`
 --> src/main.rs:8:10
  |
8 | pub enum Foo<V> where Self: 'static {  // This line fails
  |          ^^^ -
help: add missing generic argument
  |
8 | pub enum Foo<V><V> where Self: 'static {  // This line fails
  |  

Expanding the macro manually I believe it's coming from this line:

type Error = ::derive_more::TryIntoError<Foo>;