Auto merge of #27134 - fhartwig:derive, r=huonw

Fixes #25022

This adapts the deriving mechanism to not repeat bounds for the same type parameter. To give an example: for the following code:

```rust
#[derive(Clone)]
pub struct FlatMap<I, U: IntoIterator, F> {
    iter: I,
    f: F,
    frontiter: Option<U::IntoIter>,
    backiter: Option<U::IntoIter>,
}
```
the latest nightly generates the following impl signature:

```rust
impl <I: ::std::clone::Clone,
      U: ::std::clone::Clone + IntoIterator,
      F: ::std::clone::Clone>
::std::clone::Clone for FlatMap<I, U, F> where
    I: ::std::clone::Clone,
    F: ::std::clone::Clone,
    U::IntoIter: ::std::clone::Clone,
    U::IntoIter: ::std::clone::Clone
```

With these changes, the signature changes to this:
```rust
impl <I, U: IntoIterator, F> ::std::clone::Clone for FlatMap<I, U, F> where
    I: ::std::clone::Clone,
    F: ::std::clone::Clone,
    U::IntoIter: ::std::clone::Clone
```
(Nothing in the body of the impl changes)
Note that the second impl is more permissive, as it doesn't have a `Clone` bound on `U` at all. There was a compile-fail test that failed due to this. I don't understand why we would want the old behaviour (and nobody on IRC could tell me either), so please tell me if there is a good reason that I missed.

0 files changed, 0 insertions, 0 deletions