Rollup merge of #145550 - nnethercote:derive_from-no-unit, r=Kobzol

Avoid using `()` in `derive(From)` output.

Using an error type instead of `()` avoids the duplicated errors on `struct SUnsizedField` in `deriving-from-wrong-target.rs`. It also improves the expanded output from this:
```
struct S2(u32, u32);
impl ::core::convert::From<()> for S2 {
    #[inline]
    fn from(value: ()) -> S2 { (/*ERROR*/) }
}
```
to this:
```
struct S2(u32, u32);
impl ::core::convert::From<(/*ERROR*/)> for S2 {
    #[inline]
    fn from(value: (/*ERROR*/)) -> S2 { (/*ERROR*/) }
}
```
The new code also only matchs on `item.kind` once.

r? ``@Kobzol``

0 files changed, 0 insertions, 0 deletions