Auto merge of #45379 - cuviper:unit_from_iter, r=alexcrichton

impl FromIterator<()> for ()

This just collapses all unit items from an iterator into one.  This is
more useful when combined with higher-level abstractions, like
collecting to a `Result<(), E>` where you only care about errors:

```rust
use std::io::*;
data = vec![1, 2, 3, 4, 5];
let res: Result<()> = data.iter()
    .map(|x| writeln!(stdout(), "{}", x))
    .collect();
assert!(res.is_ok());
```

1 files changed, 1 insertions, 0 deletions

diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index 5659d5b8d66..4a57417e86a 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -189,3 +189,4 @@ pub mod fmt;
 mod char_private;
 mod iter_private;
 mod tuple;
+mod unit;