Rollup merge of #34828 - seanmonstar:into-opton, r=alexcrichton

core: impl From<T> for Option<T>

First, the semantics of this `impl` seem spot on. If I have a value `T`, and I wish to make a `Option<T>`, then `Option::from(val)` should always give `Some(val)`.

Second, this allows improvement for several APIs that currently take `Option<T>` as arguments. Consider:

```rust
fn set_read_timeout(&mut self, timeout: Option<u32>) {
    // ...
}

x.set_read_timeout(Some(30));
x.set_read_timeout(Some(10));
x.set_read_timeout(None);
```

With this `impl`:

```rust
fn set_read_timeout<T: Into<Option<u32>>>(&mut self, timeout: T) {
    let timeout = timeout.into();
    // ...
}

x.set_read_timeout(30);
x.set_read_timeout(10);
x.set_read_timeout(Some(10)); // backwards compatible
x.set_read_timeout(None);
```

The change to those methods aren't included, but could be modified later.

r? @sfackler

src/libcore/option.rs

@@ -142,6 +142,7 @@
 use self::Option::*;
 
 use clone::Clone;
+use convert::From;
 use default::Default;
 use iter::ExactSizeIterator;
 use iter::{Iterator, DoubleEndedIterator, FromIterator, IntoIterator};
@@ -754,6 +755,13 @@ impl<'a, T> IntoIterator for &'a mut Option<T> {
     }
 }
 
+#[stable(since = "1.12.0", feature = "option_from")]
+impl<T> From<T> for Option<T> {
+    fn from(val: T) -> Option<T> {
+        Some(val)
+    }
+}
+
 /////////////////////////////////////////////////////////////////////////////
 // The Option Iterators
 /////////////////////////////////////////////////////////////////////////////