iobuf: add unsafe IOBufShared::from_owner API

Summary:
Currently, `iobuf` depends on `minibytes` just to do:

    unsafe impl BytesToIOufSharedExt for minibytes::Bytes

It makes the OSS world's `Cargo.toml` story tricky. I don't think `minibytes`
should be moved to `folly/rust` (`folly/rust` should be just pure bindings to
folly C++).

Switching (making `minibytes` depend on `iobuf`) is annoying too, partially
because there is no `Cargo.toml` for `iobuf` right now. So let's make it
possible to construct `IOBufShared` without explicitly
`unsafe impl BytesToIOufSharedExt`.

Reviewed By: muirdm

Differential Revision: D79678311

fbshipit-source-id: f79ea3c5312908ee26887f49c88cb2d91fe28b58

Author: quark-zju

folly/rust/iobuf/src/iobuf.rs

@@ -175,6 +175,20 @@ where
     // with `iobuf_take_ownership`. We box it up so that we can pass it to
     // a freeing function, called by C++ when it wants to release the memory.
     fn from(bytes: T) -> Self {
+        // safety: `T` satisfies the contract of `from_owner` because the `BytesToIOBufSharedExt`
+        // trait bound.
+        unsafe { Self::from_owner(bytes) }
+    }
+}
+
+impl IOBufShared {
+    /// This takes the Bytes, boxes it up, and passes ownership to the IOBuf
+    /// with `iobuf_take_ownership`. We box it up so that we can pass it to
+    /// a freeing function, called by C++ when it wants to release the memory.
+    ///
+    /// SAFETY: Similar to `BytesToIOBufSharedExt`, `T` must ensure that its
+    /// deref's slice is valid for the lifetime of `T`.
+    pub unsafe fn from_owner<T: std::ops::Deref<Target = [u8]>>(bytes: T) -> Self {
         let len = bytes.len();
         let b = Box::new(bytes);