From c809c87174868d3ad9b894877da35e0bce50e41f Mon Sep 17 00:00:00 2001
From: Josh Stone <cuviper@gmail.com>
Date: Fri, 29 Jul 2022 16:58:13 -0700
Subject: [PATCH] Add `Pin` projections and implement `Future` (MSRV 1.36)

---
 .github/workflows/ci.yml |  8 +------
 Cargo.toml               |  2 +-
 README.rst               |  9 ++++++++
 src/lib.rs               | 47 ++++++++++++++++++++++++++++++++++++++++
 4 files changed, 58 insertions(+), 8 deletions(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 072430f..905cbf5 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -13,7 +13,7 @@ jobs:
       fail-fast: false
       matrix:
         rust:
-          - 1.31.0  # MSRV
+          - 1.36.0  # MSRV
           - stable
           - beta
           - nightly
@@ -30,12 +30,6 @@ jobs:
         with:
           toolchain: ${{ matrix.rust }}
 
-      - name: (1.31.0) Downgrade serde_json
-        if: ${{ matrix.rust == '1.31.0' }}
-        run: |
-          cargo generate-lockfile
-          cargo update -p serde_json --precise 1.0.39
-
       - name: Build (no_std)
         run: cargo build --no-default-features
 
diff --git a/Cargo.toml b/Cargo.toml
index a5beca5..c95bcdd 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -3,7 +3,7 @@ name = "either"
 version = "1.7.0"
 authors = ["bluss"]
 edition = "2018"
-rust-version = "1.31"
+rust-version = "1.36"
 
 license = "MIT/Apache-2.0"
 repository = "https://github.com/bluss/either"
diff --git a/README.rst b/README.rst
index 992e49c..25b2832 100644
--- a/README.rst
+++ b/README.rst
@@ -31,6 +31,15 @@ How to use with cargo::
 Recent Changes
 --------------
 
+- UNRELEASED
+
+  - **MSRV**: ``either`` now requires Rust 1.36 or later.
+
+  - Add new methods ``.as_pin_ref()`` and ``.as_pin_mut()`` to project a
+    pinned ``Either`` as inner ``Pin`` variants, by @cuviper (#77)
+
+  - Implement the ``Future`` trait, by @cuviper (#77)
+
 - 1.7.0
 
   - **MSRV**: ``either`` now requires Rust 1.31 or later.
diff --git a/src/lib.rs b/src/lib.rs
index a4f2a54..b7036b2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -26,9 +26,11 @@ pub mod serde_untagged_optional;
 
 use core::convert::{AsMut, AsRef};
 use core::fmt;
+use core::future::Future;
 use core::iter;
 use core::ops::Deref;
 use core::ops::DerefMut;
+use core::pin::Pin;
 
 #[cfg(any(test, feature = "use_std"))]
 use std::error::Error;
@@ -255,6 +257,35 @@ impl<L, R> Either<L, R> {
         }
     }
 
+    /// Convert `Pin<&Either<L, R>>` to `Either<Pin<&L>, Pin<&R>>`,
+    /// pinned projections of the inner variants.
+    pub fn as_pin_ref(self: Pin<&Self>) -> Either<Pin<&L>, Pin<&R>> {
+        // SAFETY: We can use `new_unchecked` because the `inner` parts are
+        // guaranteed to be pinned, as they come from `self` which is pinned.
+        unsafe {
+            match *Pin::get_ref(self) {
+                Left(ref inner) => Left(Pin::new_unchecked(inner)),
+                Right(ref inner) => Right(Pin::new_unchecked(inner)),
+            }
+        }
+    }
+
+    /// Convert `Pin<&mut Either<L, R>>` to `Either<Pin<&mut L>, Pin<&mut R>>`,
+    /// pinned projections of the inner variants.
+    pub fn as_pin_mut(self: Pin<&mut Self>) -> Either<Pin<&mut L>, Pin<&mut R>> {
+        // SAFETY: `get_unchecked_mut` is fine because we don't move anything.
+        // We can use `new_unchecked` because the `inner` parts are guaranteed
+        // to be pinned, as they come from `self` which is pinned, and we never
+        // offer an unpinned `&mut L` or `&mut R` through `Pin<&mut Self>`. We
+        // also don't have an implementation of `Drop`, nor manual `Unpin`.
+        unsafe {
+            match *Pin::get_unchecked_mut(self) {
+                Left(ref mut inner) => Left(Pin::new_unchecked(inner)),
+                Right(ref mut inner) => Right(Pin::new_unchecked(inner)),
+            }
+        }
+    }
+
     /// Convert `Either<L, R>` to `Either<R, L>`.
     ///
     /// ```
@@ -1038,6 +1069,22 @@ where
 {
 }
 
+/// `Either<L, R>` is a future if both `L` and `R` are futures.
+impl<L, R> Future for Either<L, R>
+where
+    L: Future,
+    R: Future<Output = L::Output>,
+{
+    type Output = L::Output;
+
+    fn poll(
+        self: Pin<&mut Self>,
+        cx: &mut core::task::Context<'_>,
+    ) -> core::task::Poll<Self::Output> {
+        for_both!(self.as_pin_mut(), inner => inner.poll(cx))
+    }
+}
+
 #[cfg(any(test, feature = "use_std"))]
 /// `Either<L, R>` implements `Read` if both `L` and `R` do.
 ///