provide a non-destructive mechanism to determine if a sink/stream are paired #2363

Job	Run time
codegen	26s
cargo bench	55s
cargo +stable build	59s
cargo clippy	35s
cargo +beta build	44s
cargo +1.36 build (futures-{core, io, sink})	2m 33s
cargo doc	19s
cargo +nightly build	1m 29s
cargo hack check --feature-powerset	3m 30s
cargo fmt	4s
cargo minimal-versions build	41s
cargo miri test	11m 56s
cargo build --target thumbv7m-none-eabi	32s
cargo build --target thumbv6m-none-eabi	27s
cargo test -Z sanitizer=address	2m 57s
cargo test (ubuntu-latest)	4m 47s
cargo test -Z sanitizer=memory	3m 18s
cargo +1.56 build	1m 49s
cargo test (macos-latest)	4m 39s
cargo test -Z sanitizer=thread	2m 20s
cargo test (windows-latest)	4m 8s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	4m 25s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	10m 10s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 39s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	5m 39s
	1h 16m 1s

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