io: remove unsafe pin-projections and remove manual Unpin implementations

[taiki-e]

Based on #1586 (First 2 commits are the same as #1586)

This removes most pin-projection related unsafe code: 16b06e3

And removes manual Unpin implementations and add tests to check that Unpin requirement does not change accidentally. see commit message for more: 7ad962d

[carllerche]

Thanks for this. Do we know how much this impacts compile time?

[taiki-e]

I didn’t actually time it, but the master branch already depends on pin-project, so I think merging this should have little impact on compile time.

Impact of adding pin-project to dependencies: As pin-project is a proc-macro, it increases the compilation time. If the dependencies have other proc-macro crates, the impact will not be so big. (As far as I know, whether or not the "full" feature of syn is enabled will particularly affect compilation time. In fact, a crate has been created to handle this issue. pin-project enables "full" feature.)

[taiki-e]

@carllerche I found the useful tool -Z timings: https://internals.rust-lang.org/t/exploring-crate-graph-build-times-with-cargo-build-ztimings/10975

Build times (clean build):

master:

this pr:

without pin-project (613fde2)

Dependencies are compiled in parallel, so I don't think this has much impact if another proc-macro crate (tokio-macros, tracing-attribute, etc.) is already enabled.

[taiki-e]

FWIW: I have written a lightweight version of pin-project written with declarative macros: https://github.com/taiki-e/pin-project-lite

[carllerche]

@taiki-e btw, how did you gen that graph?

[taiki-e]

@carllerche I used -Z timings flag of cargo: cargo build -Z timings

[carllerche]

Oh, nice, i missed all that 👍