Revisit docsrs settings now that cfg implies doc(cfg)

[asomers]

Description
A recent change to rustdoc makes #[cfg] now imply #[doc(cfg)]. That's handy for most crates, but makes Tokio's various cfg macros now partially redundant and somewhat confusing. In the best case, Tokio may be able to simply delete all of the docsrs stuff.
rust-lang/rust#89596

[Darksonn]

I worry if this will make some things documented incorrectly. For example, we might have a #[cfg(not(loom))] on something, or maybe we use cfgs to choose between two implementations depending on a feature flag. E.g. the #[tokio::main] macro that is visible in the documentation has an #[cfg(feature = "rt-multi-thread")] even though it exists without that feature flag.

[hawkw]

I worry if this will make some things documented incorrectly. For example, we might have a #[cfg(not(loom))] on something, or maybe we use cfgs to choose between two implementations depending on a feature flag. E.g. the #[tokio::main] macro that is visible in the documentation has an #[cfg(feature = "rt-multi-thread")] even though it exists without that feature flag.

According to the linked PR, we can use #![doc(cfg_hide(loom))] to disable generating docs for the loom cfg, at least. I'm not sure how this will behave for definitions where two separate versions of that definition are enabled/disabled based on the cfg attribute, though...

[Noah-Kennedy]

Does the change in rustdoc break anything for us, or just allow us to potentially simplify things?

[hawkw]

Does the change in rustdoc break anything for us, or just allow us to potentially simplify things?

It shouldn't break builds, but it may result in incorrect docs in some places, I think? Per @Darksonn's comment, there are some places where we use #[doc(cfg(...))] attributes for APIs that require a cfg but are not actually compiled out based on that cfg.

[Darksonn]

I ran cargo doc on Tokio using the nightly before and after this change. The following files were not identical:

doc/enum.NotDefinedHere.html
doc/index.html
doc/os/index.html
doc/os/windows/index.html
doc/os/windows/io/index.html
doc/os/windows/io/trait.AsRawHandle.html
doc/os/windows/io/trait.FromRawHandle.html
doc/os/windows/io/type.RawHandle.html
doc/winapi/index.html
doc/winapi/shared/index.html
doc/winapi/shared/winerror/index.html
doc/winapi/shared/winerror/type.ERROR_ACCESS_DENIED.html
doc/winapi/shared/winerror/type.ERROR_MORE_DATA.html
doc/winapi/shared/winerror/type.ERROR_PIPE_BUSY.html
doc/winapi/um/index.html
doc/winapi/um/minwinbase/index.html
doc/winapi/um/minwinbase/type.SECURITY_ATTRIBUTES.html
doc/winapi/um/winbase/index.html
doc/winapi/um/winbase/type.PIPE_CLIENT_END.html
doc/winapi/um/winbase/type.PIPE_SERVER_END.html
doc/winapi/um/winbase/type.PIPE_TYPE_BYTE.html
doc/winapi/um/winbase/type.PIPE_TYPE_MESSAGE.html
doc/winapi/um/winbase/type.SECURITY_IDENTIFICATION.html
index.html
io/struct.Interest.html
io/struct.Ready.html
net/fn.lookup_host.html
net/index.html
net/struct.TcpListener.html
net/struct.TcpSocket.html
net/struct.TcpStream.html
net/struct.UdpSocket.html
net/struct.UnixDatagram.html
net/struct.UnixListener.html
net/struct.UnixStream.html
net/tcp/index.html
net/tcp/struct.OwnedReadHalf.html
net/tcp/struct.OwnedWriteHalf.html
net/tcp/struct.ReadHalf.html
net/tcp/struct.ReuniteError.html
net/tcp/struct.WriteHalf.html
net/trait.ToSocketAddrs.html
net/unix/index.html
net/unix/struct.OwnedReadHalf.html
net/unix/struct.OwnedWriteHalf.html
net/unix/struct.ReadHalf.html
net/unix/struct.ReuniteError.html
net/unix/struct.SocketAddr.html
net/unix/struct.UCred.html
net/unix/struct.WriteHalf.html
net/windows/index.html
net/windows/named_pipe/enum.PipeEnd.html
net/windows/named_pipe/enum.PipeMode.html
net/windows/named_pipe/index.html
net/windows/named_pipe/struct.ClientOptions.html
net/windows/named_pipe/struct.NamedPipeClient.html
net/windows/named_pipe/struct.NamedPipeServer.html
net/windows/named_pipe/struct.PipeInfo.html
net/windows/named_pipe/struct.ServerOptions.html
runtime/struct.Builder.html