Stabilize library features for 1.18.0

[sfackler]

Closes #38863
Closes #38980
Closes #38903
Closes #36648

r? @alexcrichton

@rust-lang/libs

[WiSaGaN]

I think you mean "Closes #33417" instead of "Closes #34417"?

[sfackler]

Fixed, thanks

[alexcrichton]

Looks like some doc building failed? Other than that r=me, thanks @sfackler!

[ollie27]

I don't agree with stabilising TryFrom without impl<T, U> TryFrom<U> for T where T: From<U>. However if it really needs to be stabilised now for whatever reason then it should at least be properly documented. I'd like to see the documentation explaining at least the following:

1. Which types should get an impl of the form TryFrom<T> for T? For example, why is there TryFrom<u8> for u8 but not TryFrom<char> for char?
2. Which From<U> for T impls should get a TryFrom<U> for T impl. For exmple, why is there TryFrom<u8> for u32 but no TryFrom<u8> for char?
3. Do the error types for converting to a certain type have to be the same? That was implied by several comments in Tracking issue for TryFrom/TryInto traits #33417 but TryFrom<u8> for u32 and TryFrom<&str> for u32 have different error types.

Some example impls and uses would also be nice.

[sfackler]

@ollie27 If people want to wait for however many months or years it takes for ! to stabilize before landing TryFrom, then so be it, I've removed it from this PR - I'm done dealing with that mess. It's not even clear to me at this point that a single trait makes sense for all of the things we're asking of it. I'm frankly starting to think we should just deprecate and remove the whole thing.

[alexcrichton]

@sfackler it looks like tidy failed here, but I'm ok handling TryFrom elsewhere if you'd like. Want to fixup tidy and r=me?

[Mark-Simulacrum]

Looks like this is ready for another review; I think @sfackler removed TryFrom with ! as the error but I'm not really sure...

[sfackler]

I just hopefully fixed tidy

[Mark-Simulacrum]

And the mighty tidy says ... no:

[00:02:25] tidy error: The Unstable Book has a 'library feature' section 'try_from' which doesn't correspond to an unstable library feature
[00:02:25] some tidy checks failed
[00:02:25] 
[00:02:25] 
[00:02:25] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools/x86_64-unknown-linux-gnu/release/tidy" "/checkout/src" "--no-vendor"
[00:02:25] expected success, got: exit code: 1
[00:02:25] 

[sfackler]

@bors r=alexcrichton

[bors]

📌 Commit 61ce6a4 has been approved by alexcrichton

[bors]

⌛ Testing commit 61ce6a4 with merge aa5b6fc...

[bors]

💔 Test failed - status-travis

[Mark-Simulacrum]

[01:01:34] failures:
[01:01:34]     collections/hash/map.rs - collections::hash::map::HashMap<K, V, S>::retain (line 1233)
[01:01:34]     collections/hash/set.rs - collections::hash::set::HashSet<T, S>::retain (line 660)
[01:01:34]     net/tcp.rs - net::tcp::TcpStream::peek (line 345)
[01:01:34]     net/udp.rs - net::udp::UdpSocket::peek (line 640)
[01:01:34]     net/udp.rs - net::udp::UdpSocket::peek_from (line 114)

[sfackler]

@bors r=alexcrichton

[bors]

📌 Commit 7c2cd93 has been approved by alexcrichton

[bors]

⌛ Testing commit 7c2cd93 with merge f6cc40f...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing f6cc40f to master...