Add key and value methods to DebugMap

[KodrAus]

Implementation PR for an active (not approved) RFC: rust-lang/rfcs#2696.

Add two new methods to std::fmt::DebugMap for writing the key and value part of a map entry separately:

impl<'a,'b:'a>DebugMap<'a,'b>{pubfnkey(&mutself,key:&dynDebug) -> &mutSelf;pubfnvalue(&mutself,value:&dynDebug) -> &mutSelf;}

I want to do this so that I can write a serde::Serializer that forwards to our format builders, so that any T: Serialize can also be treated like a T: Debug.

[rust-highfive]

Some changes occurred in HTML/CSS.

cc @GuillaumeGomez

[rust-highfive]

r? @rkruppe

(rust_highfive has picked a reviewer for you, use r? to override)

[KodrAus]

Shifting the state of the PadAdapter into a dedicated struct here lets value pick up where key left off without having to guess what the state was before.

[GuillaumeGomez]

@rust-lang/infra: seems like a condition is broken:

Some changes occurred in HTML/CSS.

cc @GuillaumeGomez

[emilyalbini]

Highfive is working as expected though:

https://github.com/rust-lang/highfive/blob/748c035ca2a913cb3bc95404f8665985736f165c/highfive/configs/rust-lang/rust.json#L50-L53

[GuillaumeGomez]

Ah indeed! My bad.

[KodrAus]

Very quick, very dirty benchmark:

#![feature(test)]externcrate test;use std::{
cell::Cell,
fmt::{self,Write},};structNoOpWrite;implWriteforNoOpWrite{fnwrite_str(&mutself, _:&str) -> fmt::Result{Ok(())}}#[bench]fndebugmap_entry(b:&mut test::Bencher){structMap<'a>(Cell<Option<&'amut test::Bencher>>);impl<'a> fmt::DebugforMap<'a>{fnfmt(&self,f:&mut fmt::Formatter) -> fmt::Result{letmut map = f.debug_map();let b = self.0.take().unwrap();
b.iter(|| {
map.entry(&"a",&42);});
map.finish()}}let _ = write!(NoOpWrite,"{:?}",Map(Cell::new(Some(b))));}

On my machine, running this on the current nightly and this PR yields:

Before	After
36 ns/iter (+/- 7)	44 ns/iter (+/- 2)

So there's a bit of a performance impact like you'd expect, but I don't think it's too significant, and if it is we can inline the implementation in entry.

[KodrAus]

r? @alexcrichton

[alexcrichton]

This all looks great to me! I think this is still waiting on the RFC, though, right?

(FWIW I wouldn't personally consider this as requiring an RFC, but I don't mind waiting if one's open!)

[KodrAus]

Thanks! Yeh, there's an RFC for this at the moment, so I'm also happy to let this sit and wait on that.

[KodrAus]

Alrighty, the RFC has been merged and I've updated the stability attributes to point to the tracking issue.

I think this should be ready to go now!

[alexcrichton]

r=me, thanks!

Looks like CI may be failing though?

[KodrAus]

Looks like we're all good!

@bors r=alexcrichton

[Centril]

@bors r=alexcrichton

[bors]

📌 Commit 70d630f has been approved by alexcrichton

[bors]

⌛ Testing commit 70d630f with merge 909f5a0...