Rename the 2018 edition lint names

[alexcrichton]

• rust_2018_breakage -> rust_2018_compatibility - the lint for ensuring
  that your code, in the 2015 edition, is compatible with the 2018 edition's
  semantics. This is required to pass before you enable the 2018 edition.
• rust_2018_migration -> rust_2018_idioms - the lint for writing idiomatic
  code after you've already enabled the 2018 edition

[rust-highfive]

r? @nikomatsakis

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

[Mark-Simulacrum]

What about lints that can change 2015 code to 2015 code while updating certain idioms? For example, I think there's a few features (maybe dyn Trait?) which will work in the 2015 edition, and authors might want to have rust_2018_idioms mean different things depending on the current edition: in the 2015 edition these are non-2018 edition dependent lints, and in 2018 they are all the lints, both those dependent on the edition and not.

[Manishearth]

Those lints can fit in either category, I don't think there are strong opinions on which category they belong in.

@aturon had written a doc about this but I can't find it rn

[aturon]

https://paper.dropbox.com/doc/Edition-preview-cheat-sheet-HhfGRmTX4mByEOWWI2P7o at the bottom

[alexcrichton]

@Mark-Simulacrum I'd personally expect lints like that to get turned on by default over time in the sense that yeah we'd even want 2015 (and really encourage 2018) code to migrate. I may be alone in the opinion of turning these eventually on by default though!

For the most part these lints are actually internal implementation details that others won't access. Rather cargo-fix will translate --prepare-for 2018 to -W rust-2018-compatibility and --edition 2018 would translate to -W rust-2018-idioms (or something like that). In general users won't have to look too closely at these names.

[nikomatsakis]

@bors r+

[bors]

📌 Commit d636aec has been approved by nikomatsakis

[aturon]

@bors: p=1

prioritizing for edition preview

[bors]

⌛ Testing commit d636aec with merge 41707d8...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing 41707d8 to master...