Cargo fix --prepare-for should only apply edition fixes

[alexcrichton]

Originally reported as rust-lang/rustfix#129

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As per rust-lang/rustfix#120 it applies all suggestions, which include the builtin lints.

[alexcrichton]

@Mark-Simulacrum on IRC suggested we pass -A warnings -W rust-2018-compatibility which I think will work like a charm here! (we should test though)

[killercup]

@alexcrichton I'll give it a try, but this previously didn't work

[alexcrichton]

Oh bah you are indeed correct (still!).

That's due to this block of code.

That means probably we could either:

• Fix this as a bug in rustc (maybe having lots of unintended fallout)
• Pass a whole slew of -A and hope we get most of the ones that matter
• Simply don't auto-fix anything that doesn't pass our whitelist of what to look at

I'm leaning towards the last option now, but it'd probably be easier to go with the second. @killercup do you have a preference?

[killercup]

Great research!

I think the last option might be one with the least issues going forward – adjusting an allow-list when people add new edition specific lints is trivial. What do you mean with "easier to go with the second" – tracking (and keeping track of!) all enabled-by-default lints in rustc for the foreseeable future? Sheesh!

[alexcrichton]

Oh more just easier to implement because it's just modifying a string that's already in the code :)

I'm cool w/ the third option though!

[alexcrichton]

Bumping this to the RC1 milestone