Add Three point error handling to borrowck

[spastorino]

This is for rust-lang#45988

[nikomatsakis]

Let's fix the pub thing -- as for the UI tests, if you modify the code in region_infer/mod.rs and find the place where it enables tracing only if nll_dump_cause is true -- just make it enable tracing unconditionally, then we don't need to pass -Znll-dump-cause

[nikomatsakis]

this probably does not need to be pub...? in particular, submodules can still access it when it is priv

[spastorino]

error[E0446]: private type `borrow_check::Context` in public interface
  --> src/librustc_mir/borrow_check/nll/explain_borrow/mod.rs:21:5
   |
21 | /     pub fn explain_why_borrow_contains_point(
22 | |         &self,
23 | |         context: Context,
24 | |         borrow: &BorrowData<'_>,
...  |
78 | |         }
79 | |     }
   | |_____^ can't leak private type

error: aborting due to previous error

[nikomatsakis]

change that pub fn explain to pub(super) fn explain

[spastorino]

👍

[nikomatsakis]

maybe we should move this call to only execute if cause.root_cause() returns something other than LiveVar or DropVar... that would avoid dump extra causal information when we can give a more user-friendly version. But really I guess we just want to make the "cause-gathering" stuff enabled or lazy, so never mind.

[spastorino]

👍, so should just leave as is ?

[spastorino]

Let's fix the pub thing -- as for the UI tests, if you modify the code in region_infer/mod.rs and find the
place where it enables tracing only if nll_dump_cause is true -- just make it enable tracing
unconditionally, then we don't need to pass -Znll-dump-cause

I don't understand what you mean here. Enabling this unconditionally won't in some way rollback your work? in that case isn't it better to remove the commit that adds is from code

[nikomatsakis]

I don't understand what you mean here. Enabling this unconditionally won't in some way rollback your work? in that case isn't it better to remove the commit that adds is from code

Sure. Just make a single commit and I'll rebase it later and fixup.