Hand-written Debug implementation for TypeTest

[amandasystems]

This adds a hand-written debug format for TypeTests that at least was helpful for me when debugging because I always struggle to remember which component is which. It formats a type test using the Unicode turnstile symbol (for "computes") to illustrate that the test encodes a typing rule that, if it holds, produces a conclusion.

The format is: TypeTest from {originating span} {bound} ⊢ T: 'lower_bound, where T is the generic type being tested and lower_bound is the lower bound. Bounds are formatted as you would expect, but where the region for 'lower_bound is included in the outlives constraints for context. I resisted the urge to turn ALL [A, ..., Z] into [A ∧ ... ∧ Z] etc.

What it looks like

Here's an example of a simple type test from the test suite that says that some type I/#0 will be lower-bounded (outlive) ?4 iff '?1: '?4:

Before:

$ RUSTC_LOG="rustc_borrowck::region_infer=debug" rustc +stage1 tests/ui/associated-types/associated-types-eq-3.rs
...
DEBUG rustc_borrowck::region_infer type tests: [
     TypeTest {
         generic_kind: I/#0,
         lower_bound: '?4,
         span: tests/ui/associated-types/associated-types-eq-3.rs:21:18: 21:25 (#0),
         verify_bound: OutlivedBy(
             '?1,
         ),
     },
     TypeTest {
         generic_kind: I/#0,
         lower_bound: '?4,
         span: tests/ui/associated-types/associated-types-eq-3.rs:21:18: 21:25 (#0),
         verify_bound: OutlivedBy(
             '?1,
         ),
     },
 ]
...

After:

$ RUSTC_LOG="rustc_borrowck::region_infer=debug" rustc +stage1 tests/ui/associated-types/associated-types-eq-3.rs
...
 DEBUG rustc_borrowck::region_infer type tests: [
     TypeTest from tests/ui/associated-types/associated-types-eq-3.rs:21:18: 21:25 (#0)['?1: '?4] ⊢ I/#0: '?4,
     TypeTest from tests/ui/associated-types/associated-types-eq-3.rs:21:18: 21:25 (#0)['?1: '?4] ⊢ I/#0: '?4,
 ]
...

[rustbot]

r? @madsmtm

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[Enselic]

For easier comparison, would you mind providing side-by-side comparisons of old vs new output for some illustrative examples, please?

[amandasystems]

I added one above! It also uh helped me notice an unterminated bracked, so that's good.