🛠️ refactor away the `inferred_obligations` from the trait selector

[nikomatsakis]

The SelectionContext currently has this vector, inferred_obligations:

rust/src/librustc/traits/select.rs

Line 95 in 9cb18a9

inferred_obligations: SnapshotVec<InferredObligationsSnapshotVecDelegate<'tcx>>,

It is used to accumulate obligations during matching operations and so forth that takes in a select, for example here:

rust/src/librustc/traits/select.rs

Line 1559 in 9cb18a9

self.inferred_obligations.extend(obligations);

These are then stored in the vector and later added to the candidate currently under construction:

rust/src/librustc/traits/select.rs

Lines 565 to 566 in 9cb18a9

	let inferred_obligations = (*self.inferred_obligations).into_iter().cloned();
	candidate.nested_obligations_mut().extend(inferred_obligations);

It's a kinda confusing setup, since the flow of information is hidden, and it's not really needed. This commit factors it away, but it does so in the context of a long branch, and can't really be directly cherry-picked:

nikomatsakis@627a2b4

Still, the general strategy should be visible. Instead of using the inferred_obligations vector, we basically just thread obligations around explicitly.

[nikomatsakis]

cc @rust-lang/wg-traits -- this would be a good refactoring to get to know how select.rs works. This chapter of the rustc-guide should help a bit too to get oriented.

[aravind-pg]

I'll try my hand at this.