Expose more information in get_body_with_borrowck_facts

[voidc]

Verification tools for Rust such as, for example, Creusot or Prusti would benefit from having access to more information computed by the borrow checker.
As a first step in that direction, #86977 added the get_body_with_borrowck_facts API, allowing compiler consumers to obtain a mir::Body with accompanying borrow checker information.
At RustVerify 2023, multiple people working on verification tools expressed their need for a more comprehensive API.
While eventually borrow information could be part of Stable MIR, in the meantime, this PR proposes a more limited approach, extending the existing get_body_with_borrowck_facts API.
In summary, we propose the following changes:

• Permit obtaining the borrow-checked body without necessarily running Polonius
• Return the BorrowSet and the RegionInferenceContext in BodyWithBorrowckFacts
• Provide a way to compute the borrows_out_of_scope_at_location map
• Make some helper methods public

This is similar to #108328 but smaller in scope.
@smoelius Do you think these changes would also be sufficient for your needs?

r? @oli-obk
cc @JonasAlaif

[oli-obk]

These changes look reasonable to me. I cannot promise we'll keep the API around, but I do not foresee this being a maintenance issue for us, so likely some form of it will keep getting provided.

[smoelius]

@smoelius Do you think these changes would also be sufficient for your needs?

I think they might. They look very similar to some local changes that I have. Thank you very much for the ping.

[oli-obk]

please rebase, then we can land this

[oli-obk]

@bors r+

[bors]

📌 Commit 6a8112e1f8c02d89fe78667209183f570c502fac has been approved by oli-obk

It is now in the queue for this repository.

[JonasAlaif]

@voidc would it be possible to slightly extend this and make the constraints field of RegionInferenceContext here public, along with the corresponding OutlivesConstraintSet struct definition and its fn outlives. This seems like a small change to be able to get all of the OutlivesConstraints out, which already happen to be public.

[smoelius]

Sorry if it's not my place to chime in, but will the right commit be pushed? (I can't tell from the queue.)

I think @voidc intends for it to be 8dac074.

But that was pushed after @oli-obk approved.

[oli-obk]

The moment a change is done to a PR it is taken off the queue, so nothing is happening for now until I actually review and r+ it again

[JonasAlaif]

I think that the PR should be ready to merge again now, but let's wait for confirmation from @voidc

[voidc]

Yes, the PR is ready

@rustbot ready

[oli-obk]

@bors r+

[bors]

📌 Commit 8dac074 has been approved by oli-obk

It is now in the queue for this repository.

[lqd]

This seems like a small change to be able to get all of the OutlivesConstraints out, which already happen to be public.

Depending on what you want to achieve, I don't believe OutlivesConstraints would be enough to have a somewhat "complete outlives graph" view of things, since they will not contain member constraints whenever opaque types are involved. (They're not separate in the NLL MIR dump, nor visible in the graphviz region and SCC graphs, and neither present in the polonius input facts IIRC)

pub trait T {}

struct S<'a>(&'a ());

impl<'a> T for S<'a> {}

fn foo() -> impl T {
    let x = ();
    S(&x) //~ ERROR `x` does not live long enough
}

error[E0597]: `x` does not live long enough
  --> test.rs:9:7
   |
8  |     let x = ();
   |         - binding `x` declared here
9  |     S(&x) //~ ERROR `x` does not live long enough
   |     --^^-
   |     | |
   |     | borrowed value does not live long enough
   |     opaque type requires that `x` is borrowed for `'static`
10 | }
   | - `x` dropped here while still borrowed

The constraints field won't have an outlives constraint flowing into 'static here.