thir-unsafeck query loops when trying to compute frozenness

[syvb]

I tried this code with -Z thir-unsafeck:

// from src/test/ui/impl-trait/recursive-impl-trait-type-indirect.rs
fn closure_ref_capture() -> impl Sized {
    let x = closure_ref_capture();
    move || { &x; }
}
fn main() {}

I expected to see this happen: It errors with cannot resolve opaque type and a warning

Instead, this happened: There is also a query loop:

error[E0391]: cycle detected when computing type of `closure_ref_capture::{opaque#0}`
 --> freeze2.rs:2:29
  |
2 | fn closure_ref_capture() -> impl Sized {
  |                             ^^^^^^^^^^
  |
note: ...which requires borrow-checking `closure_ref_capture`...
 --> freeze2.rs:2:1
  |
2 | fn closure_ref_capture() -> impl Sized {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires processing `closure_ref_capture`...
 --> freeze2.rs:2:1
  |
2 | fn closure_ref_capture() -> impl Sized {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires processing MIR for `closure_ref_capture`...
 --> freeze2.rs:2:1
  |
2 | fn closure_ref_capture() -> impl Sized {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires unsafety-checking `closure_ref_capture`...
 --> freeze2.rs:2:1
  |
2 | fn closure_ref_capture() -> impl Sized {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires building MIR for `closure_ref_capture`...
 --> freeze2.rs:2:1
  |
2 | fn closure_ref_capture() -> impl Sized {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires unsafety-checking `closure_ref_capture`...
 --> freeze2.rs:4:16
  |
4 |     move || { &x; }
  |                ^
  = note: ...which requires computing whether `impl Sized` is freeze...
  = note: ...which requires evaluating trait selection obligation `impl Sized: std::marker::Freeze`...
  = note: ...which again requires computing type of `closure_ref_capture::{opaque#0}`, completing the cycle
note: cycle used when checking item types in top-level module
 --> freeze2.rs:2:1
  |
2 | fn closure_ref_capture() -> impl Sized {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This also happens when using the new unsafety checker with ui/generator/layout-error.rs, ui/impl-trait/recursive-impl-trait-type-indirect.rs, ui/impl-trait/unsafety-checking-cycle.rs, and ui/type-alias-impl-traiissue-52843-closure-constrain.rs.

Meta

rustc --version --verbose:

rustc 1.56.0-dev

[syvb]

@rustbot label thir-unsafeck

[rustbot]

Error: Label thir-unsafeck can only be set by Rust team members

Please let @rust-lang/release know if you're having trouble with this bot.

[syvb]

This was fixed by #87737.

[LeSeulArtichaut]

For the record, #87737 didn't solve all cases of this. There is also #87850 which affects both the THIR and MIR unsafety checkers.
Interestingly the MIR unsafeck also had this issue which was solved in a similar way with 4af0952.