thir-unsafeck query loops on closures in const-generics

[syvb]

I tried this code with -Z thir-unsafeck=on:

fn bad<T>() -> Box<dyn Iterator<Item = [(); { |x: u32| { x }; 4 }]>> { todo!() }
fn main() {}

I expected to see this happen: It compiles, as it does under the MIR checker

Instead, this happened:

error[E0391]: cycle detected when simplifying constant for the type system `bad::{constant#0}`
 --> y.rs:1:45
  |
1 | fn bad<T>() -> Box<dyn Iterator<Item = [(); { |x: u32| { x }; 4 }]>> { todo!() }
  |                                             ^^^^^^^^^^^^^^^^^^^^^
  |
note: ...which requires simplifying constant for the type system `bad::{constant#0}`...
 --> y.rs:1:45
  |
1 | fn bad<T>() -> Box<dyn Iterator<Item = [(); { |x: u32| { x }; 4 }]>> { todo!() }
  |                                             ^^^^^^^^^^^^^^^^^^^^^
note: ...which requires const-evaluating + checking `bad::{constant#0}`...
 --> y.rs:1:45
  |
1 | fn bad<T>() -> Box<dyn Iterator<Item = [(); { |x: u32| { x }; 4 }]>> { todo!() }
  |                                             ^^^^^^^^^^^^^^^^^^^^^
note: ...which requires const checking `bad::{constant#0}`...
 --> y.rs:1:45
  |
1 | fn bad<T>() -> Box<dyn Iterator<Item = [(); { |x: u32| { x }; 4 }]>> { todo!() }
  |                                             ^^^^^^^^^^^^^^^^^^^^^
note: ...which requires processing MIR for `bad::{constant#0}`...
 --> y.rs:1:45
  |
1 | fn bad<T>() -> Box<dyn Iterator<Item = [(); { |x: u32| { x }; 4 }]>> { todo!() }
  |                                             ^^^^^^^^^^^^^^^^^^^^^
note: ...which requires unsafety-checking `bad::{constant#0}`...
 --> y.rs:1:45
  |
1 | fn bad<T>() -> Box<dyn Iterator<Item = [(); { |x: u32| { x }; 4 }]>> { todo!() }
  |                                             ^^^^^^^^^^^^^^^^^^^^^
note: ...which requires unsafety-checking `bad::{constant#0}::{closure#0}`...
 --> y.rs:1:47
  |
1 | fn bad<T>() -> Box<dyn Iterator<Item = [(); { |x: u32| { x }; 4 }]>> { todo!() }
  |                                               ^^^^^^^^
note: ...which requires building MIR for `bad::{constant#0}::{closure#0}`...
 --> y.rs:1:47
  |
1 | fn bad<T>() -> Box<dyn Iterator<Item = [(); { |x: u32| { x }; 4 }]>> { todo!() }
  |                                               ^^^^^^^^
note: ...which requires unsafety-checking `bad::{constant#0}::{closure#0}`...
 --> y.rs:1:47
  |
1 | fn bad<T>() -> Box<dyn Iterator<Item = [(); { |x: u32| { x }; 4 }]>> { todo!() }
  |                                               ^^^^^^^^
note: ...which requires unsafety-checking `bad`...
 --> y.rs:1:1
  |
1 | fn bad<T>() -> Box<dyn Iterator<Item = [(); { |x: u32| { x }; 4 }]>> { todo!() }
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires building THIR for `bad`...
 --> y.rs:1:1
  |
1 | fn bad<T>() -> Box<dyn Iterator<Item = [(); { |x: u32| { x }; 4 }]>> { todo!() }
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires type-checking `bad`...
 --> y.rs:1:1
  |
1 | fn bad<T>() -> Box<dyn Iterator<Item = [(); { |x: u32| { x }; 4 }]>> { todo!() }
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: ...which again requires simplifying constant for the type system `bad::{constant#0}`, completing the cycle
note: cycle used when checking that `bad` is well-formed
 --> y.rs:1:1
  |
1 | fn bad<T>() -> Box<dyn Iterator<Item = [(); { |x: u32| { x }; 4 }]>> { todo!() }
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Meta

rustc --version --verbose:

$ rustc +local --version --verbose
rustc 1.55.0-dev
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.55.0-dev
LLVM version: 12.0.1

[FabianWolff]

Reduced (either of the following two functions reproduces the issue):

fn foo() -> [(); { |x: u32| { x }; 4 }] { todo!() }
fn bar() { let _: [(); { |x: u32| { x }; 4 }]; }

This is caused by the fact that closures are handled by their owner:

rust/compiler/rustc_mir_build/src/check_unsafety.rs

Lines 600 to 609 in fc24bce

	// Closures are handled by their owner, if it has a body
	if tcx.is_closure(def.did.to_def_id()) {
	let owner = tcx.hir().local_def_id_to_hir_id(def.did).owner;
	let owner_hir_id = tcx.hir().local_def_id_to_hir_id(owner);
	if tcx.hir().maybe_body_owned_by(owner_hir_id).is_some() {
	tcx.ensure().thir_check_unsafety(owner);
	return;
	}
	}

@rustbot claim