Treat cycle errors as bug by default

[lcnr]

We should treat unexpected query cycles as bugs, not user-facing errors. Ideally we'd add an attribute to queries to indicate that they are expected to cycle and otherwise ICE on cycles.

Affected queries are crate_variances or effective_visibilities.

---

original issue:

when fetching effective_visibilities for Adt self types of a local trait while proving a where-clause.

rust/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

Lines 2871 to 2889 in 8c32e31

	if let ty::PredicateKind::Clause(clause) = predicate.kind().skip_binder() {
	match clause {
	ty::ClauseKind::Trait(trait_pred) => {
	let def_id = trait_pred.def_id();
	let visible_item = if let Some(local) = def_id.as_local() {
	let ty = trait_pred.self_ty();
	// when `TraitA: TraitB` and `S` only impl TraitA,
	// we check if `TraitB` can be reachable from `S`
	// to determine whether to note `TraitA` is sealed trait.
	if let ty::Adt(adt, _) = ty.kind() {
	let visibilities = tcx.effective_visibilities(());
	visibilities.effective_vis(local).is_none_or(|v| {
	v.at_level(Level::Reexported)
	.is_accessible_from(adt.did(), tcx)
	})
	} else {
	// FIXME(xizheyin): if the type is not ADT, we should not suggest it
	true
	}

This query fetches the hidden type of all opaque types

rust/compiler/rustc_privacy/src/lib.rs

Lines 1841 to 1846 in 8c32e31

	let pub_ev = EffectiveVisibility::from_vis(ty::Visibility::Public);
	visitor
	.reach_through_impl_trait(opaque.def_id, pub_ev)
	.generics()
	.predicates()
	.ty();

introduced in #143431, cc @xizheyin @compiler-errors

trait LocalTrait {}
struct SomeType;
fn impls_trait<T: LocalTrait>() {}
fn foo() -> impl Sized {
    impls_trait::<SomeType>();
}

results in

error[E0391]: cycle detected when computing type of `foo::{opaque#0}`
 --> src/lib.rs:4:13
  |
4 | fn foo() -> impl Sized {
  |             ^^^^^^^^^^
  |
note: ...which requires computing type of opaque `foo::{opaque#0}`...
 --> src/lib.rs:4:13
  |
4 | fn foo() -> impl Sized {
  |             ^^^^^^^^^^
note: ...which requires borrow-checking `foo`...
 --> src/lib.rs:4:1
  |
4 | fn foo() -> impl Sized {
  | ^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires promoting constants in MIR for `foo`...
 --> src/lib.rs:4:1
  |
4 | fn foo() -> impl Sized {
  | ^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires checking if `foo` contains FFI-unwind calls...
 --> src/lib.rs:4:1
  |
4 | fn foo() -> impl Sized {
  | ^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires building MIR for `foo`...
 --> src/lib.rs:4:1
  |
4 | fn foo() -> impl Sized {
  | ^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires match-checking `foo`...
 --> src/lib.rs:4:1
  |
4 | fn foo() -> impl Sized {
  | ^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires type-checking `foo`...
 --> src/lib.rs:4:1
  |
4 | fn foo() -> impl Sized {
  | ^^^^^^^^^^^^^^^^^^^^^^
  = note: ...which requires checking effective visibilities...
  = note: ...which again requires computing type of `foo::{opaque#0}`, completing the cycle
note: cycle used when checking that `foo::{opaque#0}` is well-formed
 --> src/lib.rs:4:13
  |
4 | fn foo() -> impl Sized {
  |             ^^^^^^^^^^
  = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

error[E0277]: the trait bound `SomeType: LocalTrait` is not satisfied
 --> src/lib.rs:5:19
  |
5 |     impls_trait::<SomeType>();
  |                   ^^^^^^^^ the trait `LocalTrait` is not implemented for `SomeType`
  |
help: this trait has no implementations, consider adding one
 --> src/lib.rs:1:1
  |
1 | trait LocalTrait {}
  | ^^^^^^^^^^^^^^^^
note: required by a bound in `impls_trait`
 --> src/lib.rs:3:19
  |
3 | fn impls_trait<T: LocalTrait>() {}
  |                   ^^^^^^^^^^ required by this bound in `impls_trait`

note the query cycle before the actually desired error

[xizheyin]

Regression has been fixed by #145642.
But reopen for unimplemented "Delay cycle error as bug for crate_variances or effective_visibilities"

See: #t-compiler > query cycles: ICE if unexpected queries are involved