Rollup merge of rust-lang#153709 - zetanumbers:from_cycle_error_cleanup, r=nnethercote

Fix hypothetical ICE in `variances_of`

A cleanup based on rust-lang#153694.

~~This PR changes panic messages in `fn_sig`'s `value_from_cycle_error` to more fitting ones.~~ Also this PR makes `variances_of`'s `value_from_cycle_error` to utilize new query key argument instead of extracting it from `CycleError`.

Closes rust-lang#127971

r? @nnethercote

Author: Zalathar

compiler/rustc_query_impl/src/from_cycle_error.rs

@@ -8,13 +8,13 @@ use rustc_errors::codes::*;
 use rustc_errors::{Applicability, Diag, MultiSpan, pluralize, struct_span_code_err};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
+use rustc_middle::bug;
 use rustc_middle::dep_graph::DepKind;
 use rustc_middle::queries::{QueryVTables, TaggedQueryKey};
 use rustc_middle::query::CycleError;
 use rustc_middle::query::erase::erase_val;
 use rustc_middle::ty::layout::LayoutError;
 use rustc_middle::ty::{self, Ty, TyCtxt};
-use rustc_middle::{bug, span_bug};
 use rustc_span::def_id::{DefId, LocalDefId};
 use rustc_span::{ErrorGuaranteed, Span};
 
@@ -32,9 +32,9 @@ pub(crate) fn specialize_query_vtables<'tcx>(vtables: &mut QueryVTables<'tcx>) {
     vtables.check_representability_adt_ty.value_from_cycle_error =
         |tcx, _, cycle, _err| check_representability(tcx, cycle);
 
-    vtables.variances_of.value_from_cycle_error = |tcx, _, cycle, err| {
+    vtables.variances_of.value_from_cycle_error = |tcx, key, _, err| {
         let _guar = err.delay_as_bug();
-        erase_val(variances_of(tcx, cycle))
+        erase_val(variances_of(tcx, key))
     };
 
     vtables.layout_of.value_from_cycle_error = |tcx, _, cycle, err| {
@@ -105,27 +105,9 @@ fn check_representability<'tcx>(tcx: TyCtxt<'tcx>, cycle_error: CycleError<'tcx>
     guar.raise_fatal()
 }
 
-fn variances_of<'tcx>(tcx: TyCtxt<'tcx>, cycle_error: CycleError<'tcx>) -> &'tcx [ty::Variance] {
-    search_for_cycle_permutation(
-        &cycle_error.cycle,
-        |cycle| {
-            if let Some(frame) = cycle.get(0)
-                && frame.node.dep_kind == DepKind::variances_of
-                && let Some(def_id) = frame.node.def_id
-            {
-                let n = tcx.generics_of(def_id).own_params.len();
-                ControlFlow::Break(tcx.arena.alloc_from_iter(iter::repeat_n(ty::Bivariant, n)))
-            } else {
-                ControlFlow::Continue(())
-            }
-        },
-        || {
-            span_bug!(
-                cycle_error.usage.as_ref().unwrap().span,
-                "only `variances_of` returns `&[ty::Variance]`"
-            )
-        },
-    )
+fn variances_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> &'tcx [ty::Variance] {
+    let n = tcx.generics_of(def_id).own_params.len();
+    tcx.arena.alloc_from_iter(iter::repeat_n(ty::Bivariant, n))
 }
 
 // Take a cycle of `Q` and try `try_cycle` on every permutation, falling back to `otherwise`.