Make sure the new aggregate equality comparison is excluded from const contexts

This is unless the `const_cmp` feature is enabled, in which case `PartialEq`
becomes available in said contexts.

Author: jakubadamw

compiler/rustc_mir_build/src/builder/matches/match_pair.rs

@@ -5,6 +5,7 @@ use rustc_middle::mir::*;
 use rustc_middle::span_bug;
 use rustc_middle::thir::*;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
+use rustc_span::sym;
 
 use crate::builder::Builder;
 use crate::builder::expr::as_place::{PlaceBase, PlaceBuilder};
@@ -39,6 +40,21 @@ fn try_reconstruct_aggregate_constant<'tcx>(
     Some(ty::Value { ty: aggregate_ty, valtree })
 }
 
+impl<'a, 'tcx> Builder<'a, 'tcx> {
+    /// Check if we can use aggregate `PartialEq::eq` comparisons for constant array/slice patterns.
+    /// This is not possible in const contexts unless `#![feature(const_cmp, const_trait_impl)]` are enabled,
+    /// because`PartialEq` is not const-stable.
+    fn can_use_aggregate_eq(&self) -> bool {
+        let const_partial_eq_enabled = {
+            let features = self.tcx.features();
+            features.enabled(sym::const_trait_impl) && features.enabled(sym::const_cmp)
+        };
+        let in_const_context = self.tcx.is_const_fn(self.def_id.to_def_id())
+            || !self.tcx.hir_body_owner_kind(self.def_id).is_fn_or_closure();
+        !in_const_context || const_partial_eq_enabled
+    }
+}
+
 /// For an array or slice pattern's subpatterns (prefix/slice/suffix), returns a list
 /// of those subpatterns, each paired with a suitably-projected [`PlaceBuilder`].
 fn prefix_slice_suffix<'a, 'tcx>(
@@ -252,6 +268,7 @@ impl<'tcx> MatchPairTree<'tcx> {
                     // `PartialEq::eq` rather than element by element.
                     if slice.is_none()
                         && suffix.is_empty()
+                        && cx.can_use_aggregate_eq()
                         && let Some(aggregate_value) =
                             try_reconstruct_aggregate_constant(cx.tcx, pattern.ty, prefix)
                     {
@@ -297,6 +314,7 @@ impl<'tcx> MatchPairTree<'tcx> {
                 // is performed after the length check.
                 if slice.is_none()
                     && suffix.is_empty()
+                    && cx.can_use_aggregate_eq()
                     && let Some(aggregate_value) =
                         try_reconstruct_aggregate_constant(cx.tcx, pattern.ty, prefix)
                 {

compiler/rustc_span/src/symbol.rs

@@ -648,6 +648,7 @@ symbols! {
         const_block_items,
         const_c_variadic,
         const_closures,
+        const_cmp,
         const_compare_raw_pointers,
         const_constructor,
         const_continue,