Check where-clauses during monomorphization

compiler/rustc_middle/src/query/mod.rs

@@ -2483,6 +2483,12 @@ rustc_queries! {
         cache_on_disk_if { true }
     }
 
+    // UwU
+    query check_instance_wf(key: ty::Instance<'tcx>) {
+        desc { "monomorphization-time checking of where-clauses" }
+        cache_on_disk_if { true }
+    }
+
     /// Builds the set of functions that should be skipped for the move-size check.
     query skip_move_check_fns(_: ()) -> &'tcx FxIndexSet<DefId> {
         arena_cache

compiler/rustc_monomorphize/src/mono_checks/mod.rs

@@ -12,6 +12,7 @@ fn check_mono_item<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) {
     let body = tcx.instance_mir(instance.def);
     abi_check::check_feature_dependent_abi(tcx, instance, body);
     move_check::check_moves(tcx, instance, body);
+    tcx.ensure().check_instance_wf(instance);
 }
 
 pub(super) fn provide(providers: &mut Providers) {

compiler/rustc_traits/src/check_instance_wf.rs

@@ -0,0 +1,28 @@
+use rustc_infer::infer::TyCtxtInferExt as _;
+use rustc_middle::ty::{self, TyCtxt, TypingEnv};
+use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
+use rustc_trait_selection::traits::{Obligation, ObligationCause, ObligationCtxt};
+
+pub(crate) fn check_instance_wf<'tcx>(tcx: TyCtxt<'tcx>, instance: ty::Instance<'tcx>) {
+    let ty::InstanceKind::Item(def_id) = instance.def else {
+        // Probably want other instances too...:
+        return;
+    };
+
+    let (infcx, param_env) =
+        tcx.infer_ctxt().build_with_typing_env(TypingEnv::fully_monomorphized());
+    let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
+
+    ocx.register_obligations(
+        tcx.predicates_of(def_id).instantiate(tcx, instance.args).into_iter().map(
+            |(clause, span)| {
+                Obligation::new(tcx, ObligationCause::dummy_with_span(span), param_env, clause)
+            },
+        ),
+    );
+
+    let errors = ocx.select_all_or_error();
+    if !errors.is_empty() {
+        infcx.err_ctxt().report_fulfillment_errors(errors);
+    }
+}

compiler/rustc_traits/src/lib.rs

@@ -5,6 +5,7 @@
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
+mod check_instance_wf;
 mod codegen;
 mod dropck_outlives;
 mod evaluate_obligation;
@@ -25,4 +26,5 @@ pub fn provide(p: &mut Providers) {
     normalize_erasing_regions::provide(p);
     type_op::provide(p);
     p.codegen_select_candidate = codegen::codegen_select_candidate;
+    p.check_instance_wf = check_instance_wf::check_instance_wf;
 }