diff --git a/compiler/rustc_hir_analysis/messages.ftl b/compiler/rustc_hir_analysis/messages.ftl
index bcf4d7f59e34..8022a48ee137 100644
--- a/compiler/rustc_hir_analysis/messages.ftl
+++ b/compiler/rustc_hir_analysis/messages.ftl
@@ -165,6 +165,10 @@ hir_analysis_drop_impl_reservation = reservation `Drop` impls are not supported
 hir_analysis_duplicate_precise_capture = cannot capture parameter `{$name}` twice
     .label = parameter captured again here
 
+hir_analysis_eii_with_generics =
+    #[{$eii_name}] cannot have generic parameters other than lifetimes
+    .label = required by this attribute
+
 hir_analysis_empty_specialization = specialization impl does not specialize any associated items
     .note = impl is a specialization of this impl
 
@@ -300,6 +304,13 @@ hir_analysis_lifetime_not_captured = `impl Trait` captures lifetime parameter, b
     .label = lifetime captured due to being mentioned in the bounds of the `impl Trait`
     .param_label = this lifetime parameter is captured
 
+hir_analysis_lifetimes_or_bounds_mismatch_on_eii =
+    lifetime parameters or bounds of `{$ident}` do not match the declaration
+    .label = lifetimes do not match
+    .generics_label = lifetimes in impl do not match this signature
+    .where_label = this `where` clause might not match the one in the declaration
+    .bounds_label = this bound might be missing in the implementation
+
 hir_analysis_lifetimes_or_bounds_mismatch_on_trait =
     lifetime parameters or bounds on {$item_kind} `{$ident}` do not match the trait declaration
     .label = lifetimes do not match {$item_kind} in trait
diff --git a/compiler/rustc_hir_analysis/src/check/compare_eii.rs b/compiler/rustc_hir_analysis/src/check/compare_eii.rs
new file mode 100644
index 000000000000..06b7f4952957
--- /dev/null
+++ b/compiler/rustc_hir_analysis/src/check/compare_eii.rs
@@ -0,0 +1,424 @@
+use std::borrow::Cow;
+use std::iter;
+
+use rustc_data_structures::fx::FxIndexSet;
+use rustc_errors::{Applicability, E0805, struct_span_code_err};
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::{self as hir, FnSig, HirId, ItemKind};
+use rustc_infer::infer::{self, InferCtxt, TyCtxtInferExt};
+use rustc_infer::traits::{ObligationCause, ObligationCauseCode};
+use rustc_middle::ty::error::{ExpectedFound, TypeError};
+use rustc_middle::ty::{self, TyCtxt, TypingMode};
+use rustc_span::{ErrorGuaranteed, Ident, Span, Symbol};
+use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
+use rustc_trait_selection::regions::InferCtxtRegionExt;
+use rustc_trait_selection::traits::ObligationCtxt;
+use tracing::{debug, instrument};
+
+use super::potentially_plural_count;
+use crate::errors::{EiiWithGenerics, LifetimesOrBoundsMismatchOnEii};
+
+/// checks whether the signature of some `external_impl`, matches
+/// the signature of `declaration`, which it is supposed to be compatible
+/// with in order to implement the item.
+pub(crate) fn compare_eii_function_types<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    external_impl: LocalDefId,
+    declaration: DefId,
+    eii_name: Symbol,
+    eii_attr_span: Span,
+) -> Result<(), ErrorGuaranteed> {
+    check_is_structurally_compatible(tcx, external_impl, declaration, eii_name, eii_attr_span)?;
+
+    let external_impl_span = tcx.def_span(external_impl);
+    let cause = ObligationCause::new(
+        external_impl_span,
+        external_impl,
+        ObligationCauseCode::CompareEii { external_impl, declaration },
+    );
+
+    // FIXME(eii): even if we don't support generic functions, we should support explicit outlive bounds here
+    let param_env = tcx.param_env(declaration);
+
+    let infcx = &tcx.infer_ctxt().build(TypingMode::non_body_analysis());
+    let ocx = ObligationCtxt::new_with_diagnostics(infcx);
+
+    // We now need to check that the signature of the impl method is
+    // compatible with that of the trait method. We do this by
+    // checking that `impl_fty <: trait_fty`.
+    //
+    // FIXME: We manually instantiate the trait method here as we need
+    // to manually compute its implied bounds. Otherwise this could just
+    // be ocx.sub(impl_sig, trait_sig).
+
+    let wf_tys = FxIndexSet::default();
+    let norm_cause = ObligationCause::misc(external_impl_span, external_impl);
+
+    let declaration_sig = tcx.fn_sig(declaration).instantiate_identity();
+    let declaration_sig = infcx.enter_forall_and_leak_universe(declaration_sig);
+    let declaration_sig = ocx.normalize(&norm_cause, param_env, declaration_sig);
+
+    let external_impl_sig = infcx.instantiate_binder_with_fresh_vars(
+        external_impl_span,
+        infer::BoundRegionConversionTime::HigherRankedType,
+        tcx.fn_sig(external_impl).instantiate(
+            tcx,
+            infcx.fresh_args_for_item(external_impl_span, external_impl.to_def_id()),
+        ),
+    );
+    let external_impl_sig = ocx.normalize(&norm_cause, param_env, external_impl_sig);
+    debug!(?external_impl_sig);
+
+    // FIXME: We'd want to keep more accurate spans than "the method signature" when
+    // processing the comparison between the trait and impl fn, but we sadly lose them
+    // and point at the whole signature when a trait bound or specific input or output
+    // type would be more appropriate. In other places we have a `Vec<Span>`
+    // corresponding to their `Vec<Predicate>`, but we don't have that here.
+    // Fixing this would improve the output of test `issue-83765.rs`.
+    let result = ocx.sup(&cause, param_env, declaration_sig, external_impl_sig);
+
+    if let Err(terr) = result {
+        debug!(?external_impl_sig, ?declaration_sig, ?terr, "sub_types failed");
+
+        let emitted = report_eii_mismatch(
+            infcx,
+            cause,
+            param_env,
+            terr,
+            (declaration, declaration_sig),
+            (external_impl, external_impl_sig),
+            eii_attr_span,
+            eii_name,
+        );
+        return Err(emitted);
+    }
+
+    // Check that all obligations are satisfied by the implementation's
+    // version.
+    let errors = ocx.evaluate_obligations_error_on_ambiguity();
+    if !errors.is_empty() {
+        let reported = infcx.err_ctxt().report_fulfillment_errors(errors);
+        return Err(reported);
+    }
+
+    // Finally, resolve all regions. This catches wily misuses of
+    // lifetime parameters.
+    let errors = infcx.resolve_regions(external_impl, param_env, wf_tys);
+    if !errors.is_empty() {
+        return Err(infcx
+            .tainted_by_errors()
+            .unwrap_or_else(|| infcx.err_ctxt().report_region_errors(external_impl, &errors)));
+    }
+
+    Ok(())
+}
+
+/// Checks a bunch of different properties of the impl/trait methods for
+/// compatibility, such as asyncness, number of argument, self receiver kind,
+/// and number of early- and late-bound generics.
+fn check_is_structurally_compatible<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    external_impl: LocalDefId,
+    declaration: DefId,
+    eii_name: Symbol,
+    eii_attr_span: Span,
+) -> Result<(), ErrorGuaranteed> {
+    check_no_generics(tcx, external_impl, declaration, eii_name, eii_attr_span)?;
+    compare_number_of_method_arguments(tcx, external_impl, declaration, eii_name, eii_attr_span)?;
+    check_region_bounds_on_impl_item(tcx, external_impl, declaration, eii_attr_span)?;
+    Ok(())
+}
+
+fn check_no_generics<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    external_impl: LocalDefId,
+    _declaration: DefId,
+    eii_name: Symbol,
+    eii_attr_span: Span,
+) -> Result<(), ErrorGuaranteed> {
+    let generics = tcx.generics_of(external_impl);
+    if generics.own_requires_monomorphization() {
+        tcx.dcx().emit_err(EiiWithGenerics {
+            span: tcx.def_span(external_impl),
+            attr: eii_attr_span,
+            eii_name,
+        });
+    }
+
+    Ok(())
+}
+
+fn compare_number_of_method_arguments<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    external_impl: LocalDefId,
+    declaration: DefId,
+    eii_name: Symbol,
+    eii_attr_span: Span,
+) -> Result<(), ErrorGuaranteed> {
+    let external_impl_fty = tcx.fn_sig(external_impl);
+    let declaration_fty = tcx.fn_sig(declaration);
+    let declaration_number_args = declaration_fty.skip_binder().inputs().skip_binder().len();
+    let external_impl_number_args = external_impl_fty.skip_binder().inputs().skip_binder().len();
+    let external_impl_name = tcx.item_name(external_impl.to_def_id());
+
+    if declaration_number_args != external_impl_number_args {
+        let declaration_span = declaration
+            .as_local()
+            .and_then(|def_id| {
+                let declaration_sig = get_declaration_sig(tcx, def_id).expect("foreign item sig");
+                let pos = declaration_number_args.saturating_sub(1);
+                declaration_sig.decl.inputs.get(pos).map(|arg| {
+                    if pos == 0 {
+                        arg.span
+                    } else {
+                        arg.span.with_lo(declaration_sig.decl.inputs[0].span.lo())
+                    }
+                })
+            })
+            .or_else(|| tcx.hir_span_if_local(declaration))
+            .unwrap_or_else(|| tcx.def_span(declaration));
+
+        let (_, external_impl_sig, _, _) = &tcx.hir_expect_item(external_impl).expect_fn();
+        let pos = external_impl_number_args.saturating_sub(1);
+        let impl_span = external_impl_sig
+            .decl
+            .inputs
+            .get(pos)
+            .map(|arg| {
+                if pos == 0 {
+                    arg.span
+                } else {
+                    arg.span.with_lo(external_impl_sig.decl.inputs[0].span.lo())
+                }
+            })
+            .unwrap_or_else(|| tcx.def_span(external_impl));
+
+        let mut err = struct_span_code_err!(
+            tcx.dcx(),
+            impl_span,
+            E0805,
+            "`{external_impl_name}` has {} but #[{eii_name}] requires it to have {}",
+            potentially_plural_count(external_impl_number_args, "parameter"),
+            declaration_number_args
+        );
+
+        // if let Some(declaration_span) = declaration_span {
+        err.span_label(
+            declaration_span,
+            format!("requires {}", potentially_plural_count(declaration_number_args, "parameter")),
+        );
+        // }
+
+        err.span_label(
+            impl_span,
+            format!(
+                "expected {}, found {}",
+                potentially_plural_count(declaration_number_args, "parameter"),
+                external_impl_number_args
+            ),
+        );
+
+        err.span_label(eii_attr_span, format!("required because of this attribute"));
+
+        return Err(err.emit());
+    }
+
+    Ok(())
+}
+
+fn check_region_bounds_on_impl_item<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    external_impl: LocalDefId,
+    declaration: DefId,
+    eii_attr_span: Span,
+) -> Result<(), ErrorGuaranteed> {
+    let external_impl_generics = tcx.generics_of(external_impl.to_def_id());
+    let external_impl_params = external_impl_generics.own_counts().lifetimes;
+
+    let declaration_generics = tcx.generics_of(declaration);
+    let declaration_params = declaration_generics.own_counts().lifetimes;
+
+    debug!(?declaration_generics, ?external_impl_generics);
+
+    // Must have same number of early-bound lifetime parameters.
+    // Unfortunately, if the user screws up the bounds, then this
+    // will change classification between early and late. E.g.,
+    // if in trait we have `<'a,'b:'a>`, and in impl we just have
+    // `<'a,'b>`, then we have 2 early-bound lifetime parameters
+    // in trait but 0 in the impl. But if we report "expected 2
+    // but found 0" it's confusing, because it looks like there
+    // are zero. Since I don't quite know how to phrase things at
+    // the moment, give a kind of vague error message.
+    if declaration_params != external_impl_params {
+        let span = tcx
+            .hir_get_generics(external_impl)
+            .expect("expected impl item to have generics or else we can't compare them")
+            .span;
+
+        let mut generics_span = None;
+        let mut bounds_span = vec![];
+        let mut where_span = None;
+
+        if let Some(declaration_node) = tcx.hir_get_if_local(declaration)
+            && let Some(declaration_generics) = declaration_node.generics()
+        {
+            generics_span = Some(declaration_generics.span);
+            // FIXME: we could potentially look at the impl's bounds to not point at bounds that
+            // *are* present in the impl.
+            for p in declaration_generics.predicates {
+                if let hir::WherePredicateKind::BoundPredicate(pred) = p.kind {
+                    for b in pred.bounds {
+                        if let hir::GenericBound::Outlives(lt) = b {
+                            bounds_span.push(lt.ident.span);
+                        }
+                    }
+                }
+            }
+            if let Some(implementation_generics) = tcx.hir_get_generics(external_impl) {
+                let mut impl_bounds = 0;
+                for p in implementation_generics.predicates {
+                    if let hir::WherePredicateKind::BoundPredicate(pred) = p.kind {
+                        for b in pred.bounds {
+                            if let hir::GenericBound::Outlives(_) = b {
+                                impl_bounds += 1;
+                            }
+                        }
+                    }
+                }
+                if impl_bounds == bounds_span.len() {
+                    bounds_span = vec![];
+                } else if implementation_generics.has_where_clause_predicates {
+                    where_span = Some(implementation_generics.where_clause_span);
+                }
+            }
+        }
+        let mut diag = tcx.dcx().create_err(LifetimesOrBoundsMismatchOnEii {
+            span,
+            ident: tcx.item_name(external_impl.to_def_id()),
+            generics_span,
+            bounds_span,
+            where_span,
+        });
+
+        diag.span_label(eii_attr_span, format!("required because of this attribute"));
+        return Err(diag.emit());
+    }
+
+    Ok(())
+}
+
+fn report_eii_mismatch<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    mut cause: ObligationCause<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    terr: TypeError<'tcx>,
+    (declaration_did, declaration_sig): (DefId, ty::FnSig<'tcx>),
+    (external_impl_did, external_impl_sig): (LocalDefId, ty::FnSig<'tcx>),
+    eii_attr_span: Span,
+    eii_name: Symbol,
+) -> ErrorGuaranteed {
+    let tcx = infcx.tcx;
+    let (impl_err_span, trait_err_span, external_impl_name) =
+        extract_spans_for_error_reporting(infcx, terr, &cause, declaration_did, external_impl_did);
+
+    let mut diag = struct_span_code_err!(
+        tcx.dcx(),
+        impl_err_span,
+        E0805,
+        "function `{}` has a type that is incompatible with the declaration of `#[{eii_name}]`",
+        external_impl_name
+    );
+
+    diag.span_note(eii_attr_span, "expected this because of this attribute");
+
+    match &terr {
+        TypeError::ArgumentMutability(i) | TypeError::ArgumentSorts(_, i) => {
+            if declaration_sig.inputs().len() == *i {
+                // Suggestion to change output type. We do not suggest in `async` functions
+                // to avoid complex logic or incorrect output.
+                if let ItemKind::Fn { sig, .. } = &tcx.hir_expect_item(external_impl_did).kind
+                    && !sig.header.asyncness.is_async()
+                {
+                    let msg = "change the output type to match the declaration";
+                    let ap = Applicability::MachineApplicable;
+                    match sig.decl.output {
+                        hir::FnRetTy::DefaultReturn(sp) => {
+                            let sugg = format!(" -> {}", declaration_sig.output());
+                            diag.span_suggestion_verbose(sp, msg, sugg, ap);
+                        }
+                        hir::FnRetTy::Return(hir_ty) => {
+                            let sugg = declaration_sig.output();
+                            diag.span_suggestion_verbose(hir_ty.span, msg, sugg, ap);
+                        }
+                    };
+                };
+            } else if let Some(trait_ty) = declaration_sig.inputs().get(*i) {
+                diag.span_suggestion_verbose(
+                    impl_err_span,
+                    "change the parameter type to match the declaration",
+                    trait_ty,
+                    Applicability::MachineApplicable,
+                );
+            }
+        }
+        _ => {}
+    }
+
+    cause.span = impl_err_span;
+    infcx.err_ctxt().note_type_err(
+        &mut diag,
+        &cause,
+        trait_err_span.map(|sp| (sp, Cow::from("type in declaration"), false)),
+        Some(param_env.and(infer::ValuePairs::PolySigs(ExpectedFound {
+            expected: ty::Binder::dummy(declaration_sig),
+            found: ty::Binder::dummy(external_impl_sig),
+        }))),
+        terr,
+        false,
+        None,
+    );
+
+    diag.emit()
+}
+
+#[instrument(level = "debug", skip(infcx))]
+fn extract_spans_for_error_reporting<'tcx>(
+    infcx: &infer::InferCtxt<'tcx>,
+    terr: TypeError<'_>,
+    cause: &ObligationCause<'tcx>,
+    declaration: DefId,
+    external_impl: LocalDefId,
+) -> (Span, Option<Span>, Ident) {
+    let tcx = infcx.tcx;
+    let (mut external_impl_args, external_impl_name) = {
+        let item = tcx.hir_expect_item(external_impl);
+        let (ident, sig, _, _) = item.expect_fn();
+        (sig.decl.inputs.iter().map(|t| t.span).chain(iter::once(sig.decl.output.span())), ident)
+    };
+
+    let declaration_args = declaration.as_local().map(|def_id| {
+        if let Some(sig) = get_declaration_sig(tcx, def_id) {
+            sig.decl.inputs.iter().map(|t| t.span).chain(iter::once(sig.decl.output.span()))
+        } else {
+            panic!("expected {def_id:?} to be a foreign function");
+        }
+    });
+
+    match terr {
+        TypeError::ArgumentMutability(i) | TypeError::ArgumentSorts(ExpectedFound { .. }, i) => (
+            external_impl_args.nth(i).unwrap(),
+            declaration_args.and_then(|mut args| args.nth(i)),
+            external_impl_name,
+        ),
+        _ => (
+            cause.span,
+            tcx.hir_span_if_local(declaration).or_else(|| Some(tcx.def_span(declaration))),
+            external_impl_name,
+        ),
+    }
+}
+
+fn get_declaration_sig<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Option<&'tcx FnSig<'tcx>> {
+    let hir_id: HirId = tcx.local_def_id_to_hir_id(def_id);
+    tcx.hir_fn_sig_by_hir_id(hir_id)
+}
diff --git a/compiler/rustc_hir_analysis/src/check/mod.rs b/compiler/rustc_hir_analysis/src/check/mod.rs
index f1c84a14de30..d6ae14a7acfe 100644
--- a/compiler/rustc_hir_analysis/src/check/mod.rs
+++ b/compiler/rustc_hir_analysis/src/check/mod.rs
@@ -64,6 +64,7 @@ a type parameter).
 
 pub mod always_applicable;
 mod check;
+mod compare_eii;
 mod compare_impl_item;
 mod entry;
 pub mod intrinsic;
diff --git a/compiler/rustc_hir_analysis/src/check/wfcheck.rs b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
index 0e8859facf58..97097f4a0bfc 100644
--- a/compiler/rustc_hir_analysis/src/check/wfcheck.rs
+++ b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
@@ -6,7 +6,7 @@ use rustc_abi::ExternAbi;
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
 use rustc_errors::codes::*;
 use rustc_errors::{Applicability, ErrorGuaranteed, pluralize, struct_span_code_err};
-use rustc_hir::attrs::AttributeKind;
+use rustc_hir::attrs::{AttributeKind, EiiDecl, EiiImpl};
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::lang_items::LangItem;
@@ -39,6 +39,7 @@ use rustc_trait_selection::traits::{
 use tracing::{debug, instrument};
 use {rustc_ast as ast, rustc_hir as hir};
 
+use super::compare_eii::compare_eii_function_types;
 use crate::autoderef::Autoderef;
 use crate::constrained_generic_params::{Parameter, identify_constrained_generic_params};
 use crate::errors::InvalidReceiverTyHint;
@@ -1171,6 +1172,31 @@ fn check_item_fn(
     decl: &hir::FnDecl<'_>,
 ) -> Result<(), ErrorGuaranteed> {
     enter_wf_checking_ctxt(tcx, def_id, |wfcx| {
+        // does the function have an EiiImpl attribute? that contains the defid of a *macro*
+        // that was used to mark the implementation. This is a two step process.
+        for EiiImpl { eii_macro, span, .. } in
+            find_attr!(tcx.get_all_attrs(def_id), AttributeKind::EiiImpls(impls) => impls)
+                .into_iter()
+                .flatten()
+        {
+            // we expect this macro to have the `EiiMacroFor` attribute, that points to a function
+            // signature that we'd like to compare the function we're currently checking with
+            if let Some(eii_extern_target) = find_attr!(tcx.get_all_attrs(*eii_macro), AttributeKind::EiiExternTarget(EiiDecl {eii_extern_target, ..}) => *eii_extern_target)
+            {
+                let _ = compare_eii_function_types(
+                    tcx,
+                    def_id,
+                    eii_extern_target,
+                    tcx.item_name(*eii_macro),
+                    *span,
+                );
+            } else {
+                panic!(
+                    "EII impl macro {eii_macro:?} did not have an eii extern target attribute pointing to a foreign function"
+                )
+            }
+        }
+
         let sig = tcx.fn_sig(def_id).instantiate_identity();
         check_fn_or_method(wfcx, sig, decl, def_id);
         Ok(())
diff --git a/compiler/rustc_hir_analysis/src/errors.rs b/compiler/rustc_hir_analysis/src/errors.rs
index 34ad4e720eb5..865abdbd32c2 100644
--- a/compiler/rustc_hir_analysis/src/errors.rs
+++ b/compiler/rustc_hir_analysis/src/errors.rs
@@ -1697,3 +1697,28 @@ pub(crate) struct AsyncDropWithoutSyncDrop {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_lifetimes_or_bounds_mismatch_on_eii)]
+pub(crate) struct LifetimesOrBoundsMismatchOnEii {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    #[label(hir_analysis_generics_label)]
+    pub generics_span: Option<Span>,
+    #[label(hir_analysis_where_label)]
+    pub where_span: Option<Span>,
+    #[label(hir_analysis_bounds_label)]
+    pub bounds_span: Vec<Span>,
+    pub ident: Symbol,
+}
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_eii_with_generics)]
+pub(crate) struct EiiWithGenerics {
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub attr: Span,
+    pub eii_name: Symbol,
+}
diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index 0c7bddf60d97..c064313f67b2 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -420,6 +420,13 @@ pub enum ObligationCauseCode<'tcx> {
     /// Only reachable if the `unsized_fn_params` feature is used. Unsized function arguments must
     /// be place expressions because we can't store them in MIR locals as temporaries.
     UnsizedNonPlaceExpr(Span),
+
+    /// Error derived when checking an impl item is compatible with
+    /// its corresponding trait item's definition
+    CompareEii {
+        external_impl: LocalDefId,
+        declaration: DefId,
+    },
 }
 
 /// Whether a value can be extracted into a const.
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
index fd845e31c909..d14ff2c3b7e2 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
@@ -3843,6 +3843,9 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     "unsized values must be place expressions and cannot be put in temporaries",
                 );
             }
+            ObligationCauseCode::CompareEii { .. } => {
+                panic!("trait bounds on EII not yet supported ")
+            }
         }
     }