Move `FnPtrAddr` error to `SessionDiagnostic`

compiler/rustc_ty_utils/messages.ftl

@@ -45,3 +45,5 @@ ty_utils_control_flow_not_supported = control flow is not supported in generic c
 ty_utils_inline_asm_not_supported = assembly is not supported in generic constants
 
 ty_utils_operation_not_supported = unsupported operation in generic constants
+
+ty_utils_unexpected_fnptr_associated_item = `FnPtr` trait with unexpected associated item

compiler/rustc_ty_utils/src/errors.rs

@@ -67,3 +67,10 @@ pub enum GenericConstantTooComplexSub {
     #[label(ty_utils_operation_not_supported)]
     OperationNotSupported(#[primary_span] Span),
 }
+
+#[derive(Diagnostic)]
+#[diag(ty_utils_unexpected_fnptr_associated_item)]
+pub struct UnexpectedFnPtrAssociatedItem {
+    #[primary_span]
+    pub span: Span,
+}

compiler/rustc_ty_utils/src/instance.rs

@@ -8,6 +8,8 @@ use rustc_span::sym;
 use rustc_trait_selection::traits;
 use traits::{translate_substs, Reveal};
 
+use crate::errors::UnexpectedFnPtrAssociatedItem;
+
 fn resolve_instance<'tcx>(
     tcx: TyCtxt<'tcx>,
     key: ty::ParamEnvAnd<'tcx, (DefId, SubstsRef<'tcx>)>,
@@ -282,10 +284,9 @@ fn resolve_associated_item<'tcx>(
                         substs: rcvr_substs,
                     })
                 } else {
-                    tcx.sess.span_fatal(
-                        tcx.def_span(trait_item_id),
-                        "`FnPtrAddr` trait with unexpected assoc item",
-                    )
+                    tcx.sess.emit_fatal(UnexpectedFnPtrAssociatedItem {
+                        span: tcx.def_span(trait_item_id),
+                    })
                 }
             } else {
                 None