diff --git a/compiler/rustc_hir_analysis/src/astconv/errors.rs b/compiler/rustc_hir_analysis/src/astconv/errors.rs
index 7ced37e5694e5..8f4b83966dff8 100644
--- a/compiler/rustc_hir_analysis/src/astconv/errors.rs
+++ b/compiler/rustc_hir_analysis/src/astconv/errors.rs
@@ -4,7 +4,7 @@ use crate::errors::{
     ParenthesizedFnTraitExpansion,
 };
 use crate::traits::error_reporting::report_object_safety_error;
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::{FxHashMap, FxIndexMap, FxIndexSet};
 use rustc_errors::{pluralize, struct_span_err, Applicability, Diagnostic, ErrorGuaranteed};
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
@@ -16,8 +16,6 @@ use rustc_span::symbol::{sym, Ident};
 use rustc_span::{Span, Symbol, DUMMY_SP};
 use rustc_trait_selection::traits::object_safety_violations_for_assoc_item;
 
-use std::collections::BTreeSet;
-
 impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     /// On missing type parameters, emit an E0393 error and provide a structured suggestion using
     /// the type parameter's name as a placeholder.
@@ -504,7 +502,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     /// emit a generic note suggesting using a `where` clause to constraint instead.
     pub(crate) fn complain_about_missing_associated_types(
         &self,
-        associated_types: FxHashMap<Span, BTreeSet<DefId>>,
+        associated_types: FxIndexMap<Span, FxIndexSet<DefId>>,
         potential_assoc_types: Vec<Span>,
         trait_bounds: &[hir::PolyTraitRef<'_>],
     ) {
@@ -514,13 +512,13 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let tcx = self.tcx();
         // FIXME: Marked `mut` so that we can replace the spans further below with a more
         // appropriate one, but this should be handled earlier in the span assignment.
-        let mut associated_types: FxHashMap<Span, Vec<_>> = associated_types
+        let mut associated_types: FxIndexMap<Span, Vec<_>> = associated_types
             .into_iter()
             .map(|(span, def_ids)| {
                 (span, def_ids.into_iter().map(|did| tcx.associated_item(did)).collect())
             })
             .collect();
-        let mut names: FxHashMap<String, Vec<Symbol>> = Default::default();
+        let mut names: FxIndexMap<String, Vec<Symbol>> = Default::default();
         let mut names_len = 0;
 
         // Account for things like `dyn Foo + 'a`, like in tests `issue-22434.rs` and
diff --git a/compiler/rustc_hir_analysis/src/astconv/object_safety.rs b/compiler/rustc_hir_analysis/src/astconv/object_safety.rs
index 00ff3f836adf3..78c5809f8b4f9 100644
--- a/compiler/rustc_hir_analysis/src/astconv/object_safety.rs
+++ b/compiler/rustc_hir_analysis/src/astconv/object_safety.rs
@@ -1,7 +1,7 @@
 use crate::astconv::{GenericArgCountMismatch, GenericArgCountResult, OnlySelfBounds};
 use crate::bounds::Bounds;
 use crate::errors::TraitObjectDeclaredWithNoTraits;
-use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
 use rustc_errors::struct_span_err;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
@@ -14,7 +14,6 @@ use rustc_trait_selection::traits::error_reporting::report_object_safety_error;
 use rustc_trait_selection::traits::{self, astconv_object_safety_violations};
 
 use smallvec::{smallvec, SmallVec};
-use std::collections::BTreeSet;
 
 use super::AstConv;
 
@@ -148,8 +147,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             }
         }
 
-        // Use a `BTreeSet` to keep output in a more consistent order.
-        let mut associated_types: FxHashMap<Span, BTreeSet<DefId>> = FxHashMap::default();
+        let mut associated_types: FxIndexMap<Span, FxIndexSet<DefId>> = FxIndexMap::default();
 
         let regular_traits_refs_spans = trait_bounds
             .into_iter()
diff --git a/tests/ui/associated-types/missing-associated-types.stderr b/tests/ui/associated-types/missing-associated-types.stderr
index e9669afe8c704..0636667ea1fd5 100644
--- a/tests/ui/associated-types/missing-associated-types.stderr
+++ b/tests/ui/associated-types/missing-associated-types.stderr
@@ -49,7 +49,7 @@ LL |     type B;
 LL | type Bar<Rhs> = dyn Add<Rhs> + Sub<Rhs> + X<Rhs> + Z<Rhs>;
    |                     ^^^^^^^^   ^^^^^^^^   ^^^^^^   ^^^^^^ associated types `A`, `B`, `Output` must be specified
    |                     |          |          |
-   |                     |          |          associated types `Output` (from trait `Mul`), `Output` (from trait `Div`) must be specified
+   |                     |          |          associated types `Output` (from trait `Div`), `Output` (from trait `Mul`) must be specified
    |                     |          associated type `Output` must be specified
    |                     associated type `Output` must be specified
    |
@@ -119,7 +119,7 @@ error[E0191]: the value of the associated types `Output` in `Div`, `Output` in `
   --> $DIR/missing-associated-types.rs:24:21
    |
 LL | type Bal<Rhs> = dyn X<Rhs>;
-   |                     ^^^^^^ associated types `Output` (from trait `Mul`), `Output` (from trait `Div`) must be specified
+   |                     ^^^^^^ associated types `Output` (from trait `Div`), `Output` (from trait `Mul`) must be specified
    |
    = help: consider introducing a new type parameter, adding `where` constraints using the fully-qualified path to the associated types