diff --git a/compiler/rustc_data_structures/src/unord.rs b/compiler/rustc_data_structures/src/unord.rs
index ec779854f36..47c56eba7ad 100644
--- a/compiler/rustc_data_structures/src/unord.rs
+++ b/compiler/rustc_data_structures/src/unord.rs
@@ -168,6 +168,14 @@ pub fn into_sorted_stable_ord(self) -> Vec<T>
     }
 }
 
+/// A marker trait specifying that `Self` can consume `UnordItems<_>` without
+/// exposing any internal ordering.
+///
+/// Note: right now this is just a marker trait. It could be extended to contain
+/// some useful, common methods though, like `len`, `clear`, or the various
+/// kinds of `to_sorted`.
+trait UnordCollection {}
+
 /// This is a set collection type that tries very hard to not expose
 /// any internal iteration. This is a useful property when trying to
 /// uphold the determinism invariants imposed by the query system.
@@ -182,6 +190,8 @@ pub struct UnordSet<V: Eq + Hash> {
     inner: FxHashSet<V>,
 }
 
+impl<V: Eq + Hash> UnordCollection for UnordSet<V> {}
+
 impl<V: Eq + Hash> Default for UnordSet<V> {
     #[inline]
     fn default() -> Self {
@@ -285,19 +295,31 @@ pub fn into_sorted<HCX>(self, hcx: &HCX, cache_sort_key: bool) -> Vec<V>
         to_sorted_vec(hcx, self.inner.into_iter(), cache_sort_key, |x| x)
     }
 
-    // We can safely extend this UnordSet from a set of unordered values because that
-    // won't expose the internal ordering anywhere.
-    #[inline]
-    pub fn extend_unord<I: Iterator<Item = V>>(&mut self, items: UnordItems<V, I>) {
-        self.inner.extend(items.0)
-    }
-
     #[inline]
     pub fn clear(&mut self) {
         self.inner.clear();
     }
 }
 
+pub trait ExtendUnord<T> {
+    /// Extend this unord collection with the given `UnordItems`.
+    /// This method is called `extend_unord` instead of just `extend` so it
+    /// does not conflict with `Extend::extend`. Otherwise there would be many
+    /// places where the two methods would have to be explicitly disambiguated
+    /// via UFCS.
+    fn extend_unord<I: Iterator<Item = T>>(&mut self, items: UnordItems<T, I>);
+}
+
+// Note: it is important that `C` implements `UnordCollection` in addition to
+// `Extend`, otherwise this impl would leak the internal iteration order of
+// `items`, e.g. when calling `some_vec.extend_unord(some_unord_items)`.
+impl<C: Extend<T> + UnordCollection, T> ExtendUnord<T> for C {
+    #[inline]
+    fn extend_unord<I: Iterator<Item = T>>(&mut self, items: UnordItems<T, I>) {
+        self.extend(items.0)
+    }
+}
+
 impl<V: Hash + Eq> Extend<V> for UnordSet<V> {
     #[inline]
     fn extend<T: IntoIterator<Item = V>>(&mut self, iter: T) {
@@ -345,6 +367,8 @@ pub struct UnordMap<K: Eq + Hash, V> {
     inner: FxHashMap<K, V>,
 }
 
+impl<K: Eq + Hash, V> UnordCollection for UnordMap<K, V> {}
+
 impl<K: Eq + Hash, V> Default for UnordMap<K, V> {
     #[inline]
     fn default() -> Self {
@@ -445,13 +469,6 @@ pub fn into_items(self) -> UnordItems<(K, V), impl Iterator<Item = (K, V)>> {
         UnordItems(self.inner.into_iter())
     }
 
-    // We can safely extend this UnordMap from a set of unordered values because that
-    // won't expose the internal ordering anywhere.
-    #[inline]
-    pub fn extend<I: Iterator<Item = (K, V)>>(&mut self, items: UnordItems<(K, V), I>) {
-        self.inner.extend(items.0)
-    }
-
     /// Returns the entries of this map in stable sort order (as defined by `ToStableHashKey`).
     ///
     /// The `cache_sort_key` parameter controls if [slice::sort_by_cached_key] or
@@ -571,15 +588,10 @@ pub fn items(&self) -> UnordItems<&V, impl Iterator<Item = &V>> {
     pub fn into_items(self) -> UnordItems<V, impl Iterator<Item = V>> {
         UnordItems(self.inner.into_iter())
     }
-
-    // We can safely extend this UnordSet from a set of unordered values because that
-    // won't expose the internal ordering anywhere.
-    #[inline]
-    pub fn extend<I: Iterator<Item = V>>(&mut self, items: UnordItems<V, I>) {
-        self.inner.extend(items.0)
-    }
 }
 
+impl<T> UnordCollection for UnordBag<T> {}
+
 impl<T> Extend<T> for UnordBag<T> {
     fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
         self.inner.extend(iter)
diff --git a/compiler/rustc_hir_analysis/src/check_unused.rs b/compiler/rustc_hir_analysis/src/check_unused.rs
index 3471d5f1e77..5318e637fc7 100644
--- a/compiler/rustc_hir_analysis/src/check_unused.rs
+++ b/compiler/rustc_hir_analysis/src/check_unused.rs
@@ -1,4 +1,4 @@
-use rustc_data_structures::unord::UnordSet;
+use rustc_data_structures::unord::{ExtendUnord, UnordSet};
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::LocalDefId;
 use rustc_middle::ty::TyCtxt;
diff --git a/compiler/rustc_hir_typeck/src/upvar.rs b/compiler/rustc_hir_typeck/src/upvar.rs
index 14c073707e6..a3838275dc5 100644
--- a/compiler/rustc_hir_typeck/src/upvar.rs
+++ b/compiler/rustc_hir_typeck/src/upvar.rs
@@ -33,7 +33,7 @@
 use super::FnCtxt;
 
 use crate::expr_use_visitor as euv;
-use rustc_data_structures::unord::UnordSet;
+use rustc_data_structures::unord::{ExtendUnord, UnordSet};
 use rustc_errors::{Applicability, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def_id::LocalDefId;
diff --git a/compiler/rustc_hir_typeck/src/writeback.rs b/compiler/rustc_hir_typeck/src/writeback.rs
index eef2b835cbb..fab6090ff19 100644
--- a/compiler/rustc_hir_typeck/src/writeback.rs
+++ b/compiler/rustc_hir_typeck/src/writeback.rs
@@ -4,6 +4,7 @@
 
 use crate::FnCtxt;
 use hir::def_id::LocalDefId;
+use rustc_data_structures::unord::ExtendUnord;
 use rustc_errors::{ErrorGuaranteed, StashKey};
 use rustc_hir as hir;
 use rustc_hir::intravisit::{self, Visitor};
@@ -526,7 +527,7 @@ fn visit_user_provided_sigs(&mut self) {
         let fcx_typeck_results = self.fcx.typeck_results.borrow();
         assert_eq!(fcx_typeck_results.hir_owner, self.typeck_results.hir_owner);
 
-        self.typeck_results.user_provided_sigs.extend(
+        self.typeck_results.user_provided_sigs.extend_unord(
             fcx_typeck_results.user_provided_sigs.items().map(|(&def_id, c_sig)| {
                 if cfg!(debug_assertions) && c_sig.has_infer() {
                     span_bug!(
diff --git a/compiler/rustc_middle/src/ty/typeck_results.rs b/compiler/rustc_middle/src/ty/typeck_results.rs
index 3a63e28d97e..9c25c01b056 100644
--- a/compiler/rustc_middle/src/ty/typeck_results.rs
+++ b/compiler/rustc_middle/src/ty/typeck_results.rs
@@ -9,7 +9,7 @@
 };
 use rustc_data_structures::{
     fx::FxIndexMap,
-    unord::{UnordItems, UnordSet},
+    unord::{ExtendUnord, UnordItems, UnordSet},
 };
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir as hir;
@@ -635,7 +635,7 @@ pub fn extend(
         &mut self,
         items: UnordItems<(hir::HirId, V), impl Iterator<Item = (hir::HirId, V)>>,
     ) {
-        self.data.extend(items.map(|(id, value)| {
+        self.data.extend_unord(items.map(|(id, value)| {
             validate_hir_id_for_typeck_results(self.hir_owner, id);
             (id.local_id, value)
         }))
diff --git a/compiler/rustc_mir_transform/src/check_unsafety.rs b/compiler/rustc_mir_transform/src/check_unsafety.rs
index 70812761e88..58e9786ec1a 100644
--- a/compiler/rustc_mir_transform/src/check_unsafety.rs
+++ b/compiler/rustc_mir_transform/src/check_unsafety.rs
@@ -1,4 +1,4 @@
-use rustc_data_structures::unord::{UnordItems, UnordSet};
+use rustc_data_structures::unord::{ExtendUnord, UnordItems, UnordSet};
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId};