diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index d684a0e67e20..833a21c1394d 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -159,7 +159,7 @@ struct LoweringContext<'a, 'hir: 'a> {
 }
 
 pub trait ResolverAstLowering {
-    fn def_key(&mut self, id: DefId) -> DefKey;
+    fn def_key(&self, id: DefId) -> DefKey;
 
     fn def_span(&self, id: LocalDefId) -> Span;
 
@@ -171,17 +171,15 @@ pub trait ResolverAstLowering {
     fn get_partial_res(&self, id: NodeId) -> Option<PartialRes>;
 
     /// Obtains per-namespace resolutions for `use` statement with the given `NodeId`.
-    fn get_import_res(&mut self, id: NodeId) -> PerNS<Option<Res<NodeId>>>;
+    fn get_import_res(&self, id: NodeId) -> PerNS<Option<Res<NodeId>>>;
 
     /// Obtains resolution for a label with the given `NodeId`.
-    fn get_label_res(&mut self, id: NodeId) -> Option<NodeId>;
-
-    /// We must keep the set of definitions up to date as we add nodes that weren't in the AST.
-    /// This should only return `None` during testing.
-    fn definitions(&mut self) -> &mut Definitions;
+    fn get_label_res(&self, id: NodeId) -> Option<NodeId>;
 
     fn create_stable_hashing_context(&self) -> StableHashingContext<'_>;
 
+    fn definitions(&self) -> &Definitions;
+
     fn lint_buffer(&mut self) -> &mut LintBuffer;
 
     fn next_node_id(&mut self) -> NodeId;
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index 48675fa88270..0393a391c8a5 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -1160,9 +1160,9 @@ fn parent(self, id: DefId) -> Option<DefId> {
 /// This interface is used through the AST→HIR step, to embed full paths into the HIR. After that
 /// the resolver is no longer needed as all the relevant information is inline.
 impl ResolverAstLowering for Resolver<'_> {
-    fn def_key(&mut self, id: DefId) -> DefKey {
+    fn def_key(&self, id: DefId) -> DefKey {
         if let Some(id) = id.as_local() {
-            self.definitions().def_key(id)
+            self.definitions.def_key(id)
         } else {
             self.cstore().def_key(id)
         }
@@ -1189,22 +1189,22 @@ fn get_partial_res(&self, id: NodeId) -> Option<PartialRes> {
         self.partial_res_map.get(&id).cloned()
     }
 
-    fn get_import_res(&mut self, id: NodeId) -> PerNS<Option<Res>> {
+    fn get_import_res(&self, id: NodeId) -> PerNS<Option<Res>> {
         self.import_res_map.get(&id).cloned().unwrap_or_default()
     }
 
-    fn get_label_res(&mut self, id: NodeId) -> Option<NodeId> {
+    fn get_label_res(&self, id: NodeId) -> Option<NodeId> {
         self.label_res_map.get(&id).cloned()
     }
 
-    fn definitions(&mut self) -> &mut Definitions {
-        &mut self.definitions
-    }
-
     fn create_stable_hashing_context(&self) -> StableHashingContext<'_> {
         StableHashingContext::new(self.session, &self.definitions, self.crate_loader.cstore())
     }
 
+    fn definitions(&self) -> &Definitions {
+        &self.definitions
+    }
+
     fn lint_buffer(&mut self) -> &mut LintBuffer {
         &mut self.lint_buffer
     }