diff --git a/compiler/rustc_data_structures/src/vec_map.rs b/compiler/rustc_data_structures/src/vec_map.rs
index e3fa587985df0..cc7ec9432faed 100644
--- a/compiler/rustc_data_structures/src/vec_map.rs
+++ b/compiler/rustc_data_structures/src/vec_map.rs
@@ -1,4 +1,5 @@
 use std::borrow::Borrow;
+use std::fmt::Debug;
 use std::iter::FromIterator;
 use std::slice::Iter;
 use std::vec::IntoIter;
@@ -12,7 +13,8 @@ pub struct VecMap<K, V>(Vec<(K, V)>);
 
 impl<K, V> VecMap<K, V>
 where
-    K: PartialEq,
+    K: Debug + PartialEq,
+    V: Debug,
 {
     pub fn new() -> Self {
         VecMap(Default::default())
@@ -37,14 +39,31 @@ where
         self.0.iter().find(|(key, _)| k == key.borrow()).map(|elem| &elem.1)
     }
 
-    /// Returns the value corresponding to the supplied predicate filter.
+    /// Returns the any value corresponding to the supplied predicate filter.
     ///
     /// The supplied predicate will be applied to each (key, value) pair and it will return a
     /// reference to the values where the predicate returns `true`.
-    pub fn get_by(&self, mut predicate: impl FnMut(&(K, V)) -> bool) -> Option<&V> {
+    pub fn any_value_matching(&self, mut predicate: impl FnMut(&(K, V)) -> bool) -> Option<&V> {
         self.0.iter().find(|kv| predicate(kv)).map(|elem| &elem.1)
     }
 
+    /// Returns the value corresponding to the supplied predicate filter. It crashes if there's
+    /// more than one matching element.
+    ///
+    /// The supplied predicate will be applied to each (key, value) pair and it will return a
+    /// reference to the value where the predicate returns `true`.
+    pub fn get_value_matching(&self, mut predicate: impl FnMut(&(K, V)) -> bool) -> Option<&V> {
+        let mut filter = self.0.iter().filter(|kv| predicate(kv));
+        let (_, value) = filter.next()?;
+        // This should return just one element, otherwise it's a bug
+        assert!(
+            filter.next().is_none(),
+            "Collection {:?} should have just one matching element",
+            self
+        );
+        Some(value)
+    }
+
     /// Returns `true` if the map contains a value for the specified key.
     ///
     /// The key may be any borrowed form of the map's key type,
@@ -131,7 +150,7 @@ impl<K, V> IntoIterator for VecMap<K, V> {
     }
 }
 
-impl<K: PartialEq, V> Extend<(K, V)> for VecMap<K, V> {
+impl<K: PartialEq + Debug, V: Debug> Extend<(K, V)> for VecMap<K, V> {
     fn extend<I: IntoIterator<Item = (K, V)>>(&mut self, iter: I) {
         for (k, v) in iter {
             self.insert(k, v);
diff --git a/compiler/rustc_typeck/src/collect/type_of.rs b/compiler/rustc_typeck/src/collect/type_of.rs
index 15469cb0066c5..50e4ba4fe6c3e 100644
--- a/compiler/rustc_typeck/src/collect/type_of.rs
+++ b/compiler/rustc_typeck/src/collect/type_of.rs
@@ -364,7 +364,7 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                     let concrete_ty = tcx
                         .mir_borrowck(owner.expect_local())
                         .concrete_opaque_types
-                        .get_by(|(key, _)| key.def_id == def_id.to_def_id())
+                        .get_value_matching(|(key, _)| key.def_id == def_id.to_def_id())
                         .map(|concrete_ty| *concrete_ty)
                         .unwrap_or_else(|| {
                             tcx.sess.delay_span_bug(
@@ -512,8 +512,15 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Ty<'_> {
 
     struct ConstraintLocator<'tcx> {
         tcx: TyCtxt<'tcx>,
+
+        /// def_id of the opaque type whose defining uses are being checked
         def_id: DefId,
-        // (first found type span, actual type)
+
+        /// as we walk the defining uses, we are checking that all of them
+        /// define the same hidden type. This variable is set to `Some`
+        /// with the first type that we find, and then later types are
+        /// checked against it (we also carry the span of that first
+        /// type).
         found: Option<(Span, Ty<'tcx>)>,
     }
 
@@ -531,7 +538,7 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Ty<'_> {
                 .tcx
                 .typeck(def_id)
                 .concrete_opaque_types
-                .get_by(|(key, _)| key.def_id == self.def_id)
+                .any_value_matching(|(key, _)| key.def_id == self.def_id)
                 .is_none()
             {
                 debug!("no constraints in typeck results");