diff --git a/compiler/rustc_data_structures/src/sync/vec.rs b/compiler/rustc_data_structures/src/sync/vec.rs
index 64b0aff6ca2..aefaa8519d5 100644
--- a/compiler/rustc_data_structures/src/sync/vec.rs
+++ b/compiler/rustc_data_structures/src/sync/vec.rs
@@ -75,20 +75,31 @@ impl<T: Copy> AppendOnlyVec<T> {
         #[cfg(parallel_compiler)]
         return self.vec.get(i);
     }
+
+    pub fn iter_enumerated(&self) -> impl Iterator<Item = (usize, T)> + '_ {
+        (0..)
+            .map(|i| (i, self.get(i)))
+            .take_while(|(_, o)| o.is_some())
+            .filter_map(|(i, o)| Some((i, o?)))
+    }
+
+    pub fn iter(&self) -> impl Iterator<Item = T> + '_ {
+        (0..).map(|i| self.get(i)).take_while(|o| o.is_some()).filter_map(|o| o)
+    }
 }
 
 impl<T: Copy + PartialEq> AppendOnlyVec<T> {
     pub fn contains(&self, val: T) -> bool {
-        for i in 0.. {
-            match self.get(i) {
-                None => return false,
-                Some(v) => {
-                    if val == v {
-                        return true;
-                    }
-                }
-            }
-        }
-        false
+        self.iter_enumerated().any(|(_, v)| v == val)
+    }
+}
+
+impl<A: Copy> FromIterator<A> for AppendOnlyVec<A> {
+    fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
+        let this = Self::new();
+        for val in iter {
+            this.push(val);
+        }
+        this
     }
 }
diff --git a/compiler/rustc_metadata/src/creader.rs b/compiler/rustc_metadata/src/creader.rs
index 23c1aebb8ae..23aceca0622 100644
--- a/compiler/rustc_metadata/src/creader.rs
+++ b/compiler/rustc_metadata/src/creader.rs
@@ -185,7 +185,7 @@ impl CStore {
     fn push_dependencies_in_postorder(&self, deps: &mut Vec<CrateNum>, cnum: CrateNum) {
         if !deps.contains(&cnum) {
             let data = self.get_crate_data(cnum);
-            for &dep in data.dependencies().iter() {
+            for dep in data.dependencies() {
                 if dep != cnum {
                     self.push_dependencies_in_postorder(deps, dep);
                 }
@@ -605,7 +605,7 @@ impl<'a, 'tcx> CrateLoader<'a, 'tcx> {
         if cmeta.update_extern_crate(extern_crate) {
             // Propagate the extern crate info to dependencies if it was updated.
             let extern_crate = ExternCrate { dependency_of: cnum, ..extern_crate };
-            for &dep_cnum in cmeta.dependencies().iter() {
+            for dep_cnum in cmeta.dependencies() {
                 self.update_extern_crate(dep_cnum, extern_crate);
             }
         }
diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index 43e5946f313..21ac7e201f9 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -7,7 +7,7 @@ use rustc_ast as ast;
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::svh::Svh;
-use rustc_data_structures::sync::{Lock, LockGuard, Lrc, OnceCell};
+use rustc_data_structures::sync::{AppendOnlyVec, Lock, Lrc, OnceCell};
 use rustc_data_structures::unhash::UnhashMap;
 use rustc_expand::base::{SyntaxExtension, SyntaxExtensionKind};
 use rustc_expand::proc_macro::{AttrProcMacro, BangProcMacro, DeriveProcMacro};
@@ -109,7 +109,7 @@ pub(crate) struct CrateMetadata {
     /// IDs as they are seen from the current compilation session.
     cnum_map: CrateNumMap,
     /// Same ID set as `cnum_map` plus maybe some injected crates like panic runtime.
-    dependencies: Lock<Vec<CrateNum>>,
+    dependencies: AppendOnlyVec<CrateNum>,
     /// How to link (or not link) this crate to the currently compiled crate.
     dep_kind: Lock<CrateDepKind>,
     /// Filesystem location of this crate.
@@ -1594,7 +1594,7 @@ impl CrateMetadata {
             .collect();
         let alloc_decoding_state =
             AllocDecodingState::new(root.interpret_alloc_index.decode(&blob).collect());
-        let dependencies = Lock::new(cnum_map.iter().cloned().collect());
+        let dependencies = cnum_map.iter().copied().collect();
 
         // Pre-decode the DefPathHash->DefIndex table. This is a cheap operation
         // that does not copy any data. It just does some data verification.
@@ -1634,12 +1634,12 @@ impl CrateMetadata {
         cdata
     }
 
-    pub(crate) fn dependencies(&self) -> LockGuard<'_, Vec<CrateNum>> {
-        self.dependencies.borrow()
+    pub(crate) fn dependencies(&self) -> impl Iterator<Item = CrateNum> + '_ {
+        self.dependencies.iter()
     }
 
     pub(crate) fn add_dependency(&self, cnum: CrateNum) {
-        self.dependencies.borrow_mut().push(cnum);
+        self.dependencies.push(cnum);
     }
 
     pub(crate) fn update_extern_crate(&self, new_extern_crate: ExternCrate) -> bool {