diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index 85c0d28d5ea11..39aa1844d6911 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -1033,45 +1033,33 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
 
     /// Iterates over all the stability attributes in the given crate.
     fn get_lib_features(self, tcx: TyCtxt<'tcx>) -> &'tcx [(Symbol, Option<Symbol>)] {
-        // FIXME: For a proc macro crate, not sure whether we should return the "host"
-        // features or an empty Vec. Both don't cause ICEs.
         tcx.arena.alloc_from_iter(self.root.lib_features.decode(self))
     }
 
     /// Iterates over the language items in the given crate.
     fn get_lang_items(self, tcx: TyCtxt<'tcx>) -> &'tcx [(DefId, usize)] {
-        if self.root.is_proc_macro_crate() {
-            // Proc macro crates do not export any lang-items to the target.
-            &[]
-        } else {
-            tcx.arena.alloc_from_iter(
-                self.root
-                    .lang_items
-                    .decode(self)
-                    .map(|(def_index, index)| (self.local_def_id(def_index), index)),
-            )
-        }
+        tcx.arena.alloc_from_iter(
+            self.root
+                .lang_items
+                .decode(self)
+                .map(|(def_index, index)| (self.local_def_id(def_index), index)),
+        )
     }
 
     /// Iterates over the diagnostic items in the given crate.
     fn get_diagnostic_items(self) -> DiagnosticItems {
-        if self.root.is_proc_macro_crate() {
-            // Proc macro crates do not export any diagnostic-items to the target.
-            Default::default()
-        } else {
-            let mut id_to_name = FxHashMap::default();
-            let name_to_id = self
-                .root
-                .diagnostic_items
-                .decode(self)
-                .map(|(name, def_index)| {
-                    let id = self.local_def_id(def_index);
-                    id_to_name.insert(id, name);
-                    (name, id)
-                })
-                .collect();
-            DiagnosticItems { id_to_name, name_to_id }
-        }
+        let mut id_to_name = FxHashMap::default();
+        let name_to_id = self
+            .root
+            .diagnostic_items
+            .decode(self)
+            .map(|(name, def_index)| {
+                let id = self.local_def_id(def_index);
+                id_to_name.insert(id, name);
+                (name, id)
+            })
+            .collect();
+        DiagnosticItems { id_to_name, name_to_id }
     }
 
     /// Iterates over all named children of the given module,
@@ -1346,26 +1334,28 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             .decode((self, sess))
     }
 
-    fn get_struct_field_names(self, id: DefIndex, sess: &Session) -> Vec<Spanned<Symbol>> {
+    fn get_struct_field_names(
+        self,
+        id: DefIndex,
+        sess: &'a Session,
+    ) -> impl Iterator<Item = Spanned<Symbol>> + 'a {
         self.root
             .tables
             .children
             .get(self, id)
             .unwrap_or_else(Lazy::empty)
             .decode(self)
-            .map(|index| respan(self.get_span(index, sess), self.item_ident(index, sess).name))
-            .collect()
+            .map(move |index| respan(self.get_span(index, sess), self.item_ident(index, sess).name))
     }
 
-    fn get_struct_field_visibilities(self, id: DefIndex) -> Vec<Visibility> {
+    fn get_struct_field_visibilities(self, id: DefIndex) -> impl Iterator<Item = Visibility> + 'a {
         self.root
             .tables
             .children
             .get(self, id)
             .unwrap_or_else(Lazy::empty)
             .decode(self)
-            .map(|field_index| self.get_visibility(field_index))
-            .collect()
+            .map(move |field_index| self.get_visibility(field_index))
     }
 
     fn get_inherent_implementations_for_type(
@@ -1401,8 +1391,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         tcx: TyCtxt<'tcx>,
         trait_def_id: DefId,
     ) -> &'tcx [(DefId, Option<SimplifiedType>)] {
-        if self.root.is_proc_macro_crate() {
-            // proc-macro crates export no trait impls.
+        if self.trait_impls.is_empty() {
             return &[];
         }
 
@@ -1437,13 +1426,8 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         })
     }
 
-    fn get_native_libraries(self, sess: &Session) -> Vec<NativeLib> {
-        if self.root.is_proc_macro_crate() {
-            // Proc macro crates do not have any *target* native libraries.
-            vec![]
-        } else {
-            self.root.native_libraries.decode((self, sess)).collect()
-        }
+    fn get_native_libraries(self, sess: &'a Session) -> impl Iterator<Item = NativeLib> + 'a {
+        self.root.native_libraries.decode((self, sess))
     }
 
     fn get_proc_macro_quoted_span(self, index: usize, sess: &Session) -> Span {
@@ -1455,15 +1439,8 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             .decode((self, sess))
     }
 
-    fn get_foreign_modules(self, tcx: TyCtxt<'tcx>) -> Lrc<FxHashMap<DefId, ForeignModule>> {
-        if self.root.is_proc_macro_crate() {
-            // Proc macro crates do not have any *target* foreign modules.
-            Lrc::new(FxHashMap::default())
-        } else {
-            let modules: FxHashMap<DefId, ForeignModule> =
-                self.root.foreign_modules.decode((self, tcx.sess)).map(|m| (m.def_id, m)).collect();
-            Lrc::new(modules)
-        }
+    fn get_foreign_modules(self, sess: &'a Session) -> impl Iterator<Item = ForeignModule> + '_ {
+        self.root.foreign_modules.decode((self, sess))
     }
 
     fn get_dylib_dependency_formats(
@@ -1479,12 +1456,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
     }
 
     fn get_missing_lang_items(self, tcx: TyCtxt<'tcx>) -> &'tcx [lang_items::LangItem] {
-        if self.root.is_proc_macro_crate() {
-            // Proc macro crates do not depend on any target weak lang-items.
-            &[]
-        } else {
-            tcx.arena.alloc_from_iter(self.root.lang_items_missing.decode(self))
-        }
+        tcx.arena.alloc_from_iter(self.root.lang_items_missing.decode(self))
     }
 
     fn get_fn_param_names(self, tcx: TyCtxt<'tcx>, id: DefIndex) -> &'tcx [Ident] {
@@ -1500,13 +1472,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         self,
         tcx: TyCtxt<'tcx>,
     ) -> &'tcx [(ExportedSymbol<'tcx>, SymbolExportLevel)] {
-        if self.root.is_proc_macro_crate() {
-            // If this crate is a custom derive crate, then we're not even going to
-            // link those in so we skip those crates.
-            &[]
-        } else {
-            tcx.arena.alloc_from_iter(self.root.exported_symbols.decode((self, tcx)))
-        }
+        tcx.arena.alloc_from_iter(self.root.exported_symbols.decode((self, tcx)))
     }
 
     fn get_rendered_const(self, id: DefIndex) -> String {
diff --git a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
index a639772fde709..9b59c14c2fec8 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
@@ -179,8 +179,10 @@ provide! { <'tcx> tcx, def_id, other, cdata,
 
         reachable_non_generics
     }
-    native_libraries => { Lrc::new(cdata.get_native_libraries(tcx.sess)) }
-    foreign_modules => { cdata.get_foreign_modules(tcx) }
+    native_libraries => { Lrc::new(cdata.get_native_libraries(tcx.sess).collect()) }
+    foreign_modules => {
+        Lrc::new(cdata.get_foreign_modules(tcx.sess).map(|m| (m.def_id, m)).collect())
+    }
     crate_hash => { cdata.root.hash }
     crate_host_hash => { cdata.host_hash }
     crate_name => { cdata.root.name }
@@ -371,11 +373,18 @@ pub(in crate::rmeta) fn provide(providers: &mut Providers) {
 }
 
 impl CStore {
-    pub fn struct_field_names_untracked(&self, def: DefId, sess: &Session) -> Vec<Spanned<Symbol>> {
+    pub fn struct_field_names_untracked<'a>(
+        &'a self,
+        def: DefId,
+        sess: &'a Session,
+    ) -> impl Iterator<Item = Spanned<Symbol>> + 'a {
         self.get_crate_data(def.krate).get_struct_field_names(def.index, sess)
     }
 
-    pub fn struct_field_visibilities_untracked(&self, def: DefId) -> Vec<Visibility> {
+    pub fn struct_field_visibilities_untracked(
+        &self,
+        def: DefId,
+    ) -> impl Iterator<Item = Visibility> + '_ {
         self.get_crate_data(def.krate).get_struct_field_visibilities(def.index)
     }
 
@@ -460,8 +469,12 @@ impl CStore {
         self.get_crate_data(cnum).num_def_ids()
     }
 
-    pub fn item_attrs_untracked(&self, def_id: DefId, sess: &Session) -> Vec<ast::Attribute> {
-        self.get_crate_data(def_id.krate).get_item_attrs(def_id.index, sess).collect()
+    pub fn item_attrs_untracked<'a>(
+        &'a self,
+        def_id: DefId,
+        sess: &'a Session,
+    ) -> impl Iterator<Item = ast::Attribute> + 'a {
+        self.get_crate_data(def_id.krate).get_item_attrs(def_id.index, sess)
     }
 
     pub fn get_proc_macro_quoted_span_untracked(
@@ -473,15 +486,15 @@ impl CStore {
         self.get_crate_data(cnum).get_proc_macro_quoted_span(id, sess)
     }
 
-    pub fn traits_in_crate_untracked(&self, cnum: CrateNum) -> Vec<DefId> {
-        self.get_crate_data(cnum).get_traits().collect()
+    pub fn traits_in_crate_untracked(&self, cnum: CrateNum) -> impl Iterator<Item = DefId> + '_ {
+        self.get_crate_data(cnum).get_traits()
     }
 
     pub fn trait_impls_in_crate_untracked(
         &self,
         cnum: CrateNum,
-    ) -> Vec<(DefId, Option<SimplifiedType>)> {
-        self.get_crate_data(cnum).get_trait_impls().collect()
+    ) -> impl Iterator<Item = (DefId, Option<SimplifiedType>)> + '_ {
+        self.get_crate_data(cnum).get_trait_impls()
     }
 }
 
diff --git a/compiler/rustc_resolve/src/build_reduced_graph.rs b/compiler/rustc_resolve/src/build_reduced_graph.rs
index 052770b201a5b..631b8fef668db 100644
--- a/compiler/rustc_resolve/src/build_reduced_graph.rs
+++ b/compiler/rustc_resolve/src/build_reduced_graph.rs
@@ -999,12 +999,14 @@ impl<'a, 'b> BuildReducedGraphVisitor<'a, 'b> {
         let cstore = self.r.cstore();
         match res {
             Res::Def(DefKind::Struct, def_id) => {
-                let field_names = cstore.struct_field_names_untracked(def_id, self.r.session);
+                let field_names =
+                    cstore.struct_field_names_untracked(def_id, self.r.session).collect();
                 let ctor = cstore.ctor_def_id_and_kind_untracked(def_id);
                 if let Some((ctor_def_id, ctor_kind)) = ctor {
                     let ctor_res = Res::Def(DefKind::Ctor(CtorOf::Struct, ctor_kind), ctor_def_id);
                     let ctor_vis = cstore.visibility_untracked(ctor_def_id);
-                    let field_visibilities = cstore.struct_field_visibilities_untracked(def_id);
+                    let field_visibilities =
+                        cstore.struct_field_visibilities_untracked(def_id).collect();
                     self.r
                         .struct_constructors
                         .insert(def_id, (ctor_res, ctor_vis, field_visibilities));
@@ -1012,7 +1014,8 @@ impl<'a, 'b> BuildReducedGraphVisitor<'a, 'b> {
                 self.insert_field_names(def_id, field_names);
             }
             Res::Def(DefKind::Union, def_id) => {
-                let field_names = cstore.struct_field_names_untracked(def_id, self.r.session);
+                let field_names =
+                    cstore.struct_field_names_untracked(def_id, self.r.session).collect();
                 self.insert_field_names(def_id, field_names);
             }
             Res::Def(DefKind::AssocFn, def_id) => {
diff --git a/compiler/rustc_resolve/src/diagnostics.rs b/compiler/rustc_resolve/src/diagnostics.rs
index 941c2a70d11ef..c7edcb077b917 100644
--- a/compiler/rustc_resolve/src/diagnostics.rs
+++ b/compiler/rustc_resolve/src/diagnostics.rs
@@ -914,11 +914,8 @@ impl<'a> Resolver<'a> {
                             // a note about editions
                             let note = if let Some(did) = did {
                                 let requires_note = !did.is_local()
-                                    && this
-                                        .cstore()
-                                        .item_attrs_untracked(did, this.session)
-                                        .iter()
-                                        .any(|attr| {
+                                    && this.cstore().item_attrs_untracked(did, this.session).any(
+                                        |attr| {
                                             if attr.has_name(sym::rustc_diagnostic_item) {
                                                 [sym::TryInto, sym::TryFrom, sym::FromIterator]
                                                     .map(|x| Some(x))
@@ -926,7 +923,8 @@ impl<'a> Resolver<'a> {
                                             } else {
                                                 false
                                             }
-                                        });
+                                        },
+                                    );
 
                                 requires_note.then(|| {
                                     format!(
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index 25137c6eda763..f5b2ba8fd72eb 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -3442,7 +3442,6 @@ impl<'a> Resolver<'a> {
                 let attr = self
                     .cstore()
                     .item_attrs_untracked(def_id, self.session)
-                    .into_iter()
                     .find(|a| a.has_name(sym::rustc_legacy_const_generics))?;
                 let mut ret = Vec::new();
                 for meta in attr.meta_item_list()? {