diff --git a/src/librustc/hir/intravisit.rs b/src/librustc/hir/intravisit.rs
index 37ee672291ec..cae813582cd2 100644
--- a/src/librustc/hir/intravisit.rs
+++ b/src/librustc/hir/intravisit.rs
@@ -247,7 +247,7 @@ pub trait Visitor<'v>: Sized {
     fn visit_ident(&mut self, ident: Ident) {
         walk_ident(self, ident)
     }
-    fn visit_mod(&mut self, m: &'v Mod, _s: Span, n: HirId) {
+    fn visit_mod(&mut self, m: &'v Mod<'v>, _s: Span, n: HirId) {
         walk_mod(self, m, n)
     }
     fn visit_foreign_item(&mut self, i: &'v ForeignItem<'v>) {
@@ -394,9 +394,9 @@ pub fn walk_macro_def<'v, V: Visitor<'v>>(visitor: &mut V, macro_def: &'v MacroD
     walk_list!(visitor, visit_attribute, macro_def.attrs);
 }
 
-pub fn walk_mod<'v, V: Visitor<'v>>(visitor: &mut V, module: &'v Mod, mod_hir_id: HirId) {
+pub fn walk_mod<'v, V: Visitor<'v>>(visitor: &mut V, module: &'v Mod<'v>, mod_hir_id: HirId) {
     visitor.visit_id(mod_hir_id);
-    for &item_id in &module.item_ids {
+    for &item_id in module.item_ids {
         visitor.visit_nested_item(item_id);
     }
 }
diff --git a/src/librustc/hir/lowering/item.rs b/src/librustc/hir/lowering/item.rs
index d79f1bfa6809..c49242485e15 100644
--- a/src/librustc/hir/lowering/item.rs
+++ b/src/librustc/hir/lowering/item.rs
@@ -161,10 +161,10 @@ impl LoweringContext<'_, 'hir> {
         res
     }
 
-    pub(super) fn lower_mod(&mut self, m: &Mod) -> hir::Mod {
+    pub(super) fn lower_mod(&mut self, m: &Mod) -> hir::Mod<'hir> {
         hir::Mod {
             inner: m.inner,
-            item_ids: m.items.iter().flat_map(|x| self.lower_item_id(x)).collect(),
+            item_ids: self.arena.alloc_from_iter(m.items.iter().flat_map(|x| self.lower_item_id(x))),
         }
     }
 
diff --git a/src/librustc/hir/map/mod.rs b/src/librustc/hir/map/mod.rs
index c6409f8d4651..cd96bedf4bdb 100644
--- a/src/librustc/hir/map/mod.rs
+++ b/src/librustc/hir/map/mod.rs
@@ -580,7 +580,7 @@ impl<'hir> Map<'hir> {
         &self.forest.krate.attrs
     }
 
-    pub fn get_module(&self, module: DefId) -> (&'hir Mod, Span, HirId) {
+    pub fn get_module(&self, module: DefId) -> (&'hir Mod<'hir>, Span, HirId) {
         let hir_id = self.as_local_hir_id(module).unwrap();
         self.read(hir_id);
         match self.find_entry(hir_id).unwrap().node {
diff --git a/src/librustc/hir/mod.rs b/src/librustc/hir/mod.rs
index 25fc365b4ec0..c89d93337ebc 100644
--- a/src/librustc/hir/mod.rs
+++ b/src/librustc/hir/mod.rs
@@ -743,7 +743,7 @@ pub struct ModuleItems {
 /// [rustc guide]: https://rust-lang.github.io/rustc-guide/hir.html
 #[derive(RustcEncodable, RustcDecodable, Debug)]
 pub struct Crate<'hir> {
-    pub module: Mod,
+    pub module: Mod<'hir>,
     pub attrs: &'hir [Attribute],
     pub span: Span,
     pub exported_macros: &'hir [MacroDef<'hir>],
@@ -2243,12 +2243,12 @@ impl FunctionRetTy {
 }
 
 #[derive(RustcEncodable, RustcDecodable, Debug)]
-pub struct Mod {
+pub struct Mod<'hir> {
     /// A span from the first token past `{` to the last token until `}`.
     /// For `mod foo;`, the inner span ranges from the first token
     /// to the last token in the external file.
     pub inner: Span,
-    pub item_ids: HirVec<ItemId>,
+    pub item_ids: &'hir [ItemId],
 }
 
 #[derive(RustcEncodable, RustcDecodable, Debug, HashStable)]
@@ -2489,7 +2489,7 @@ pub enum ItemKind<'hir> {
     /// A function declaration.
     Fn(FnSig, Generics, BodyId),
     /// A module.
-    Mod(Mod),
+    Mod(Mod<'hir>),
     /// An external module, e.g. `extern { .. }`.
     ForeignMod(ForeignMod<'hir>),
     /// Module-level inline assembly (from `global_asm!`).
diff --git a/src/librustc/hir/print.rs b/src/librustc/hir/print.rs
index 63d8afc7ec99..f0499e5853f3 100644
--- a/src/librustc/hir/print.rs
+++ b/src/librustc/hir/print.rs
@@ -259,9 +259,9 @@ impl<'a> State<'a> {
         self.commasep_cmnt(b, exprs, |s, e| s.print_expr(&e), |e| e.span)
     }
 
-    pub fn print_mod(&mut self, _mod: &hir::Mod, attrs: &[ast::Attribute]) {
+    pub fn print_mod(&mut self, _mod: &hir::Mod<'_>, attrs: &[ast::Attribute]) {
         self.print_inner_attributes(attrs);
-        for &item_id in &_mod.item_ids {
+        for &item_id in _mod.item_ids {
             self.ann.nested(self, Nested::Item(item_id));
         }
     }
diff --git a/src/librustc/ich/impls_hir.rs b/src/librustc/ich/impls_hir.rs
index a91535b55162..45274b0526fe 100644
--- a/src/librustc/ich/impls_hir.rs
+++ b/src/librustc/ich/impls_hir.rs
@@ -218,7 +218,7 @@ impl<'a> HashStable<StableHashingContext<'a>> for hir::VisibilityKind {
     }
 }
 
-impl<'a> HashStable<StableHashingContext<'a>> for hir::Mod {
+impl<'a> HashStable<StableHashingContext<'a>> for hir::Mod<'_> {
     fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
         let hir::Mod {
             inner: ref inner_span,
diff --git a/src/librustc/lint/context.rs b/src/librustc/lint/context.rs
index 7f19ec60f536..5ac20f46238f 100644
--- a/src/librustc/lint/context.rs
+++ b/src/librustc/lint/context.rs
@@ -883,7 +883,7 @@ impl<'a, 'tcx, T: LateLintPass<'a, 'tcx>> LateContextAndPass<'a, 'tcx, T> {
         self.context.param_env = old_param_env;
     }
 
-    fn process_mod(&mut self, m: &'tcx hir::Mod, s: Span, n: hir::HirId) {
+    fn process_mod(&mut self, m: &'tcx hir::Mod<'tcx>, s: Span, n: hir::HirId) {
         lint_callback!(self, check_mod, m, s, n);
         hir_visit::walk_mod(self, m, n);
         lint_callback!(self, check_mod_post, m, s, n);
@@ -1027,7 +1027,7 @@ for LateContextAndPass<'a, 'tcx, T> {
         lint_callback!(self, check_name, sp, name);
     }
 
-    fn visit_mod(&mut self, m: &'tcx hir::Mod, s: Span, n: hir::HirId) {
+    fn visit_mod(&mut self, m: &'tcx hir::Mod<'tcx>, s: Span, n: hir::HirId) {
         if !self.context.only_module {
             self.process_mod(m, s, n);
         }
diff --git a/src/librustc/lint/mod.rs b/src/librustc/lint/mod.rs
index 86c7d5515607..a5d6cf9dbb72 100644
--- a/src/librustc/lint/mod.rs
+++ b/src/librustc/lint/mod.rs
@@ -92,8 +92,8 @@ macro_rules! late_lint_methods {
             fn check_name(a: Span, b: ast::Name);
             fn check_crate(a: &$hir hir::Crate<$hir>);
             fn check_crate_post(a: &$hir hir::Crate<$hir>);
-            fn check_mod(a: &$hir hir::Mod, b: Span, c: hir::HirId);
-            fn check_mod_post(a: &$hir hir::Mod, b: Span, c: hir::HirId);
+            fn check_mod(a: &$hir hir::Mod<$hir>, b: Span, c: hir::HirId);
+            fn check_mod_post(a: &$hir hir::Mod<$hir>, b: Span, c: hir::HirId);
             fn check_foreign_item(a: &$hir hir::ForeignItem<$hir>);
             fn check_foreign_item_post(a: &$hir hir::ForeignItem<$hir>);
             fn check_item(a: &$hir hir::Item<$hir>);
diff --git a/src/librustc_lint/nonstandard_style.rs b/src/librustc_lint/nonstandard_style.rs
index 63a5f8681d6f..9b9203083ee7 100644
--- a/src/librustc_lint/nonstandard_style.rs
+++ b/src/librustc_lint/nonstandard_style.rs
@@ -246,7 +246,7 @@ impl NonSnakeCase {
 }
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NonSnakeCase {
-    fn check_mod(&mut self, cx: &LateContext<'_, '_>, _: &'tcx hir::Mod, _: Span, id: hir::HirId) {
+    fn check_mod(&mut self, cx: &LateContext<'_, '_>, _: &'tcx hir::Mod<'tcx>, _: Span, id: hir::HirId) {
         if id != hir::CRATE_HIR_ID {
             return;
         }
diff --git a/src/librustc_metadata/rmeta/encoder.rs b/src/librustc_metadata/rmeta/encoder.rs
index e9c5f90d9dcf..cfe5ea65fca5 100644
--- a/src/librustc_metadata/rmeta/encoder.rs
+++ b/src/librustc_metadata/rmeta/encoder.rs
@@ -682,7 +682,7 @@ impl EncodeContext<'tcx> {
     fn encode_info_for_mod(
         &mut self,
         id: hir::HirId,
-        md: &hir::Mod,
+        md: &hir::Mod<'_>,
         attrs: &[ast::Attribute],
         vis: &hir::Visibility,
     ) {
diff --git a/src/librustc_passes/hir_stats.rs b/src/librustc_passes/hir_stats.rs
index 9ea742354014..776c1aff1118 100644
--- a/src/librustc_passes/hir_stats.rs
+++ b/src/librustc_passes/hir_stats.rs
@@ -128,7 +128,7 @@ impl<'v> hir_visit::Visitor<'v> for StatCollector<'v> {
         hir_visit::walk_item(self, i)
     }
 
-    fn visit_mod(&mut self, m: &'v hir::Mod, _s: Span, n: hir::HirId) {
+    fn visit_mod(&mut self, m: &'v hir::Mod<'v>, _s: Span, n: hir::HirId) {
         self.record("Mod", Id::None, m);
         hir_visit::walk_mod(self, m, n)
     }
diff --git a/src/librustc_privacy/lib.rs b/src/librustc_privacy/lib.rs
index d8d554bad2ca..83f2fe7e0231 100644
--- a/src/librustc_privacy/lib.rs
+++ b/src/librustc_privacy/lib.rs
@@ -510,7 +510,7 @@ impl EmbargoVisitor<'tcx> {
     fn update_macro_reachable_mod(&mut self, reachable_mod: hir::HirId, defining_mod: DefId) {
         let module_def_id = self.tcx.hir().local_def_id(reachable_mod);
         let module = self.tcx.hir().get_module(module_def_id).0;
-        for item_id in &module.item_ids {
+        for item_id in module.item_ids {
             let hir_id = item_id.id;
             let item_def_id = self.tcx.hir().local_def_id(hir_id);
             if let Some(def_kind) = self.tcx.def_kind(item_def_id) {
@@ -849,7 +849,7 @@ impl Visitor<'tcx> for EmbargoVisitor<'tcx> {
         self.prev_level = orig_level;
     }
 
-    fn visit_mod(&mut self, m: &'tcx hir::Mod, _sp: Span, id: hir::HirId) {
+    fn visit_mod(&mut self, m: &'tcx hir::Mod<'tcx>, _sp: Span, id: hir::HirId) {
         // This code is here instead of in visit_item so that the
         // crate module gets processed as well.
         if self.prev_level.is_some() {
@@ -992,7 +992,7 @@ impl<'a, 'tcx> Visitor<'tcx> for NamePrivacyVisitor<'a, 'tcx> {
         NestedVisitorMap::All(&self.tcx.hir())
     }
 
-    fn visit_mod(&mut self, _m: &'tcx hir::Mod, _s: Span, _n: hir::HirId) {
+    fn visit_mod(&mut self, _m: &'tcx hir::Mod<'tcx>, _s: Span, _n: hir::HirId) {
         // Don't visit nested modules, since we run a separate visitor walk
         // for each module in `privacy_access_levels`
     }
@@ -1132,7 +1132,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypePrivacyVisitor<'a, 'tcx> {
         NestedVisitorMap::All(&self.tcx.hir())
     }
 
-    fn visit_mod(&mut self, _m: &'tcx hir::Mod, _s: Span, _n: hir::HirId) {
+    fn visit_mod(&mut self, _m: &'tcx hir::Mod<'tcx>, _s: Span, _n: hir::HirId) {
         // Don't visit nested modules, since we run a separate visitor walk
         // for each module in `privacy_access_levels`
     }
diff --git a/src/librustc_typeck/check/method/suggest.rs b/src/librustc_typeck/check/method/suggest.rs
index aecc46d31288..e2b77c01e24d 100644
--- a/src/librustc_typeck/check/method/suggest.rs
+++ b/src/librustc_typeck/check/method/suggest.rs
@@ -1093,7 +1093,7 @@ impl UsePlacementFinder<'tcx> {
 impl hir::intravisit::Visitor<'tcx> for UsePlacementFinder<'tcx> {
     fn visit_mod(
         &mut self,
-        module: &'tcx hir::Mod,
+        module: &'tcx hir::Mod<'tcx>,
         _: Span,
         hir_id: hir::HirId,
     ) {
@@ -1105,7 +1105,7 @@ impl hir::intravisit::Visitor<'tcx> for UsePlacementFinder<'tcx> {
             return;
         }
         // Find a `use` statement.
-        for item_id in &module.item_ids {
+        for item_id in module.item_ids {
             let item = self.tcx.hir().expect_item(item_id.id);
             match item.kind {
                 hir::ItemKind::Use(..) => {
diff --git a/src/librustdoc/visit_ast.rs b/src/librustdoc/visit_ast.rs
index 2c2a51928d9c..897c00f276e9 100644
--- a/src/librustdoc/visit_ast.rs
+++ b/src/librustdoc/visit_ast.rs
@@ -215,7 +215,7 @@ impl<'a, 'tcx> RustdocVisitor<'a, 'tcx> {
 
     fn visit_mod_contents(&mut self, span: Span, attrs: &'tcx [ast::Attribute],
                               vis: &'tcx hir::Visibility, id: hir::HirId,
-                              m: &'tcx hir::Mod,
+                              m: &'tcx hir::Mod<'tcx>,
                               name: Option<ast::Name>) -> Module<'tcx> {
         let mut om = Module::new(name, attrs, vis);
         om.where_outer = span;
@@ -224,7 +224,7 @@ impl<'a, 'tcx> RustdocVisitor<'a, 'tcx> {
         // Keep track of if there were any private modules in the path.
         let orig_inside_public_path = self.inside_public_path;
         self.inside_public_path &= vis.node.is_pub();
-        for i in &m.item_ids {
+        for i in m.item_ids {
             let item = self.cx.tcx.hir().expect_item(i.id);
             self.visit_item(item, None, &mut om);
         }
@@ -322,7 +322,7 @@ impl<'a, 'tcx> RustdocVisitor<'a, 'tcx> {
         let ret = match tcx.hir().get(res_hir_id) {
             Node::Item(&hir::Item { kind: hir::ItemKind::Mod(ref m), .. }) if glob => {
                 let prev = mem::replace(&mut self.inlining, true);
-                for i in &m.item_ids {
+                for i in m.item_ids {
                     let i = self.cx.tcx.hir().expect_item(i.id);
                     self.visit_item(i, None, om);
                 }