diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index e2236b3ef828e..3c8d9140b0093 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -2435,7 +2435,7 @@ pub struct ForeignMod {
     /// semantically by Rust.
     pub unsafety: Unsafe,
     pub abi: Option<StrLit>,
-    pub items: Vec<P<ForeignItem>>,
+    pub items: ThinVec<P<ForeignItem>>,
 }
 
 #[derive(Clone, Encodable, Decodable, Debug)]
@@ -2753,7 +2753,7 @@ pub struct Trait {
     pub is_auto: IsAuto,
     pub generics: Generics,
     pub bounds: GenericBounds,
-    pub items: Vec<P<AssocItem>>,
+    pub items: ThinVec<P<AssocItem>>,
 }
 
 /// The location of a where clause on a `TyAlias` (`Span`) and whether there was
@@ -2801,7 +2801,7 @@ pub struct Impl {
     /// The trait being implemented, if any.
     pub of_trait: Option<TraitRef>,
     pub self_ty: P<Ty>,
-    pub items: Vec<P<AssocItem>>,
+    pub items: ThinVec<P<AssocItem>>,
 }
 
 #[derive(Clone, Encodable, Decodable, Debug)]
@@ -3047,7 +3047,7 @@ mod size_asserts {
     static_assert_size!(ForeignItemKind, 24);
     static_assert_size!(GenericBound, 56);
     static_assert_size!(Generics, 40);
-    static_assert_size!(Impl, 152);
+    static_assert_size!(Impl, 136);
     static_assert_size!(Item, 152);
     static_assert_size!(ItemKind, 80);
     static_assert_size!(Lit, 48);
diff --git a/compiler/rustc_builtin_macros/src/deriving/mod.rs b/compiler/rustc_builtin_macros/src/deriving/mod.rs
index a65d0bad6de80..9039c15a1fb62 100644
--- a/compiler/rustc_builtin_macros/src/deriving/mod.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/mod.rs
@@ -6,6 +6,7 @@ use rustc_ast::{GenericArg, Impl, ItemKind, MetaItem};
 use rustc_expand::base::{Annotatable, ExpandResult, ExtCtxt, MultiItemModifier};
 use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::Span;
+use thin_vec::ThinVec;
 
 macro path_local($x:ident) {
     generic::ty::Path::new_local(sym::$x)
@@ -187,7 +188,7 @@ fn inject_impl_of_structural_trait(
             generics,
             of_trait: Some(trait_ref),
             self_ty: self_type,
-            items: Vec::new(),
+            items: ThinVec::new(),
         })),
     );
 
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index e77c4f9be023b..58175c37a1786 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -663,12 +663,12 @@ impl<'a> Parser<'a> {
         &mut self,
         attrs: &mut AttrVec,
         mut parse_item: impl FnMut(&mut Parser<'a>) -> PResult<'a, Option<Option<T>>>,
-    ) -> PResult<'a, Vec<T>> {
+    ) -> PResult<'a, ThinVec<T>> {
         let open_brace_span = self.token.span;
         self.expect(&token::OpenDelim(Delimiter::Brace))?;
         attrs.extend(self.parse_inner_attributes()?);
 
-        let mut items = Vec::new();
+        let mut items = ThinVec::new();
         while !self.eat(&token::CloseDelim(Delimiter::Brace)) {
             if self.recover_doc_comment_before_brace() {
                 continue;