diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 48a86b2753a..de6a0b563eb 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -1414,7 +1414,7 @@ pub enum ExprKind {
     /// `'label: loop { block }`
     Loop(P<Block>, Option<Label>, Span),
     /// A `match` block.
-    Match(P<Expr>, Vec<Arm>),
+    Match(P<Expr>, ThinVec<Arm>),
     /// A closure (e.g., `move |a, b, c| a + b + c`).
     Closure(Box<Closure>),
     /// A block (`'label: { ... }`).
diff --git a/compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs b/compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs
index b4ea7883baa..cfd36f030a1 100644
--- a/compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs
@@ -71,7 +71,7 @@ pub fn cs_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> Bl
                 let eq_arm = cx.arm(span, cx.pat_path(span, equal_path.clone()), expr1);
                 let neq_arm =
                     cx.arm(span, cx.pat_ident(span, test_id), cx.expr_ident(span, test_id));
-                cx.expr_match(span, expr2, vec![eq_arm, neq_arm])
+                cx.expr_match(span, expr2, thin_vec![eq_arm, neq_arm])
             }
             CsFold::Fieldless => cx.expr_path(equal_path.clone()),
         },
diff --git a/compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs b/compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs
index 9a98ed4e735..9f46247908d 100644
--- a/compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs
@@ -143,7 +143,7 @@ fn cs_partial_cmp(
                         cx.arm(span, cx.pat_some(span, cx.pat_path(span, equal_path.clone())), expr1);
                     let neq_arm =
                         cx.arm(span, cx.pat_ident(span, test_id), cx.expr_ident(span, test_id));
-                    cx.expr_match(span, expr2, vec![eq_arm, neq_arm])
+                    cx.expr_match(span, expr2, thin_vec![eq_arm, neq_arm])
                 }
             }
             CsFold::Fieldless => cx.expr_some(span, cx.expr_path(equal_path.clone())),
diff --git a/compiler/rustc_builtin_macros/src/deriving/debug.rs b/compiler/rustc_builtin_macros/src/deriving/debug.rs
index c99b72a2634..809f9838d20 100644
--- a/compiler/rustc_builtin_macros/src/deriving/debug.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/debug.rs
@@ -234,7 +234,7 @@ fn show_fieldless_enum(
             };
             cx.arm(span, pat, cx.expr_str(span, v.ident.name))
         })
-        .collect::<Vec<_>>();
+        .collect::<ThinVec<_>>();
     let name = cx.expr_match(span, cx.expr_self(span), arms);
     let fn_path_write_str = cx.std_path(&[sym::fmt, sym::Formatter, sym::write_str]);
     BlockOrExpr::new_expr(cx.expr_call_global(span, fn_path_write_str, thin_vec![fmt, name]))
diff --git a/compiler/rustc_builtin_macros/src/deriving/decodable.rs b/compiler/rustc_builtin_macros/src/deriving/decodable.rs
index 7c324c54707..3921533c84a 100644
--- a/compiler/rustc_builtin_macros/src/deriving/decodable.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/decodable.rs
@@ -123,7 +123,7 @@ fn decodable_substructure(
         StaticEnum(_, fields) => {
             let variant = Ident::new(sym::i, trait_span);
 
-            let mut arms = Vec::with_capacity(fields.len() + 1);
+            let mut arms = ThinVec::with_capacity(fields.len() + 1);
             let mut variants = ThinVec::with_capacity(fields.len());
 
             let fn_read_enum_variant_arg_path: Vec<_> =
diff --git a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
index 31921a0c864..1f819beeb5d 100644
--- a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
@@ -1247,7 +1247,7 @@ impl<'a> MethodDef<'a> {
         // (Variant2, Variant2, ...) => Body2
         // ...
         // where each tuple has length = selflike_args.len()
-        let mut match_arms: Vec<ast::Arm> = variants
+        let mut match_arms: ThinVec<ast::Arm> = variants
             .iter()
             .enumerate()
             .filter(|&(_, v)| !(unify_fieldless_variants && v.data.fields().is_empty()))
diff --git a/compiler/rustc_expand/src/build.rs b/compiler/rustc_expand/src/build.rs
index b09d3e0842f..eaecce91b8a 100644
--- a/compiler/rustc_expand/src/build.rs
+++ b/compiler/rustc_expand/src/build.rs
@@ -446,7 +446,7 @@ impl<'a> ExtCtxt<'a> {
         let err_arm = self.arm(sp, err_pat, err_expr);
 
         // `match head { Ok() => ..., Err() => ... }`
-        self.expr_match(sp, head, vec![ok_arm, err_arm])
+        self.expr_match(sp, head, thin_vec![ok_arm, err_arm])
     }
 
     pub fn pat(&self, span: Span, kind: PatKind) -> P<ast::Pat> {
@@ -516,7 +516,7 @@ impl<'a> ExtCtxt<'a> {
         self.arm(span, self.pat_wild(span), self.expr_unreachable(span))
     }
 
-    pub fn expr_match(&self, span: Span, arg: P<ast::Expr>, arms: Vec<ast::Arm>) -> P<Expr> {
+    pub fn expr_match(&self, span: Span, arg: P<ast::Expr>, arms: ThinVec<ast::Arm>) -> P<Expr> {
         self.expr(span, ast::ExprKind::Match(arg, arms))
     }
 
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 0c397414ba1..c92379754bf 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -2566,7 +2566,7 @@ impl<'a> Parser<'a> {
         }
         let attrs = self.parse_inner_attributes()?;
 
-        let mut arms: Vec<Arm> = Vec::new();
+        let mut arms = ThinVec::new();
         while self.token != token::CloseDelim(Delimiter::Brace) {
             match self.parse_arm() {
                 Ok(arm) => arms.push(arm),