diff --git a/Cargo.lock b/Cargo.lock
index 68ea8b51c555c..9a804d6bedcc3 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3171,6 +3171,7 @@ version = "0.0.0"
 dependencies = [
  "rustc_ast",
  "rustc_span",
+ "thin-vec",
 ]
 
 [[package]]
@@ -3461,6 +3462,7 @@ dependencies = [
  "rustc_session",
  "rustc_span",
  "smallvec",
+ "thin-vec",
  "tracing",
 ]
 
@@ -3850,6 +3852,7 @@ dependencies = [
  "rustc_macros",
  "rustc_session",
  "rustc_span",
+ "thin-vec",
  "tracing",
  "unicode-normalization",
  "unicode-width",
@@ -3984,6 +3987,7 @@ dependencies = [
  "rustc_session",
  "rustc_span",
  "smallvec",
+ "thin-vec",
  "tracing",
 ]
 
diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index f25fdc942b085..d7f123d997213 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -37,7 +37,7 @@ use std::cmp::Ordering;
 use std::convert::TryFrom;
 use std::fmt;
 use std::mem;
-use thin_vec::ThinVec;
+use thin_vec::{thin_vec, ThinVec};
 
 /// A "Label" is an identifier of some point in sources,
 /// e.g. in the following code:
@@ -91,7 +91,7 @@ pub struct Path {
     pub span: Span,
     /// The segments in the path: the things separated by `::`.
     /// Global paths begin with `kw::PathRoot`.
-    pub segments: Vec<PathSegment>,
+    pub segments: ThinVec<PathSegment>,
     pub tokens: Option<LazyTokenStream>,
 }
 
@@ -115,7 +115,7 @@ impl Path {
     // Convert a span and an identifier to the corresponding
     // one-segment path.
     pub fn from_ident(ident: Ident) -> Path {
-        Path { segments: vec![PathSegment::from_ident(ident)], span: ident.span, tokens: None }
+        Path { segments: thin_vec![PathSegment::from_ident(ident)], span: ident.span, tokens: None }
     }
 
     pub fn is_global(&self) -> bool {
@@ -414,7 +414,7 @@ impl GenericParam {
 /// a function, enum, trait, etc.
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct Generics {
-    pub params: Vec<GenericParam>,
+    pub params: ThinVec<GenericParam>,
     pub where_clause: WhereClause,
     pub span: Span,
 }
@@ -423,10 +423,10 @@ impl Default for Generics {
     /// Creates an instance of `Generics`.
     fn default() -> Generics {
         Generics {
-            params: Vec::new(),
+            params: ThinVec::new(),
             where_clause: WhereClause {
                 has_where_token: false,
-                predicates: Vec::new(),
+                predicates: ThinVec::new(),
                 span: DUMMY_SP,
             },
             span: DUMMY_SP,
@@ -441,7 +441,7 @@ pub struct WhereClause {
     /// if we parsed no predicates (e.g. `struct Foo where {}`).
     /// This allows us to pretty-print accurately.
     pub has_where_token: bool,
-    pub predicates: Vec<WherePredicate>,
+    pub predicates: ThinVec<WherePredicate>,
     pub span: Span,
 }
 
@@ -473,7 +473,7 @@ impl WherePredicate {
 pub struct WhereBoundPredicate {
     pub span: Span,
     /// Any generics from a `for` binding.
-    pub bound_generic_params: Vec<GenericParam>,
+    pub bound_generic_params: ThinVec<GenericParam>,
     /// The type being bounded.
     pub bounded_ty: P<Ty>,
     /// Trait and lifetime bounds (`Clone + Send + 'static`).
@@ -1187,7 +1187,7 @@ impl Expr {
     pub fn to_bound(&self) -> Option<GenericBound> {
         match &self.kind {
             ExprKind::Path(None, path) => Some(GenericBound::Trait(
-                PolyTraitRef::new(Vec::new(), path.clone(), self.span),
+                PolyTraitRef::new(ThinVec::new(), path.clone(), self.span),
                 TraitBoundModifier::None,
             )),
             _ => None,
@@ -1564,7 +1564,7 @@ pub enum ClosureBinder {
         /// for<'a, 'b> |_: &'a (), _: &'b ()| { ... }
         ///     ^^^^^^ -- this
         /// ```
-        generic_params: P<[GenericParam]>,
+        generic_params: ThinVec<GenericParam>,
     },
 }
 
@@ -2025,7 +2025,7 @@ impl Ty {
 pub struct BareFnTy {
     pub unsafety: Unsafe,
     pub ext: Extern,
-    pub generic_params: Vec<GenericParam>,
+    pub generic_params: ThinVec<GenericParam>,
     pub decl: P<FnDecl>,
     /// Span of the `fn(...) -> ...` part.
     pub decl_span: Span,
@@ -2468,7 +2468,7 @@ 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)]
@@ -2611,7 +2611,7 @@ pub struct TraitRef {
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct PolyTraitRef {
     /// The `'a` in `for<'a> Foo<&'a T>`.
-    pub bound_generic_params: Vec<GenericParam>,
+    pub bound_generic_params: ThinVec<GenericParam>,
 
     /// The `Foo<&'a T>` in `<'a> Foo<&'a T>`.
     pub trait_ref: TraitRef,
@@ -2620,7 +2620,7 @@ pub struct PolyTraitRef {
 }
 
 impl PolyTraitRef {
-    pub fn new(generic_params: Vec<GenericParam>, path: Path, span: Span) -> Self {
+    pub fn new(generic_params: ThinVec<GenericParam>, path: Path, span: Span) -> Self {
         PolyTraitRef {
             bound_generic_params: generic_params,
             trait_ref: TraitRef { path, ref_id: DUMMY_NODE_ID },
@@ -2786,7 +2786,7 @@ 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
@@ -2834,7 +2834,7 @@ 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)]
@@ -3075,25 +3075,25 @@ mod size_asserts {
     static_assert_size!(Block, 48);
     static_assert_size!(Expr, 104);
     static_assert_size!(ExprKind, 72);
-    static_assert_size!(Fn, 192);
+    static_assert_size!(Fn, 160);
     static_assert_size!(ForeignItem, 96);
     static_assert_size!(ForeignItemKind, 24);
     static_assert_size!(GenericArg, 24);
-    static_assert_size!(GenericBound, 88);
-    static_assert_size!(Generics, 72);
-    static_assert_size!(Impl, 200);
-    static_assert_size!(Item, 184);
-    static_assert_size!(ItemKind, 112);
+    static_assert_size!(GenericBound, 56);
+    static_assert_size!(Generics, 40);
+    static_assert_size!(Impl, 136);
+    static_assert_size!(Item, 152);
+    static_assert_size!(ItemKind, 80);
     static_assert_size!(Lit, 48);
     static_assert_size!(LitKind, 24);
     static_assert_size!(Local, 72);
     static_assert_size!(Param, 40);
-    static_assert_size!(Pat, 120);
-    static_assert_size!(PatKind, 96);
-    static_assert_size!(Path, 40);
+    static_assert_size!(Pat, 104);
+    static_assert_size!(PatKind, 80);
+    static_assert_size!(Path, 24);
     static_assert_size!(PathSegment, 24);
     static_assert_size!(Stmt, 32);
     static_assert_size!(StmtKind, 16);
-    static_assert_size!(Ty, 96);
-    static_assert_size!(TyKind, 72);
+    static_assert_size!(Ty, 80);
+    static_assert_size!(TyKind, 56);
 }
diff --git a/compiler/rustc_ast/src/attr/mod.rs b/compiler/rustc_ast/src/attr/mod.rs
index 5b72ec2b6015c..b8592e362cb0d 100644
--- a/compiler/rustc_ast/src/attr/mod.rs
+++ b/compiler/rustc_ast/src/attr/mod.rs
@@ -11,13 +11,12 @@ use crate::tokenstream::{AttrAnnotatedTokenStream, AttrAnnotatedTokenTree};
 use crate::tokenstream::{DelimSpan, Spacing, TokenTree};
 use crate::tokenstream::{LazyTokenStream, TokenStream};
 use crate::util::comments;
-
 use rustc_index::bit_set::GrowableBitSet;
 use rustc_span::source_map::BytePos;
 use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::Span;
-
 use std::iter;
+use thin_vec::thin_vec;
 
 pub struct MarkedAttrs(GrowableBitSet<AttrId>);
 
@@ -422,12 +421,12 @@ impl MetaItem {
                         tokens.peek()
                     {
                         tokens.next();
-                        vec![PathSegment::from_ident(Ident::new(name, span))]
+                        thin_vec![PathSegment::from_ident(Ident::new(name, span))]
                     } else {
                         break 'arm Path::from_ident(Ident::new(name, span));
                     }
                 } else {
-                    vec![PathSegment::path_root(span)]
+                    thin_vec![PathSegment::path_root(span)]
                 };
                 loop {
                     if let Some(TokenTree::Token(Token { kind: token::Ident(name, _), span }, _)) =
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index 458d1156ec251..bbb0e1fc74756 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -17,10 +17,10 @@ use rustc_data_structures::sync::Lrc;
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::Ident;
 use rustc_span::Span;
-
 use smallvec::{smallvec, Array, SmallVec};
 use std::ops::DerefMut;
 use std::{panic, ptr};
+use thin_vec::ThinVec;
 
 pub trait ExpectOne<A: Array> {
     fn expect_one(self, err: &'static str) -> A::Item;
@@ -325,6 +325,17 @@ where
     }
 }
 
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+#[inline]
+pub fn visit_thin_vec<T, F>(elems: &mut ThinVec<T>, mut visit_elem: F)
+where
+    F: FnMut(&mut T),
+{
+    for elem in elems {
+        visit_elem(elem);
+    }
+}
+
 // No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
 #[inline]
 pub fn visit_opt<T, F>(opt: &mut Option<T>, mut visit_elem: F)
@@ -829,9 +840,7 @@ pub fn noop_visit_closure_binder<T: MutVisitor>(binder: &mut ClosureBinder, vis:
     match binder {
         ClosureBinder::NotPresent => {}
         ClosureBinder::For { span: _, generic_params } => {
-            let mut vec = std::mem::take(generic_params).into_vec();
-            vec.flat_map_in_place(|param| vis.flat_map_generic_param(param));
-            *generic_params = P::from_vec(vec);
+            generic_params.flat_map_in_place(|param| vis.flat_map_generic_param(param));
         }
     }
 }
@@ -909,7 +918,7 @@ pub fn noop_visit_generics<T: MutVisitor>(generics: &mut Generics, vis: &mut T)
 
 pub fn noop_visit_where_clause<T: MutVisitor>(wc: &mut WhereClause, vis: &mut T) {
     let WhereClause { has_where_token: _, predicates, span } = wc;
-    visit_vec(predicates, |predicate| vis.visit_where_predicate(predicate));
+    visit_thin_vec(predicates, |predicate| vis.visit_where_predicate(predicate));
     vis.visit_span(span);
 }
 
diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index 76f63d1d78a90..90469b4239da8 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -19,8 +19,8 @@ use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::Span;
 use rustc_target::spec::abi;
 use smallvec::{smallvec, SmallVec};
-
 use std::iter;
+use thin_vec::ThinVec;
 
 pub(super) struct ItemLowerer<'a, 'hir> {
     pub(super) tcx: TyCtxt<'hir>,
@@ -235,7 +235,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             ItemKind::ExternCrate(orig_name) => hir::ItemKind::ExternCrate(orig_name),
             ItemKind::Use(ref use_tree) => {
                 // Start with an empty prefix.
-                let prefix = Path { segments: vec![], span: use_tree.span, tokens: None };
+                let prefix = Path { segments: ThinVec::new(), span: use_tree.span, tokens: None };
 
                 self.lower_use_tree(use_tree, &prefix, id, vis_span, ident, attrs)
             }
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 3a94c7a91b23f..73c9678a0e64e 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -68,9 +68,9 @@ use rustc_span::hygiene::MacroKind;
 use rustc_span::source_map::DesugaringKind;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{Span, DUMMY_SP};
-
 use smallvec::SmallVec;
 use std::collections::hash_map::Entry;
+use thin_vec::ThinVec;
 
 macro_rules! arena_vec {
     ($this:expr; $($x:expr),*) => (
@@ -1183,7 +1183,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             let (bounds, lifetime_bound) = self.with_dyn_type_scope(true, |this| {
                 let bound = this.lower_poly_trait_ref(
                     &PolyTraitRef {
-                        bound_generic_params: vec![],
+                        bound_generic_params: ThinVec::new(),
                         trait_ref: TraitRef { path: path.clone(), ref_id: t.id },
                         span: t.span
                     },
diff --git a/compiler/rustc_ast_pretty/Cargo.toml b/compiler/rustc_ast_pretty/Cargo.toml
index 5ad8714e9fec9..8a512b352c5fe 100644
--- a/compiler/rustc_ast_pretty/Cargo.toml
+++ b/compiler/rustc_ast_pretty/Cargo.toml
@@ -9,3 +9,4 @@ doctest = false
 [dependencies]
 rustc_span = { path = "../rustc_span" }
 rustc_ast = { path = "../rustc_ast" }
+thin-vec = "0.2.8"
diff --git a/compiler/rustc_ast_pretty/src/pprust/state.rs b/compiler/rustc_ast_pretty/src/pprust/state.rs
index ed5e7dace4bc4..9b449fd22971b 100644
--- a/compiler/rustc_ast_pretty/src/pprust/state.rs
+++ b/compiler/rustc_ast_pretty/src/pprust/state.rs
@@ -21,8 +21,8 @@ use rustc_span::edition::Edition;
 use rustc_span::source_map::{SourceMap, Spanned};
 use rustc_span::symbol::{kw, sym, Ident, IdentPrinter, Symbol};
 use rustc_span::{BytePos, FileName, Span};
-
 use std::borrow::Cow;
+use thin_vec::ThinVec;
 
 pub use self::delimited::IterDelimited;
 
@@ -1708,10 +1708,10 @@ impl<'a> State<'a> {
         self.ibox(INDENT_UNIT);
         self.print_formal_generic_params(generic_params);
         let generics = ast::Generics {
-            params: Vec::new(),
+            params: ThinVec::new(),
             where_clause: ast::WhereClause {
                 has_where_token: false,
-                predicates: Vec::new(),
+                predicates: ThinVec::new(),
                 span: rustc_span::DUMMY_SP,
             },
             span: rustc_span::DUMMY_SP,
diff --git a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
index 3cc160adb5397..1221892b01377 100644
--- a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
@@ -175,7 +175,7 @@ use rustc_span::Span;
 use std::cell::RefCell;
 use std::iter;
 use std::vec;
-use thin_vec::thin_vec;
+use thin_vec::{thin_vec, ThinVec};
 use ty::{Bounds, Path, Ref, Self_, Ty};
 
 pub mod ty;
@@ -293,7 +293,7 @@ pub fn combine_substructure(
 }
 
 struct TypeParameter {
-    bound_generic_params: Vec<ast::GenericParam>,
+    bound_generic_params: ThinVec<ast::GenericParam>,
     ty: P<ast::Ty>,
 }
 
@@ -360,7 +360,7 @@ fn find_type_parameters(
     struct Visitor<'a, 'b> {
         cx: &'a ExtCtxt<'b>,
         ty_param_names: &'a [Symbol],
-        bound_generic_params_stack: Vec<ast::GenericParam>,
+        bound_generic_params_stack: ThinVec<ast::GenericParam>,
         type_params: Vec<TypeParameter>,
     }
 
@@ -398,7 +398,7 @@ fn find_type_parameters(
     let mut visitor = Visitor {
         cx,
         ty_param_names,
-        bound_generic_params_stack: Vec::new(),
+        bound_generic_params_stack: ThinVec::new(),
         type_params: Vec::new(),
     };
     visit::Visitor::visit_ty(&mut visitor, ty);
diff --git a/compiler/rustc_builtin_macros/src/deriving/generic/ty.rs b/compiler/rustc_builtin_macros/src/deriving/generic/ty.rs
index 36e2e29308694..9185c4d652835 100644
--- a/compiler/rustc_builtin_macros/src/deriving/generic/ty.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/generic/ty.rs
@@ -9,6 +9,7 @@ use rustc_expand::base::ExtCtxt;
 use rustc_span::source_map::{respan, DUMMY_SP};
 use rustc_span::symbol::{kw, Ident, Symbol};
 use rustc_span::Span;
+use thin_vec::ThinVec;
 
 /// A path, e.g., `::std::option::Option::<i32>` (global). Has support
 /// for type parameters.
@@ -188,7 +189,11 @@ impl Bounds {
 
         Generics {
             params,
-            where_clause: ast::WhereClause { has_where_token: false, predicates: Vec::new(), span },
+            where_clause: ast::WhereClause {
+                has_where_token: false,
+                predicates: ThinVec::new(),
+                span,
+            },
             span,
         }
     }
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_expand/Cargo.toml b/compiler/rustc_expand/Cargo.toml
index 4ee7b6c42bbf5..192f54171cee6 100644
--- a/compiler/rustc_expand/Cargo.toml
+++ b/compiler/rustc_expand/Cargo.toml
@@ -8,20 +8,21 @@ build = false
 doctest = false
 
 [dependencies]
-rustc_serialize = { path = "../rustc_serialize" }
-tracing = "0.1"
-rustc_span = { path = "../rustc_span" }
-rustc_ast_pretty = { path = "../rustc_ast_pretty" }
+crossbeam-channel = "0.5.0"
 rustc_ast_passes = { path = "../rustc_ast_passes" }
+rustc_ast = { path = "../rustc_ast" }
+rustc_ast_pretty = { path = "../rustc_ast_pretty" }
 rustc_attr = { path = "../rustc_attr" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }
 rustc_feature = { path = "../rustc_feature" }
+rustc_lexer = { path = "../rustc_lexer" }
 rustc_lint_defs = { path = "../rustc_lint_defs" }
 rustc_macros = { path = "../rustc_macros" }
-rustc_lexer = { path = "../rustc_lexer" }
 rustc_parse = { path = "../rustc_parse" }
+rustc_serialize = { path = "../rustc_serialize" }
 rustc_session = { path = "../rustc_session" }
+rustc_span = { path = "../rustc_span" }
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
-rustc_ast = { path = "../rustc_ast" }
-crossbeam-channel = "0.5.0"
+thin-vec = "0.2.8"
+tracing = "0.1"
diff --git a/compiler/rustc_expand/src/build.rs b/compiler/rustc_expand/src/build.rs
index cf2c023c2f89f..9a024766c0aa3 100644
--- a/compiler/rustc_expand/src/build.rs
+++ b/compiler/rustc_expand/src/build.rs
@@ -1,13 +1,12 @@
 use crate::base::ExtCtxt;
-
 use rustc_ast::attr;
 use rustc_ast::ptr::P;
 use rustc_ast::{self as ast, AttrVec, BlockCheckMode, Expr, LocalKind, PatKind, UnOp};
 use rustc_data_structures::sync::Lrc;
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
-
 use rustc_span::Span;
+use thin_vec::ThinVec;
 
 impl<'a> ExtCtxt<'a> {
     pub fn path(&self, span: Span, strs: Vec<Ident>) -> ast::Path {
@@ -28,7 +27,7 @@ impl<'a> ExtCtxt<'a> {
     ) -> ast::Path {
         assert!(!idents.is_empty());
         let add_root = global && !idents[0].is_path_segment_keyword();
-        let mut segments = Vec::with_capacity(idents.len() + add_root as usize);
+        let mut segments = ThinVec::with_capacity(idents.len() + add_root as usize);
         if add_root {
             segments.push(ast::PathSegment::path_root(span));
         }
@@ -127,7 +126,7 @@ impl<'a> ExtCtxt<'a> {
 
     pub fn poly_trait_ref(&self, span: Span, path: ast::Path) -> ast::PolyTraitRef {
         ast::PolyTraitRef {
-            bound_generic_params: Vec::new(),
+            bound_generic_params: ThinVec::new(),
             trait_ref: self.trait_ref(path),
             span,
         }
diff --git a/compiler/rustc_expand/src/placeholders.rs b/compiler/rustc_expand/src/placeholders.rs
index 3b0d5ddb97b4e..921d869d1abd5 100644
--- a/compiler/rustc_expand/src/placeholders.rs
+++ b/compiler/rustc_expand/src/placeholders.rs
@@ -1,14 +1,12 @@
 use crate::expand::{AstFragment, AstFragmentKind};
-
 use rustc_ast as ast;
 use rustc_ast::mut_visit::*;
 use rustc_ast::ptr::P;
+use rustc_data_structures::fx::FxHashMap;
 use rustc_span::source_map::DUMMY_SP;
 use rustc_span::symbol::Ident;
-
 use smallvec::{smallvec, SmallVec};
-
-use rustc_data_structures::fx::FxHashMap;
+use thin_vec::ThinVec;
 
 pub fn placeholder(
     kind: AstFragmentKind,
@@ -17,7 +15,7 @@ pub fn placeholder(
 ) -> AstFragment {
     fn mac_placeholder() -> P<ast::MacCall> {
         P(ast::MacCall {
-            path: ast::Path { span: DUMMY_SP, segments: Vec::new(), tokens: None },
+            path: ast::Path { span: DUMMY_SP, segments: ThinVec::new(), tokens: None },
             args: P(ast::MacArgs::Empty),
             prior_type_ascription: None,
         })
diff --git a/compiler/rustc_parse/Cargo.toml b/compiler/rustc_parse/Cargo.toml
index c6ca260e9831e..0423b4ad557d9 100644
--- a/compiler/rustc_parse/Cargo.toml
+++ b/compiler/rustc_parse/Cargo.toml
@@ -8,15 +8,16 @@ doctest = false
 
 [dependencies]
 bitflags = "1.0"
-tracing = "0.1"
+rustc_ast = { path = "../rustc_ast" }
 rustc_ast_pretty = { path = "../rustc_ast_pretty" }
 rustc_data_structures = { path = "../rustc_data_structures" }
+rustc_errors = { path = "../rustc_errors" }
 rustc_feature = { path = "../rustc_feature" }
 rustc_lexer = { path = "../rustc_lexer" }
 rustc_macros = { path = "../rustc_macros" }
-rustc_errors = { path = "../rustc_errors" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
-rustc_ast = { path = "../rustc_ast" }
+thin-vec = "0.2.8"
+tracing = "0.1"
 unicode-normalization = "0.1.11"
 unicode-width = "0.1.4"
diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index ad49227222b2c..610ea76ea939a 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -5,6 +5,7 @@ use super::{
 };
 
 use crate::lexer::UnmatchedBrace;
+use crate::parser;
 use rustc_ast as ast;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, Lit, LitKind, TokenKind};
@@ -24,11 +25,10 @@ use rustc_macros::{SessionDiagnostic, SessionSubdiagnostic};
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::{Span, SpanSnippetError, DUMMY_SP};
-use std::ops::{Deref, DerefMut};
-
 use std::mem::take;
-
-use crate::parser;
+use std::ops::{Deref, DerefMut};
+use thin_vec::{thin_vec, ThinVec};
+use tracing::{debug, trace};
 
 const TURBOFISH_SUGGESTION_STR: &str =
     "use `::<...>` instead of `<...>` to specify lifetime, type, or const arguments";
@@ -1126,8 +1126,11 @@ impl<'a> Parser<'a> {
             //     field: value,
             // }
             let mut snapshot = self.create_snapshot_for_diagnostic();
-            let path =
-                Path { segments: vec![], span: self.prev_token.span.shrink_to_lo(), tokens: None };
+            let path = Path {
+                segments: ThinVec::new(),
+                span: self.prev_token.span.shrink_to_lo(),
+                tokens: None,
+            };
             let struct_expr = snapshot.parse_struct_expr(None, path, false);
             let block_tail = self.parse_block_tail(lo, s, AttemptLocalParseRecovery::No);
             return Some(match (struct_expr, block_tail) {
@@ -1934,7 +1937,7 @@ impl<'a> Parser<'a> {
     ) -> PResult<'a, P<T>> {
         self.expect(&token::ModSep)?;
 
-        let mut path = ast::Path { segments: Vec::new(), span: DUMMY_SP, tokens: None };
+        let mut path = ast::Path { segments: ThinVec::new(), span: DUMMY_SP, tokens: None };
         self.parse_path_segments(&mut path.segments, T::PATH_STYLE, None)?;
         path.span = ty_span.to(self.prev_token.span);
 
@@ -2976,7 +2979,7 @@ impl<'a> Parser<'a> {
                                             None,
                                             Path {
                                                 span: new_span,
-                                                segments: vec![
+                                                segments: thin_vec![
                                                     PathSegment::from_ident(*old_ident),
                                                     PathSegment::from_ident(*ident),
                                                 ],
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 7addf519872f0..6b0295414bf49 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -2047,7 +2047,7 @@ impl<'a> Parser<'a> {
 
             self.sess.gated_spans.gate(sym::closure_lifetime_binder, span);
 
-            ClosureBinder::For { span, generic_params: P::from_vec(lifetime_defs) }
+            ClosureBinder::For { span, generic_params: lifetime_defs }
         } else {
             ClosureBinder::NotPresent
         };
diff --git a/compiler/rustc_parse/src/parser/generics.rs b/compiler/rustc_parse/src/parser/generics.rs
index 4d0a8b05eb027..bda1c2012093b 100644
--- a/compiler/rustc_parse/src/parser/generics.rs
+++ b/compiler/rustc_parse/src/parser/generics.rs
@@ -4,6 +4,7 @@ use rustc_ast::token;
 use rustc_ast::{self as ast, AttrVec, GenericBounds, GenericParam, GenericParamKind, WhereClause};
 use rustc_errors::{Applicability, PResult};
 use rustc_span::symbol::kw;
+use thin_vec::ThinVec;
 
 impl<'a> Parser<'a> {
     /// Parses bounds of a lifetime parameter `BOUND + BOUND + BOUND`, possibly with trailing `+`.
@@ -76,8 +77,8 @@ impl<'a> Parser<'a> {
 
     /// Parses a (possibly empty) list of lifetime and type parameters, possibly including
     /// a trailing comma and erroneous trailing attributes.
-    pub(super) fn parse_generic_params(&mut self) -> PResult<'a, Vec<ast::GenericParam>> {
-        let mut params = Vec::new();
+    pub(super) fn parse_generic_params(&mut self) -> PResult<'a, ThinVec<ast::GenericParam>> {
+        let mut params = ThinVec::new();
         let mut done = false;
         while !done {
             let attrs = self.parse_outer_attributes()?;
@@ -195,13 +196,13 @@ impl<'a> Parser<'a> {
             self.expect_gt()?;
             (params, span_lo.to(self.prev_token.span))
         } else {
-            (vec![], self.prev_token.span.shrink_to_hi())
+            (ThinVec::new(), self.prev_token.span.shrink_to_hi())
         };
         Ok(ast::Generics {
             params,
             where_clause: WhereClause {
                 has_where_token: false,
-                predicates: Vec::new(),
+                predicates: ThinVec::new(),
                 span: self.prev_token.span.shrink_to_hi(),
             },
             span,
@@ -216,7 +217,7 @@ impl<'a> Parser<'a> {
     pub(super) fn parse_where_clause(&mut self) -> PResult<'a, WhereClause> {
         let mut where_clause = WhereClause {
             has_where_token: false,
-            predicates: Vec::new(),
+            predicates: ThinVec::new(),
             span: self.prev_token.span.shrink_to_hi(),
         };
 
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index b190a7062defd..1f3adb0101063 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -1,7 +1,6 @@
 use super::diagnostics::{dummy_arg, ConsumeClosingDelim, Error};
 use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{AttrWrapper, FollowedByType, ForceCollect, Parser, PathStyle, TrailingToken};
-
 use rustc_ast::ast::*;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, TokenKind};
@@ -19,9 +18,10 @@ use rustc_span::lev_distance::lev_distance;
 use rustc_span::source_map::{self, Span};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::DUMMY_SP;
-
 use std::convert::TryFrom;
 use std::mem;
+use thin_vec::ThinVec;
+use tracing::debug;
 
 impl<'a> Parser<'a> {
     /// Parses a source module as a crate. This is the main entry point for the parser.
@@ -662,12 +662,12 @@ 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;
@@ -929,7 +929,8 @@ impl<'a> Parser<'a> {
     fn parse_use_tree(&mut self) -> PResult<'a, UseTree> {
         let lo = self.token.span;
 
-        let mut prefix = ast::Path { segments: Vec::new(), span: lo.shrink_to_lo(), tokens: None };
+        let mut prefix =
+            ast::Path { segments: ThinVec::new(), span: lo.shrink_to_lo(), tokens: None };
         let kind = if self.check(&token::OpenDelim(Delimiter::Brace))
             || self.check(&token::BinOp(token::Star))
             || self.is_import_coupler()
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index fdc1af27f82e4..f673f307df415 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -11,8 +11,9 @@ use rustc_ast::{
 use rustc_errors::{pluralize, Applicability, PResult};
 use rustc_span::source_map::{BytePos, Span};
 use rustc_span::symbol::{kw, sym, Ident};
-
 use std::mem;
+use thin_vec::ThinVec;
+use tracing::debug;
 
 /// Specifies how to parse a path.
 #[derive(Copy, Clone, PartialEq)]
@@ -63,7 +64,7 @@ impl<'a> Parser<'a> {
             path_span = path_lo.to(self.prev_token.span);
         } else {
             path_span = self.token.span.to(self.token.span);
-            path = ast::Path { segments: Vec::new(), span: path_span, tokens: None };
+            path = ast::Path { segments: ThinVec::new(), span: path_span, tokens: None };
         }
 
         // See doc comment for `unmatched_angle_bracket_count`.
@@ -179,7 +180,7 @@ impl<'a> Parser<'a> {
         }
 
         let lo = self.token.span;
-        let mut segments = Vec::new();
+        let mut segments = ThinVec::new();
         let mod_sep_ctxt = self.token.span.ctxt();
         if self.eat(&token::ModSep) {
             segments.push(PathSegment::path_root(lo.shrink_to_lo().with_ctxt(mod_sep_ctxt)));
@@ -191,7 +192,7 @@ impl<'a> Parser<'a> {
 
     pub(super) fn parse_path_segments(
         &mut self,
-        segments: &mut Vec<PathSegment>,
+        segments: &mut ThinVec<PathSegment>,
         style: PathStyle,
         ty_generics: Option<&Generics>,
     ) -> PResult<'a, ()> {
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
index 4a2cf74905bf5..f5ce72cfd89cd 100644
--- a/compiler/rustc_parse/src/parser/ty.rs
+++ b/compiler/rustc_parse/src/parser/ty.rs
@@ -11,6 +11,7 @@ use rustc_ast::{
 use rustc_errors::{pluralize, struct_span_err, Applicability, PResult};
 use rustc_span::source_map::Span;
 use rustc_span::symbol::{kw, sym};
+use thin_vec::ThinVec;
 
 /// Any `?` or `~const` modifiers that appear at the start of a bound.
 struct BoundModifiers {
@@ -269,7 +270,7 @@ impl<'a> Parser<'a> {
             TyKind::Infer
         } else if self.check_fn_front_matter(false) {
             // Function pointer type
-            self.parse_ty_bare_fn(lo, Vec::new(), recover_return_sign)?
+            self.parse_ty_bare_fn(lo, ThinVec::new(), recover_return_sign)?
         } else if self.check_keyword(kw::For) {
             // Function pointer type or bound list (trait object type) starting with a poly-trait.
             //   `for<'lt> [unsafe] [extern "ABI"] fn (&'lt S) -> T`
@@ -343,7 +344,7 @@ impl<'a> Parser<'a> {
             match ty.kind {
                 // `(TY_BOUND_NOPAREN) + BOUND + ...`.
                 TyKind::Path(None, path) if maybe_bounds => {
-                    self.parse_remaining_bounds_path(Vec::new(), path, lo, true)
+                    self.parse_remaining_bounds_path(ThinVec::new(), path, lo, true)
                 }
                 TyKind::TraitObject(bounds, TraitObjectSyntax::None)
                     if maybe_bounds && bounds.len() == 1 && !trailing_plus =>
@@ -370,7 +371,7 @@ impl<'a> Parser<'a> {
 
     fn parse_remaining_bounds_path(
         &mut self,
-        generic_params: Vec<GenericParam>,
+        generic_params: ThinVec<GenericParam>,
         path: ast::Path,
         lo: Span,
         parse_plus: bool,
@@ -515,7 +516,7 @@ impl<'a> Parser<'a> {
     fn parse_ty_bare_fn(
         &mut self,
         lo: Span,
-        params: Vec<GenericParam>,
+        params: ThinVec<GenericParam>,
         recover_return_sign: RecoverReturnSign,
     ) -> PResult<'a, TyKind> {
         let inherited_vis = rustc_ast::Visibility {
@@ -605,7 +606,7 @@ impl<'a> Parser<'a> {
             })))
         } else if allow_plus == AllowPlus::Yes && self.check_plus() {
             // `Trait1 + Trait2 + 'a`
-            self.parse_remaining_bounds_path(Vec::new(), path, lo, true)
+            self.parse_remaining_bounds_path(ThinVec::new(), path, lo, true)
         } else {
             // Just a type path.
             Ok(TyKind::Path(None, path))
@@ -877,7 +878,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Optionally parses `for<$generic_params>`.
-    pub(super) fn parse_late_bound_lifetime_defs(&mut self) -> PResult<'a, Vec<GenericParam>> {
+    pub(super) fn parse_late_bound_lifetime_defs(&mut self) -> PResult<'a, ThinVec<GenericParam>> {
         if self.eat_keyword(kw::For) {
             self.expect_lt()?;
             let params = self.parse_generic_params()?;
@@ -886,7 +887,7 @@ impl<'a> Parser<'a> {
             // parameters, and the lifetime parameters must not have bounds.
             Ok(params)
         } else {
-            Ok(Vec::new())
+            Ok(ThinVec::new())
         }
     }
 
diff --git a/compiler/rustc_resolve/Cargo.toml b/compiler/rustc_resolve/Cargo.toml
index 5d2b606b43cd5..2f37fe784de1e 100644
--- a/compiler/rustc_resolve/Cargo.toml
+++ b/compiler/rustc_resolve/Cargo.toml
@@ -8,10 +8,8 @@ doctest = false
 
 [dependencies]
 bitflags = "1.2.1"
-tracing = "0.1"
-rustc_ast = { path = "../rustc_ast" }
 rustc_arena = { path = "../rustc_arena" }
-rustc_middle = { path = "../rustc_middle" }
+rustc_ast = { path = "../rustc_ast" }
 rustc_ast_pretty = { path = "../rustc_ast_pretty" }
 rustc_attr = { path = "../rustc_attr" }
 rustc_data_structures = { path = "../rustc_data_structures" }
@@ -21,7 +19,10 @@ rustc_feature = { path = "../rustc_feature" }
 rustc_hir = { path = "../rustc_hir" }
 rustc_index = { path = "../rustc_index" }
 rustc_metadata = { path = "../rustc_metadata" }
+rustc_middle = { path = "../rustc_middle" }
 rustc_query_system = { path = "../rustc_query_system" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
+thin-vec = "0.2.8"
+tracing = "0.1"
diff --git a/compiler/rustc_resolve/src/diagnostics.rs b/compiler/rustc_resolve/src/diagnostics.rs
index ab71fa0bc1d4d..e186fac39a6b7 100644
--- a/compiler/rustc_resolve/src/diagnostics.rs
+++ b/compiler/rustc_resolve/src/diagnostics.rs
@@ -25,6 +25,7 @@ use rustc_span::lev_distance::find_best_match_for_name;
 use rustc_span::source_map::SourceMap;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{BytePos, Span};
+use thin_vec::ThinVec;
 
 use crate::imports::{Import, ImportKind, ImportResolver};
 use crate::late::{PatternSource, Rib};
@@ -1254,7 +1255,7 @@ impl<'a> Resolver<'a> {
     {
         let mut candidates = Vec::new();
         let mut seen_modules = FxHashSet::default();
-        let mut worklist = vec![(start_module, Vec::<ast::PathSegment>::new(), true)];
+        let mut worklist = vec![(start_module, ThinVec::<ast::PathSegment>::new(), true)];
         let mut worklist_via_import = vec![];
 
         while let Some((in_module, path_segments, accessible)) = match worklist.pop() {
diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
index 99d13acbae1b9..5712d98f65669 100644
--- a/compiler/rustc_resolve/src/late/diagnostics.rs
+++ b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -33,6 +33,8 @@ use rustc_span::{BytePos, Span};
 use std::iter;
 use std::ops::Deref;
 
+use thin_vec::ThinVec;
+
 type Res = def::Res<ast::NodeId>;
 
 /// A field or associated item from self type suggested in case of resolution failure.
@@ -72,7 +74,7 @@ fn import_candidate_to_enum_paths(suggestion: &ImportSuggestion) -> (String, Str
     let path_len = suggestion.path.segments.len();
     let enum_path = ast::Path {
         span: suggestion.path.span,
-        segments: suggestion.path.segments[0..path_len - 1].to_vec(),
+        segments: suggestion.path.segments[0..path_len - 1].iter().cloned().collect(),
         tokens: None,
     };
     let enum_path_string = path_names_to_string(&enum_path);
@@ -1655,7 +1657,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
     fn find_module(&mut self, def_id: DefId) -> Option<(Module<'a>, ImportSuggestion)> {
         let mut result = None;
         let mut seen_modules = FxHashSet::default();
-        let mut worklist = vec![(self.r.graph_root, Vec::new())];
+        let mut worklist = vec![(self.r.graph_root, ThinVec::new())];
 
         while let Some((in_module, path_segments)) = worklist.pop() {
             // abort if the module is already found
diff --git a/src/test/ui-fulldeps/pprust-expr-roundtrip.rs b/src/test/ui-fulldeps/pprust-expr-roundtrip.rs
index 117b798710cf0..270c48d52fb95 100644
--- a/src/test/ui-fulldeps/pprust-expr-roundtrip.rs
+++ b/src/test/ui-fulldeps/pprust-expr-roundtrip.rs
@@ -25,6 +25,7 @@ extern crate rustc_data_structures;
 extern crate rustc_parse;
 extern crate rustc_session;
 extern crate rustc_span;
+extern crate thin_vec;
 
 use rustc_ast::mut_visit::{self, visit_clobber, MutVisitor};
 use rustc_ast::ptr::P;
@@ -35,6 +36,7 @@ use rustc_session::parse::ParseSess;
 use rustc_span::source_map::FilePathMapping;
 use rustc_span::source_map::{FileName, Spanned, DUMMY_SP};
 use rustc_span::symbol::Ident;
+use thin_vec::thin_vec;
 
 fn parse_expr(ps: &ParseSess, src: &str) -> Option<P<Expr>> {
     let src_as_string = src.to_string();
@@ -51,7 +53,7 @@ fn expr(kind: ExprKind) -> P<Expr> {
 
 fn make_x() -> P<Expr> {
     let seg = PathSegment::from_ident(Ident::from_str("x"));
-    let path = Path { segments: vec![seg], span: DUMMY_SP, tokens: None };
+    let path = Path { segments: thin_vec![seg], span: DUMMY_SP, tokens: None };
     expr(ExprKind::Path(None, path))
 }
 
diff --git a/src/test/ui/stats/hir-stats.stderr b/src/test/ui/stats/hir-stats.stderr
index 0a736f7be834e..fb7e5b42de879 100644
--- a/src/test/ui/stats/hir-stats.stderr
+++ b/src/test/ui/stats/hir-stats.stderr
@@ -2,6 +2,8 @@ ast-stats-1 PRE EXPANSION AST STATS
 ast-stats-1 Name                Accumulated Size         Count     Item Size
 ast-stats-1 ----------------------------------------------------------------
 ast-stats-1 ExprField                 48 ( 0.6%)             1            48
+ast-stats-1 WherePredicate            56 ( 0.7%)             1            56
+ast-stats-1 - BoundPredicate            56 ( 0.7%)             1
 ast-stats-1 Crate                     56 ( 0.7%)             1            56
 ast-stats-1 Attribute                 64 ( 0.8%)             2            32
 ast-stats-1 - Normal                    32 ( 0.4%)             1
@@ -9,111 +11,109 @@ ast-stats-1 - DocComment                32 ( 0.4%)             1
 ast-stats-1 GenericArgs               64 ( 0.8%)             1            64
 ast-stats-1 - AngleBracketed            64 ( 0.8%)             1
 ast-stats-1 Local                     72 ( 0.9%)             1            72
-ast-stats-1 WherePredicate            72 ( 0.9%)             1            72
-ast-stats-1 - BoundPredicate            72 ( 0.9%)             1
-ast-stats-1 Arm                       96 ( 1.1%)             2            48
-ast-stats-1 ForeignItem               96 ( 1.1%)             1            96
-ast-stats-1 - Fn                        96 ( 1.1%)             1
-ast-stats-1 FieldDef                 160 ( 1.9%)             2            80
-ast-stats-1 Stmt                     160 ( 1.9%)             5            32
+ast-stats-1 Arm                       96 ( 1.2%)             2            48
+ast-stats-1 ForeignItem               96 ( 1.2%)             1            96
+ast-stats-1 - Fn                        96 ( 1.2%)             1
+ast-stats-1 FieldDef                 160 ( 2.1%)             2            80
+ast-stats-1 Stmt                     160 ( 2.1%)             5            32
 ast-stats-1 - Local                     32 ( 0.4%)             1
 ast-stats-1 - MacCall                   32 ( 0.4%)             1
-ast-stats-1 - Expr                      96 ( 1.1%)             3
-ast-stats-1 Param                    160 ( 1.9%)             4            40
-ast-stats-1 FnDecl                   200 ( 2.4%)             5            40
-ast-stats-1 Variant                  240 ( 2.8%)             2           120
-ast-stats-1 Block                    288 ( 3.4%)             6            48
-ast-stats-1 GenericBound             352 ( 4.2%)             4            88
-ast-stats-1 - Trait                    352 ( 4.2%)             4
-ast-stats-1 AssocItem                416 ( 4.9%)             4           104
-ast-stats-1 - TyAlias                  208 ( 2.5%)             2
-ast-stats-1 - Fn                       208 ( 2.5%)             2
-ast-stats-1 GenericParam             520 ( 6.1%)             5           104
-ast-stats-1 PathSegment              720 ( 8.5%)            30            24
-ast-stats-1 Expr                     832 ( 9.8%)             8           104
-ast-stats-1 - Path                     104 ( 1.2%)             1
-ast-stats-1 - Match                    104 ( 1.2%)             1
-ast-stats-1 - Struct                   104 ( 1.2%)             1
-ast-stats-1 - Lit                      208 ( 2.5%)             2
-ast-stats-1 - Block                    312 ( 3.7%)             3
-ast-stats-1 Pat                      840 ( 9.9%)             7           120
-ast-stats-1 - Struct                   120 ( 1.4%)             1
-ast-stats-1 - Wild                     120 ( 1.4%)             1
-ast-stats-1 - Ident                    600 ( 7.1%)             5
-ast-stats-1 Ty                     1_344 (15.9%)            14            96
-ast-stats-1 - Rptr                      96 ( 1.1%)             1
-ast-stats-1 - Ptr                       96 ( 1.1%)             1
-ast-stats-1 - ImplicitSelf             192 ( 2.3%)             2
-ast-stats-1 - Path                     960 (11.4%)            10
-ast-stats-1 Item                   1_656 (19.6%)             9           184
-ast-stats-1 - Trait                    184 ( 2.2%)             1
-ast-stats-1 - Enum                     184 ( 2.2%)             1
-ast-stats-1 - ForeignMod               184 ( 2.2%)             1
-ast-stats-1 - Impl                     184 ( 2.2%)             1
-ast-stats-1 - Fn                       368 ( 4.4%)             2
-ast-stats-1 - Use                      552 ( 6.5%)             3
+ast-stats-1 - Expr                      96 ( 1.2%)             3
+ast-stats-1 Param                    160 ( 2.1%)             4            40
+ast-stats-1 FnDecl                   200 ( 2.6%)             5            40
+ast-stats-1 GenericBound             224 ( 2.9%)             4            56
+ast-stats-1 - Trait                    224 ( 2.9%)             4
+ast-stats-1 Variant                  240 ( 3.1%)             2           120
+ast-stats-1 Block                    288 ( 3.7%)             6            48
+ast-stats-1 AssocItem                416 ( 5.4%)             4           104
+ast-stats-1 - TyAlias                  208 ( 2.7%)             2
+ast-stats-1 - Fn                       208 ( 2.7%)             2
+ast-stats-1 GenericParam             520 ( 6.8%)             5           104
+ast-stats-1 PathSegment              720 ( 9.4%)            30            24
+ast-stats-1 Pat                      728 ( 9.5%)             7           104
+ast-stats-1 - Struct                   104 ( 1.4%)             1
+ast-stats-1 - Wild                     104 ( 1.4%)             1
+ast-stats-1 - Ident                    520 ( 6.8%)             5
+ast-stats-1 Expr                     832 (10.8%)             8           104
+ast-stats-1 - Path                     104 ( 1.4%)             1
+ast-stats-1 - Match                    104 ( 1.4%)             1
+ast-stats-1 - Struct                   104 ( 1.4%)             1
+ast-stats-1 - Lit                      208 ( 2.7%)             2
+ast-stats-1 - Block                    312 ( 4.1%)             3
+ast-stats-1 Ty                     1_120 (14.6%)            14            80
+ast-stats-1 - Rptr                      80 ( 1.0%)             1
+ast-stats-1 - Ptr                       80 ( 1.0%)             1
+ast-stats-1 - ImplicitSelf             160 ( 2.1%)             2
+ast-stats-1 - Path                     800 (10.4%)            10
+ast-stats-1 Item                   1_368 (17.8%)             9           152
+ast-stats-1 - Trait                    152 ( 2.0%)             1
+ast-stats-1 - Enum                     152 ( 2.0%)             1
+ast-stats-1 - ForeignMod               152 ( 2.0%)             1
+ast-stats-1 - Impl                     152 ( 2.0%)             1
+ast-stats-1 - Fn                       304 ( 4.0%)             2
+ast-stats-1 - Use                      456 ( 5.9%)             3
 ast-stats-1 ----------------------------------------------------------------
-ast-stats-1 Total                  8_456
+ast-stats-1 Total                  7_688
 ast-stats-1
 ast-stats-2 POST EXPANSION AST STATS
 ast-stats-2 Name                Accumulated Size         Count     Item Size
 ast-stats-2 ----------------------------------------------------------------
-ast-stats-2 ExprField                 48 ( 0.5%)             1            48
-ast-stats-2 Crate                     56 ( 0.6%)             1            56
-ast-stats-2 GenericArgs               64 ( 0.7%)             1            64
-ast-stats-2 - AngleBracketed            64 ( 0.7%)             1
-ast-stats-2 Local                     72 ( 0.8%)             1            72
-ast-stats-2 WherePredicate            72 ( 0.8%)             1            72
-ast-stats-2 - BoundPredicate            72 ( 0.8%)             1
-ast-stats-2 Arm                       96 ( 1.0%)             2            48
-ast-stats-2 ForeignItem               96 ( 1.0%)             1            96
-ast-stats-2 - Fn                        96 ( 1.0%)             1
-ast-stats-2 InlineAsm                120 ( 1.3%)             1           120
-ast-stats-2 Attribute                128 ( 1.4%)             4            32
-ast-stats-2 - DocComment                32 ( 0.3%)             1
-ast-stats-2 - Normal                    96 ( 1.0%)             3
-ast-stats-2 FieldDef                 160 ( 1.7%)             2            80
-ast-stats-2 Stmt                     160 ( 1.7%)             5            32
-ast-stats-2 - Local                     32 ( 0.3%)             1
-ast-stats-2 - Semi                      32 ( 0.3%)             1
-ast-stats-2 - Expr                      96 ( 1.0%)             3
-ast-stats-2 Param                    160 ( 1.7%)             4            40
-ast-stats-2 FnDecl                   200 ( 2.2%)             5            40
-ast-stats-2 Variant                  240 ( 2.6%)             2           120
-ast-stats-2 Block                    288 ( 3.1%)             6            48
-ast-stats-2 GenericBound             352 ( 3.8%)             4            88
-ast-stats-2 - Trait                    352 ( 3.8%)             4
-ast-stats-2 AssocItem                416 ( 4.5%)             4           104
-ast-stats-2 - TyAlias                  208 ( 2.3%)             2
-ast-stats-2 - Fn                       208 ( 2.3%)             2
-ast-stats-2 GenericParam             520 ( 5.7%)             5           104
-ast-stats-2 PathSegment              792 ( 8.6%)            33            24
-ast-stats-2 Pat                      840 ( 9.1%)             7           120
-ast-stats-2 - Struct                   120 ( 1.3%)             1
-ast-stats-2 - Wild                     120 ( 1.3%)             1
-ast-stats-2 - Ident                    600 ( 6.5%)             5
-ast-stats-2 Expr                     936 (10.2%)             9           104
-ast-stats-2 - Path                     104 ( 1.1%)             1
-ast-stats-2 - Match                    104 ( 1.1%)             1
-ast-stats-2 - Struct                   104 ( 1.1%)             1
-ast-stats-2 - InlineAsm                104 ( 1.1%)             1
-ast-stats-2 - Lit                      208 ( 2.3%)             2
-ast-stats-2 - Block                    312 ( 3.4%)             3
-ast-stats-2 Ty                     1_344 (14.6%)            14            96
-ast-stats-2 - Rptr                      96 ( 1.0%)             1
-ast-stats-2 - Ptr                       96 ( 1.0%)             1
-ast-stats-2 - ImplicitSelf             192 ( 2.1%)             2
-ast-stats-2 - Path                     960 (10.5%)            10
-ast-stats-2 Item                   2_024 (22.0%)            11           184
-ast-stats-2 - Trait                    184 ( 2.0%)             1
-ast-stats-2 - Enum                     184 ( 2.0%)             1
-ast-stats-2 - ExternCrate              184 ( 2.0%)             1
-ast-stats-2 - ForeignMod               184 ( 2.0%)             1
-ast-stats-2 - Impl                     184 ( 2.0%)             1
-ast-stats-2 - Fn                       368 ( 4.0%)             2
-ast-stats-2 - Use                      736 ( 8.0%)             4
+ast-stats-2 ExprField                 48 ( 0.6%)             1            48
+ast-stats-2 WherePredicate            56 ( 0.7%)             1            56
+ast-stats-2 - BoundPredicate            56 ( 0.7%)             1
+ast-stats-2 Crate                     56 ( 0.7%)             1            56
+ast-stats-2 GenericArgs               64 ( 0.8%)             1            64
+ast-stats-2 - AngleBracketed            64 ( 0.8%)             1
+ast-stats-2 Local                     72 ( 0.9%)             1            72
+ast-stats-2 Arm                       96 ( 1.1%)             2            48
+ast-stats-2 ForeignItem               96 ( 1.1%)             1            96
+ast-stats-2 - Fn                        96 ( 1.1%)             1
+ast-stats-2 InlineAsm                120 ( 1.4%)             1           120
+ast-stats-2 Attribute                128 ( 1.5%)             4            32
+ast-stats-2 - DocComment                32 ( 0.4%)             1
+ast-stats-2 - Normal                    96 ( 1.1%)             3
+ast-stats-2 FieldDef                 160 ( 1.9%)             2            80
+ast-stats-2 Stmt                     160 ( 1.9%)             5            32
+ast-stats-2 - Local                     32 ( 0.4%)             1
+ast-stats-2 - Semi                      32 ( 0.4%)             1
+ast-stats-2 - Expr                      96 ( 1.1%)             3
+ast-stats-2 Param                    160 ( 1.9%)             4            40
+ast-stats-2 FnDecl                   200 ( 2.4%)             5            40
+ast-stats-2 GenericBound             224 ( 2.7%)             4            56
+ast-stats-2 - Trait                    224 ( 2.7%)             4
+ast-stats-2 Variant                  240 ( 2.9%)             2           120
+ast-stats-2 Block                    288 ( 3.4%)             6            48
+ast-stats-2 AssocItem                416 ( 5.0%)             4           104
+ast-stats-2 - TyAlias                  208 ( 2.5%)             2
+ast-stats-2 - Fn                       208 ( 2.5%)             2
+ast-stats-2 GenericParam             520 ( 6.2%)             5           104
+ast-stats-2 Pat                      728 ( 8.7%)             7           104
+ast-stats-2 - Struct                   104 ( 1.2%)             1
+ast-stats-2 - Wild                     104 ( 1.2%)             1
+ast-stats-2 - Ident                    520 ( 6.2%)             5
+ast-stats-2 PathSegment              792 ( 9.5%)            33            24
+ast-stats-2 Expr                     936 (11.2%)             9           104
+ast-stats-2 - Path                     104 ( 1.2%)             1
+ast-stats-2 - Match                    104 ( 1.2%)             1
+ast-stats-2 - Struct                   104 ( 1.2%)             1
+ast-stats-2 - InlineAsm                104 ( 1.2%)             1
+ast-stats-2 - Lit                      208 ( 2.5%)             2
+ast-stats-2 - Block                    312 ( 3.7%)             3
+ast-stats-2 Ty                     1_120 (13.4%)            14            80
+ast-stats-2 - Rptr                      80 ( 1.0%)             1
+ast-stats-2 - Ptr                       80 ( 1.0%)             1
+ast-stats-2 - ImplicitSelf             160 ( 1.9%)             2
+ast-stats-2 - Path                     800 ( 9.6%)            10
+ast-stats-2 Item                   1_672 (20.0%)            11           152
+ast-stats-2 - Trait                    152 ( 1.8%)             1
+ast-stats-2 - Enum                     152 ( 1.8%)             1
+ast-stats-2 - ExternCrate              152 ( 1.8%)             1
+ast-stats-2 - ForeignMod               152 ( 1.8%)             1
+ast-stats-2 - Impl                     152 ( 1.8%)             1
+ast-stats-2 - Fn                       304 ( 3.6%)             2
+ast-stats-2 - Use                      608 ( 7.3%)             4
 ast-stats-2 ----------------------------------------------------------------
-ast-stats-2 Total                  9_184
+ast-stats-2 Total                  8_352
 ast-stats-2
 hir-stats HIR STATS
 hir-stats Name                Accumulated Size         Count     Item Size