Replace `rustc_data_structures::thin_vec::ThinVec` with `thin_vec::ThinVec`.

`rustc_data_structures::thin_vec::ThinVec` looks like this:
```
pub struct ThinVec<T>(Option<Box<Vec<T>>>);
```
It's just a zero word if the vector is empty, but requires two
allocations if it is non-empty. So it's only usable in cases where the
vector is empty most of the time.

This commit removes it in favour of `thin_vec::ThinVec`, which is also
word-sized, but stores the length and capacity in the same allocation as
the elements. It's good in a wider variety of situation, e.g. in enum
variants where the vector is usually/always non-empty.

The commit also:
- Sorts some `Cargo.toml` dependency lists, to make additions easier.
- Sorts some `use` item lists, to make additions easier.
- Changes `clean_trait_ref_with_bindings` to take a
  `ThinVec<TypeBinding>` rather than a `&[TypeBinding]`, because this
  avoid some unnecessary allocations.

42 files changed, 164 insertions, 356 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 100e9095703..f7d66e1c465 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3207,6 +3207,7 @@ dependencies = [
  "rustc_serialize",
  "rustc_span",
  "smallvec",
+ "thin-vec",
  "tracing",
 ]
 
@@ -3228,6 +3229,7 @@ dependencies = [
  "rustc_span",
  "rustc_target",
  "smallvec",
+ "thin-vec",
  "tracing",
 ]
 
@@ -3322,6 +3324,7 @@ dependencies = [
  "rustc_span",
  "rustc_target",
  "smallvec",
+ "thin-vec",
  "tracing",
 ]
 
@@ -3446,6 +3449,7 @@ dependencies = [
  "stable_deref_trait",
  "stacker",
  "tempfile",
+ "thin-vec",
  "tracing",
  "winapi",
 ]
@@ -3827,6 +3831,7 @@ dependencies = [
  "rustc_target",
  "rustc_type_ir",
  "smallvec",
+ "thin-vec",
  "tracing",
 ]
 
@@ -4014,6 +4019,7 @@ dependencies = [
  "rustc_session",
  "rustc_span",
  "rustc_target",
+ "thin-vec",
  "tracing",
 ]
 
@@ -4036,6 +4042,7 @@ dependencies = [
  "rustc_span",
  "rustc_target",
  "smallvec",
+ "thin-vec",
  "tracing",
 ]
 
@@ -4091,6 +4098,7 @@ dependencies = [
  "indexmap",
  "rustc_macros",
  "smallvec",
+ "thin-vec",
 ]
 
 [[package]]
@@ -4325,6 +4333,7 @@ dependencies = [
  "serde_json",
  "smallvec",
  "tempfile",
+ "thin-vec",
  "tracing",
  "tracing-subscriber",
  "tracing-tree",
@@ -4879,6 +4888,12 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b1141d4d61095b28419e22cb0bbf02755f5e54e0526f97f1e3d1d160e60885fb"
 
 [[package]]
+name = "thin-vec"
+version = "0.2.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "104c2cb3180b6fb6d5b2278768e9b88b578d32ba751ea6e8d026688a40d7ed87"
+
+[[package]]
 name = "thiserror"
 version = "1.0.30"
 source = "registry+https://github.com/rust-lang/crates.io-index"
diff --git a/compiler/rustc_ast/Cargo.toml b/compiler/rustc_ast/Cargo.toml
index 9822e9864e2..c24180bacfc 100644
--- a/compiler/rustc_ast/Cargo.toml
+++ b/compiler/rustc_ast/Cargo.toml
@@ -7,12 +7,13 @@ edition = "2021"
 doctest = false
 
 [dependencies]
-rustc_serialize = { path = "../rustc_serialize" }
-tracing = "0.1"
-rustc_span = { path = "../rustc_span" }
+bitflags = "1.2.1"
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_index = { path = "../rustc_index" }
 rustc_lexer = { path = "../rustc_lexer" }
 rustc_macros = { path = "../rustc_macros" }
+rustc_serialize = { path = "../rustc_serialize" }
+rustc_span = { path = "../rustc_span" }
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
-bitflags = "1.2.1"
+thin-vec = "0.2.8"
+tracing = "0.1"
diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index fb521073a42..3af6dee9e43 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -25,21 +25,19 @@ pub use UnsafeSource::*;
 use crate::ptr::P;
 use crate::token::{self, CommentKind, Delimiter};
 use crate::tokenstream::{DelimSpan, LazyTokenStream, TokenStream};
-
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_data_structures::sync::Lrc;
-use rustc_data_structures::thin_vec::ThinVec;
 use rustc_macros::HashStable_Generic;
 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 use rustc_span::source_map::{respan, Spanned};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{Span, DUMMY_SP};
-
 use std::cmp::Ordering;
 use std::convert::TryFrom;
 use std::fmt;
 use std::mem;
+use thin_vec::ThinVec;
 
 /// A "Label" is an identifier of some point in sources,
 /// e.g. in the following code:
diff --git a/compiler/rustc_ast/src/tokenstream.rs b/compiler/rustc_ast/src/tokenstream.rs
index 9e4a22e1fa3..3964e615d66 100644
--- a/compiler/rustc_ast/src/tokenstream.rs
+++ b/compiler/rustc_ast/src/tokenstream.rs
@@ -426,7 +426,8 @@ impl TokenStream {
         let attr_annotated = if attrs.is_empty() {
             tokens.create_token_stream()
         } else {
-            let attr_data = AttributesData { attrs: attrs.to_vec().into(), tokens: tokens.clone() };
+            let attr_data =
+                AttributesData { attrs: attrs.iter().cloned().collect(), tokens: tokens.clone() };
             AttrAnnotatedTokenStream::new(vec![(
                 AttrAnnotatedTokenTree::Attributes(attr_data),
                 Spacing::Alone,
diff --git a/compiler/rustc_ast_lowering/Cargo.toml b/compiler/rustc_ast_lowering/Cargo.toml
index 474aff2e2aa..ce1c8d4997d 100644
--- a/compiler/rustc_ast_lowering/Cargo.toml
+++ b/compiler/rustc_ast_lowering/Cargo.toml
@@ -8,17 +8,18 @@ doctest = false
 
 [dependencies]
 rustc_arena = { path = "../rustc_arena" }
-tracing = "0.1"
+rustc_ast = { path = "../rustc_ast" }
 rustc_ast_pretty = { path = "../rustc_ast_pretty" }
-rustc_hir = { path = "../rustc_hir" }
-rustc_target = { path = "../rustc_target" }
 rustc_data_structures = { path = "../rustc_data_structures" }
+rustc_errors = { path = "../rustc_errors" }
+rustc_hir = { path = "../rustc_hir" }
 rustc_index = { path = "../rustc_index" }
 rustc_middle = { path = "../rustc_middle" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_query_system = { path = "../rustc_query_system" }
-rustc_span = { path = "../rustc_span" }
-rustc_errors = { path = "../rustc_errors" }
 rustc_session = { path = "../rustc_session" }
-rustc_ast = { path = "../rustc_ast" }
+rustc_span = { path = "../rustc_span" }
+rustc_target = { path = "../rustc_target" }
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
+thin-vec = "0.2.8"
+tracing = "0.1"
diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index 61f8c0216f1..e4a36f4d9ff 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -7,7 +7,6 @@ use super::errors::{
 use super::ResolverAstLoweringExt;
 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
 use crate::{FnDeclKind, ImplTraitPosition};
-
 use rustc_ast::attr;
 use rustc_ast::ptr::P as AstP;
 use rustc_ast::*;
@@ -18,6 +17,7 @@ use rustc_hir::definitions::DefPathData;
 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::DUMMY_SP;
+use thin_vec::thin_vec;
 
 impl<'hir> LoweringContext<'_, 'hir> {
     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
@@ -1535,7 +1535,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             };
             attr::mk_attr_outer(allow)
         };
-        let attrs: AttrVec = vec![attr].into();
+        let attrs: AttrVec = thin_vec![attr];
 
         // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
         let continue_arm = {
diff --git a/compiler/rustc_builtin_macros/Cargo.toml b/compiler/rustc_builtin_macros/Cargo.toml
index 8d8e9d9b5ff..6469d0d7b88 100644
--- a/compiler/rustc_builtin_macros/Cargo.toml
+++ b/compiler/rustc_builtin_macros/Cargo.toml
@@ -7,20 +7,21 @@ edition = "2021"
 doctest = false
 
 [dependencies]
-rustc_parse_format = { path = "../rustc_parse_format" }
-tracing = "0.1"
+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_expand = { path = "../rustc_expand" }
 rustc_feature = { path = "../rustc_feature" }
 rustc_lexer = { path = "../rustc_lexer" }
 rustc_lint_defs = { path = "../rustc_lint_defs" }
 rustc_macros = { path = "../rustc_macros" }
+rustc_parse_format = { path = "../rustc_parse_format" }
 rustc_parse = { path = "../rustc_parse" }
-rustc_target = { path = "../rustc_target" }
 rustc_session = { path = "../rustc_session" }
-smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
-rustc_ast = { path = "../rustc_ast" }
-rustc_expand = { path = "../rustc_expand" }
 rustc_span = { path = "../rustc_span" }
+rustc_target = { path = "../rustc_target" }
+smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
+thin-vec = "0.2.8"
+tracing = "0.1"
diff --git a/compiler/rustc_builtin_macros/src/assert/context.rs b/compiler/rustc_builtin_macros/src/assert/context.rs
index d2ee4249989..973a8cb85c2 100644
--- a/compiler/rustc_builtin_macros/src/assert/context.rs
+++ b/compiler/rustc_builtin_macros/src/assert/context.rs
@@ -13,6 +13,7 @@ use rustc_span::{
     symbol::{sym, Ident, Symbol},
     Span,
 };
+use thin_vec::thin_vec;
 
 pub(super) struct Context<'cx, 'a> {
     // An optimization.
@@ -116,11 +117,10 @@ impl<'cx, 'a> Context<'cx, 'a> {
             self.cx.item(
                 self.span,
                 Ident::empty(),
-                vec![self.cx.attribute(attr::mk_list_item(
+                thin_vec![self.cx.attribute(attr::mk_list_item(
                     Ident::new(sym::allow, self.span),
                     vec![attr::mk_nested_word_item(Ident::new(sym::unused_imports, self.span))],
-                ))]
-                .into(),
+                ))],
                 ItemKind::Use(UseTree {
                     prefix: self.cx.path(self.span, self.cx.std_path(&[sym::asserting])),
                     kind: UseTreeKind::Nested(vec![
diff --git a/compiler/rustc_builtin_macros/src/deriving/clone.rs b/compiler/rustc_builtin_macros/src/deriving/clone.rs
index dd7989cf48c..c7f2d95e72f 100644
--- a/compiler/rustc_builtin_macros/src/deriving/clone.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/clone.rs
@@ -1,12 +1,12 @@
 use crate::deriving::generic::ty::*;
 use crate::deriving::generic::*;
 use crate::deriving::path_std;
-
 use rustc_ast::{self as ast, Generics, ItemKind, MetaItem, VariantData};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::Span;
+use thin_vec::thin_vec;
 
 pub fn expand_deriving_clone(
     cx: &mut ExtCtxt<'_>,
@@ -68,7 +68,7 @@ pub fn expand_deriving_clone(
     }
 
     let inline = cx.meta_word(span, sym::inline);
-    let attrs = vec![cx.attribute(inline)].into();
+    let attrs = thin_vec![cx.attribute(inline)];
     let trait_def = TraitDef {
         span,
         path: path_std!(clone::Clone),
diff --git a/compiler/rustc_builtin_macros/src/deriving/cmp/eq.rs b/compiler/rustc_builtin_macros/src/deriving/cmp/eq.rs
index 9b6d3e5032f..5b556c5c9b9 100644
--- a/compiler/rustc_builtin_macros/src/deriving/cmp/eq.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/cmp/eq.rs
@@ -7,6 +7,7 @@ use rustc_data_structures::fx::FxHashSet;
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
+use thin_vec::thin_vec;
 
 pub fn expand_deriving_eq(
     cx: &mut ExtCtxt<'_>,
@@ -20,7 +21,7 @@ pub fn expand_deriving_eq(
     let hidden = rustc_ast::attr::mk_nested_word_item(Ident::new(sym::hidden, span));
     let doc = rustc_ast::attr::mk_list_item(Ident::new(sym::doc, span), vec![hidden]);
     let no_coverage = cx.meta_word(span, sym::no_coverage);
-    let attrs = vec![cx.attribute(inline), cx.attribute(doc), cx.attribute(no_coverage)].into();
+    let attrs = thin_vec![cx.attribute(inline), cx.attribute(doc), cx.attribute(no_coverage)];
     let trait_def = TraitDef {
         span,
         path: path_std!(cmp::Eq),
diff --git a/compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs b/compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs
index 0e17b951787..72625869558 100644
--- a/compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs
@@ -1,11 +1,11 @@
 use crate::deriving::generic::ty::*;
 use crate::deriving::generic::*;
 use crate::deriving::path_std;
-
 use rustc_ast::MetaItem;
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
+use thin_vec::thin_vec;
 
 pub fn expand_deriving_ord(
     cx: &mut ExtCtxt<'_>,
@@ -15,7 +15,7 @@ pub fn expand_deriving_ord(
     push: &mut dyn FnMut(Annotatable),
 ) {
     let inline = cx.meta_word(span, sym::inline);
-    let attrs = vec![cx.attribute(inline)].into();
+    let attrs = thin_vec![cx.attribute(inline)];
     let trait_def = TraitDef {
         span,
         path: path_std!(cmp::Ord),
diff --git a/compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs b/compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs
index ac1325b92a6..42ee65b570a 100644
--- a/compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs
@@ -1,12 +1,12 @@
 use crate::deriving::generic::ty::*;
 use crate::deriving::generic::*;
 use crate::deriving::{path_local, path_std};
-
 use rustc_ast::ptr::P;
 use rustc_ast::{BinOpKind, BorrowKind, Expr, ExprKind, MetaItem, Mutability};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::sym;
 use rustc_span::Span;
+use thin_vec::thin_vec;
 
 pub fn expand_deriving_partial_eq(
     cx: &mut ExtCtxt<'_>,
@@ -68,7 +68,7 @@ pub fn expand_deriving_partial_eq(
     // No need to generate `ne`, the default suffices, and not generating it is
     // faster.
     let inline = cx.meta_word(span, sym::inline);
-    let attrs = vec![cx.attribute(inline)].into();
+    let attrs = thin_vec![cx.attribute(inline)];
     let methods = vec![MethodDef {
         name: sym::eq,
         generics: Bounds::empty(),
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 7763e554017..516892aeda9 100644
--- a/compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs
@@ -1,11 +1,11 @@
 use crate::deriving::generic::ty::*;
 use crate::deriving::generic::*;
 use crate::deriving::{path_std, pathvec_std};
-
 use rustc_ast::MetaItem;
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
+use thin_vec::thin_vec;
 
 pub fn expand_deriving_partial_ord(
     cx: &mut ExtCtxt<'_>,
@@ -19,7 +19,7 @@ pub fn expand_deriving_partial_ord(
         Path(Path::new_(pathvec_std!(option::Option), vec![Box::new(ordering_ty)], PathKind::Std));
 
     let inline = cx.meta_word(span, sym::inline);
-    let attrs = vec![cx.attribute(inline)].into();
+    let attrs = thin_vec![cx.attribute(inline)];
 
     let partial_cmp_def = MethodDef {
         name: sym::partial_cmp,
diff --git a/compiler/rustc_builtin_macros/src/deriving/default.rs b/compiler/rustc_builtin_macros/src/deriving/default.rs
index f316f01ef66..a94c8a996e6 100644
--- a/compiler/rustc_builtin_macros/src/deriving/default.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/default.rs
@@ -1,6 +1,5 @@
 use crate::deriving::generic::ty::*;
 use crate::deriving::generic::*;
-
 use rustc_ast as ast;
 use rustc_ast::{walk_list, EnumDef, VariantData};
 use rustc_errors::Applicability;
@@ -9,6 +8,7 @@ use rustc_span::symbol::Ident;
 use rustc_span::symbol::{kw, sym};
 use rustc_span::Span;
 use smallvec::SmallVec;
+use thin_vec::thin_vec;
 
 pub fn expand_deriving_default(
     cx: &mut ExtCtxt<'_>,
@@ -20,7 +20,7 @@ pub fn expand_deriving_default(
     item.visit_with(&mut DetectNonVariantDefaultAttr { cx });
 
     let inline = cx.meta_word(span, sym::inline);
-    let attrs = vec![cx.attribute(inline)].into();
+    let attrs = thin_vec![cx.attribute(inline)];
     let trait_def = TraitDef {
         span,
         path: Path::new(vec![kw::Default, sym::Default]),
diff --git a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
index c1bbc601560..a7917bf2ead 100644
--- a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
@@ -162,10 +162,7 @@
 pub use StaticFields::*;
 pub use SubstructureFields::*;
 
-use std::cell::RefCell;
-use std::iter;
-use std::vec;
-
+use crate::deriving;
 use rustc_ast::ptr::P;
 use rustc_ast::{self as ast, EnumDef, Expr, Generics, PatKind};
 use rustc_ast::{GenericArg, GenericParamKind, VariantData};
@@ -173,11 +170,12 @@ use rustc_attr as attr;
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::Span;
-
+use std::cell::RefCell;
+use std::iter;
+use std::vec;
+use thin_vec::thin_vec;
 use ty::{Bounds, Path, Ref, Self_, Ty};
 
-use crate::deriving;
-
 pub mod ty;
 
 pub struct TraitDef<'a> {
@@ -716,7 +714,7 @@ impl<'a> TraitDef<'a> {
         let self_type = cx.ty_path(path);
 
         let attr = cx.attribute(cx.meta_word(self.span, sym::automatically_derived));
-        let attrs = vec![attr].into();
+        let attrs = thin_vec![attr];
         let opt_trait_ref = Some(trait_ref);
 
         cx.item(
diff --git a/compiler/rustc_builtin_macros/src/global_allocator.rs b/compiler/rustc_builtin_macros/src/global_allocator.rs
index 2bad9bbce66..45b9b8ab6b6 100644
--- a/compiler/rustc_builtin_macros/src/global_allocator.rs
+++ b/compiler/rustc_builtin_macros/src/global_allocator.rs
@@ -9,6 +9,7 @@ use rustc_ast::{Fn, ItemKind, Mutability, Stmt, Ty, TyKind, Unsafe};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::Span;
+use thin_vec::thin_vec;
 
 pub fn expand(
     ecx: &mut ExtCtxt<'_>,
@@ -116,7 +117,7 @@ impl AllocFnFactory<'_, '_> {
     fn attrs(&self) -> AttrVec {
         let special = sym::rustc_std_internal_symbol;
         let special = self.cx.meta_word(self.span, special);
-        vec![self.cx.attribute(special)].into()
+        thin_vec![self.cx.attribute(special)]
     }
 
     fn arg_ty(
diff --git a/compiler/rustc_builtin_macros/src/standard_library_imports.rs b/compiler/rustc_builtin_macros/src/standard_library_imports.rs
index 90ea1e457ba..49ef538f04e 100644
--- a/compiler/rustc_builtin_macros/src/standard_library_imports.rs
+++ b/compiler/rustc_builtin_macros/src/standard_library_imports.rs
@@ -6,6 +6,7 @@ use rustc_span::edition::Edition::*;
 use rustc_span::hygiene::AstPass;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::DUMMY_SP;
+use thin_vec::thin_vec;
 
 pub fn inject(
     mut krate: ast::Crate,
@@ -51,7 +52,7 @@ pub fn inject(
             cx.item(
                 span,
                 ident,
-                vec![cx.attribute(cx.meta_word(span, sym::macro_use))].into(),
+                thin_vec![cx.attribute(cx.meta_word(span, sym::macro_use))],
                 ast::ItemKind::ExternCrate(None),
             ),
         );
@@ -78,7 +79,7 @@ pub fn inject(
     let use_item = cx.item(
         span,
         Ident::empty(),
-        vec![cx.attribute(cx.meta_word(span, sym::prelude_import))].into(),
+        thin_vec![cx.attribute(cx.meta_word(span, sym::prelude_import))],
         ast::ItemKind::Use(ast::UseTree {
             prefix: cx.path(span, import_path),
             kind: ast::UseTreeKind::Glob,
diff --git a/compiler/rustc_builtin_macros/src/test.rs b/compiler/rustc_builtin_macros/src/test.rs
index 03c84f5ec2a..7eee8073366 100644
--- a/compiler/rustc_builtin_macros/src/test.rs
+++ b/compiler/rustc_builtin_macros/src/test.rs
@@ -1,7 +1,6 @@
 /// The expansion from a test function to the appropriate test struct for libtest
 /// Ideally, this code would be in libtest but for efficiency and error messages it lives here.
 use crate::util::{check_builtin_macro_attribute, warn_on_duplicate_attribute};
-
 use rustc_ast as ast;
 use rustc_ast::attr;
 use rustc_ast::ptr::P;
@@ -11,8 +10,8 @@ use rustc_expand::base::*;
 use rustc_session::Session;
 use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::Span;
-
 use std::iter;
+use thin_vec::thin_vec;
 
 // #[test_case] is used by custom test authors to mark tests
 // When building for test, it needs to make the item public and gensym the name
@@ -219,7 +218,7 @@ pub fn expand_test_or_bench(
     let mut test_const = cx.item(
         sp,
         Ident::new(item.ident.name, sp),
-        vec![
+        thin_vec![
             // #[cfg(test)]
             cx.attribute(attr::mk_list_item(
                 Ident::new(sym::cfg, attr_sp),
@@ -227,8 +226,7 @@ pub fn expand_test_or_bench(
             )),
             // #[rustc_test_marker]
             cx.attribute(cx.meta_word(attr_sp, sym::rustc_test_marker)),
-        ]
-        .into(),
+        ],
         // const $ident: test::TestDescAndFn =
         ast::ItemKind::Const(
             ast::Defaultness::Final,
diff --git a/compiler/rustc_builtin_macros/src/test_harness.rs b/compiler/rustc_builtin_macros/src/test_harness.rs
index 093f0f10a38..89b0d2cc9be 100644
--- a/compiler/rustc_builtin_macros/src/test_harness.rs
+++ b/compiler/rustc_builtin_macros/src/test_harness.rs
@@ -14,6 +14,7 @@ use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::{Span, DUMMY_SP};
 use rustc_target::spec::PanicStrategy;
 use smallvec::{smallvec, SmallVec};
+use thin_vec::thin_vec;
 use tracing::debug;
 
 use std::{iter, mem};
@@ -335,7 +336,7 @@ fn mk_main(cx: &mut TestCtxt<'_>) -> P<ast::Item> {
 
     let main = P(ast::Item {
         ident: main_id,
-        attrs: vec![main_attr].into(),
+        attrs: thin_vec![main_attr],
         id: ast::DUMMY_NODE_ID,
         kind: main,
         vis: ast::Visibility { span: sp, kind: ast::VisibilityKind::Public, tokens: None },
diff --git a/compiler/rustc_data_structures/Cargo.toml b/compiler/rustc_data_structures/Cargo.toml
index 5c641f54f68..2d8658db5e6 100644
--- a/compiler/rustc_data_structures/Cargo.toml
+++ b/compiler/rustc_data_structures/Cargo.toml
@@ -8,25 +8,26 @@ doctest = false
 
 [dependencies]
 arrayvec = { version = "0.7", default-features = false }
+bitflags = "1.2.1"
+cfg-if = "0.1.2"
 ena = "0.14"
 indexmap = { version = "1.9.1" }
-tracing = "0.1"
 jobserver_crate = { version = "0.1.13", package = "jobserver" }
-rustc_serialize = { path = "../rustc_serialize" }
-rustc_macros = { path = "../rustc_macros" }
-rustc_graphviz = { path = "../rustc_graphviz" }
-cfg-if = "0.1.2"
-stable_deref_trait = "1.0.0"
-rayon = { version = "0.4.0", package = "rustc-rayon", optional = true }
+libc = "0.2"
+measureme = "10.0.0"
 rayon-core = { version = "0.4.0", package = "rustc-rayon-core", optional = true }
+rayon = { version = "0.4.0", package = "rustc-rayon", optional = true }
+rustc_graphviz = { path = "../rustc_graphviz" }
 rustc-hash = "1.1.0"
-smallvec = { version = "1.8.1", features = ["const_generics", "union", "may_dangle"] }
 rustc_index = { path = "../rustc_index", package = "rustc_index" }
-bitflags = "1.2.1"
-measureme = "10.0.0"
-libc = "0.2"
+rustc_macros = { path = "../rustc_macros" }
+rustc_serialize = { path = "../rustc_serialize" }
+smallvec = { version = "1.8.1", features = ["const_generics", "union", "may_dangle"] }
+stable_deref_trait = "1.0.0"
 stacker = "0.1.14"
 tempfile = "3.2"
+thin-vec = "0.2.8"
+tracing = "0.1"
 
 [dependencies.parking_lot]
 version = "0.11"
diff --git a/compiler/rustc_data_structures/src/lib.rs b/compiler/rustc_data_structures/src/lib.rs
index c8b09cffe01..a7429ed008f 100644
--- a/compiler/rustc_data_structures/src/lib.rs
+++ b/compiler/rustc_data_structures/src/lib.rs
@@ -75,7 +75,6 @@ pub mod profiling;
 pub mod sharded;
 pub mod stack;
 pub mod sync;
-pub mod thin_vec;
 pub mod tiny_list;
 pub mod transitive_relation;
 pub mod vec_linked_list;
diff --git a/compiler/rustc_data_structures/src/map_in_place.rs b/compiler/rustc_data_structures/src/map_in_place.rs
index d912211443a..a0d4b7ade1f 100644
--- a/compiler/rustc_data_structures/src/map_in_place.rs
+++ b/compiler/rustc_data_structures/src/map_in_place.rs
@@ -1,6 +1,6 @@
-use crate::thin_vec::ThinVec;
 use smallvec::{Array, SmallVec};
 use std::ptr;
+use thin_vec::ThinVec;
 
 pub trait MapInPlace<T>: Sized {
     fn map_in_place<F>(&mut self, mut f: F)
diff --git a/compiler/rustc_data_structures/src/thin_vec.rs b/compiler/rustc_data_structures/src/thin_vec.rs
deleted file mode 100644
index fce42e709ab..00000000000
--- a/compiler/rustc_data_structures/src/thin_vec.rs
+++ /dev/null
@@ -1,180 +0,0 @@
-use crate::stable_hasher::{HashStable, StableHasher};
-
-use std::iter::FromIterator;
-
-/// A vector type optimized for cases where this size is usually 0 (cf. `SmallVec`).
-/// The `Option<Box<..>>` wrapping allows us to represent a zero sized vector with `None`,
-/// which uses only a single (null) pointer.
-#[derive(Clone, Encodable, Decodable, Debug, Hash, Eq, PartialEq)]
-pub struct ThinVec<T>(Option<Box<Vec<T>>>);
-
-impl<T> ThinVec<T> {
-    pub fn new() -> Self {
-        ThinVec(None)
-    }
-
-    pub fn iter(&self) -> std::slice::Iter<'_, T> {
-        self.into_iter()
-    }
-
-    pub fn iter_mut(&mut self) -> std::slice::IterMut<'_, T> {
-        self.into_iter()
-    }
-
-    pub fn push(&mut self, item: T) {
-        match *self {
-            ThinVec(Some(ref mut vec)) => vec.push(item),
-            ThinVec(None) => *self = vec![item].into(),
-        }
-    }
-
-    /// Note: if `set_len(0)` is called on a non-empty `ThinVec`, it will
-    /// remain in the `Some` form. This is required for some code sequences
-    /// (such as the one in `flat_map_in_place`) that call `set_len(0)` before
-    /// an operation that might panic, and then call `set_len(n)` again
-    /// afterwards.
-    pub unsafe fn set_len(&mut self, new_len: usize) {
-        match *self {
-            ThinVec(None) => {
-                // A prerequisite of `Vec::set_len` is that `new_len` must be
-                // less than or equal to capacity(). The same applies here.
-                if new_len != 0 {
-                    panic!("unsafe ThinVec::set_len({})", new_len);
-                }
-            }
-            ThinVec(Some(ref mut vec)) => vec.set_len(new_len),
-        }
-    }
-
-    pub fn insert(&mut self, index: usize, value: T) {
-        match *self {
-            ThinVec(None) => {
-                if index == 0 {
-                    *self = vec![value].into();
-                } else {
-                    panic!("invalid ThinVec::insert");
-                }
-            }
-            ThinVec(Some(ref mut vec)) => vec.insert(index, value),
-        }
-    }
-
-    pub fn remove(&mut self, index: usize) -> T {
-        match self {
-            ThinVec(None) => panic!("invalid ThinVec::remove"),
-            ThinVec(Some(vec)) => vec.remove(index),
-        }
-    }
-
-    pub fn as_slice(&self) -> &[T] {
-        match self {
-            ThinVec(None) => &[],
-            ThinVec(Some(vec)) => vec.as_slice(),
-        }
-    }
-}
-
-impl<T> From<Vec<T>> for ThinVec<T> {
-    fn from(vec: Vec<T>) -> Self {
-        if vec.is_empty() { ThinVec(None) } else { ThinVec(Some(Box::new(vec))) }
-    }
-}
-
-impl<T> Into<Vec<T>> for ThinVec<T> {
-    fn into(self) -> Vec<T> {
-        match self {
-            ThinVec(None) => Vec::new(),
-            ThinVec(Some(vec)) => *vec,
-        }
-    }
-}
-
-impl<T> ::std::ops::Deref for ThinVec<T> {
-    type Target = [T];
-    fn deref(&self) -> &[T] {
-        match *self {
-            ThinVec(None) => &[],
-            ThinVec(Some(ref vec)) => vec,
-        }
-    }
-}
-
-impl<T> ::std::ops::DerefMut for ThinVec<T> {
-    fn deref_mut(&mut self) -> &mut [T] {
-        match *self {
-            ThinVec(None) => &mut [],
-            ThinVec(Some(ref mut vec)) => vec,
-        }
-    }
-}
-
-impl<T> FromIterator<T> for ThinVec<T> {
-    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
-        // `Vec::from_iter()` should not allocate if the iterator is empty.
-        let vec: Vec<_> = iter.into_iter().collect();
-        if vec.is_empty() { ThinVec(None) } else { ThinVec(Some(Box::new(vec))) }
-    }
-}
-
-impl<T> IntoIterator for ThinVec<T> {
-    type Item = T;
-    type IntoIter = std::vec::IntoIter<T>;
-
-    fn into_iter(self) -> Self::IntoIter {
-        // This is still performant because `Vec::new()` does not allocate.
-        self.0.map_or_else(Vec::new, |ptr| *ptr).into_iter()
-    }
-}
-
-impl<'a, T> IntoIterator for &'a ThinVec<T> {
-    type Item = &'a T;
-    type IntoIter = std::slice::Iter<'a, T>;
-
-    fn into_iter(self) -> Self::IntoIter {
-        self.as_ref().iter()
-    }
-}
-
-impl<'a, T> IntoIterator for &'a mut ThinVec<T> {
-    type Item = &'a mut T;
-    type IntoIter = std::slice::IterMut<'a, T>;
-
-    fn into_iter(self) -> Self::IntoIter {
-        self.as_mut().iter_mut()
-    }
-}
-
-impl<T> Extend<T> for ThinVec<T> {
-    fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
-        match *self {
-            ThinVec(Some(ref mut vec)) => vec.extend(iter),
-            ThinVec(None) => *self = iter.into_iter().collect::<Vec<_>>().into(),
-        }
-    }
-
-    fn extend_one(&mut self, item: T) {
-        self.push(item)
-    }
-
-    fn extend_reserve(&mut self, additional: usize) {
-        match *self {
-            ThinVec(Some(ref mut vec)) => vec.reserve(additional),
-            ThinVec(None) => *self = Vec::with_capacity(additional).into(),
-        }
-    }
-}
-
-impl<T: HashStable<CTX>, CTX> HashStable<CTX> for ThinVec<T> {
-    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
-        (**self).hash_stable(hcx, hasher)
-    }
-}
-
-impl<T> Default for ThinVec<T> {
-    fn default() -> Self {
-        Self(None)
-    }
-}
-
-#[cfg(test)]
-mod tests;
diff --git a/compiler/rustc_data_structures/src/thin_vec/tests.rs b/compiler/rustc_data_structures/src/thin_vec/tests.rs
deleted file mode 100644
index 0221b9912bb..00000000000
--- a/compiler/rustc_data_structures/src/thin_vec/tests.rs
+++ /dev/null
@@ -1,42 +0,0 @@
-use super::*;
-
-impl<T> ThinVec<T> {
-    fn into_vec(self) -> Vec<T> {
-        self.into()
-    }
-}
-
-#[test]
-fn test_from_iterator() {
-    assert_eq!(std::iter::empty().collect::<ThinVec<String>>().into_vec(), Vec::<String>::new());
-    assert_eq!(std::iter::once(42).collect::<ThinVec<_>>().into_vec(), vec![42]);
-    assert_eq!([1, 2].into_iter().collect::<ThinVec<_>>().into_vec(), vec![1, 2]);
-    assert_eq!([1, 2, 3].into_iter().collect::<ThinVec<_>>().into_vec(), vec![1, 2, 3]);
-}
-
-#[test]
-fn test_into_iterator_owned() {
-    assert_eq!(ThinVec::new().into_iter().collect::<Vec<String>>(), Vec::<String>::new());
-    assert_eq!(ThinVec::from(vec![1]).into_iter().collect::<Vec<_>>(), vec![1]);
-    assert_eq!(ThinVec::from(vec![1, 2]).into_iter().collect::<Vec<_>>(), vec![1, 2]);
-    assert_eq!(ThinVec::from(vec![1, 2, 3]).into_iter().collect::<Vec<_>>(), vec![1, 2, 3]);
-}
-
-#[test]
-fn test_into_iterator_ref() {
-    assert_eq!(ThinVec::new().iter().collect::<Vec<&String>>(), Vec::<&String>::new());
-    assert_eq!(ThinVec::from(vec![1]).iter().collect::<Vec<_>>(), vec![&1]);
-    assert_eq!(ThinVec::from(vec![1, 2]).iter().collect::<Vec<_>>(), vec![&1, &2]);
-    assert_eq!(ThinVec::from(vec![1, 2, 3]).iter().collect::<Vec<_>>(), vec![&1, &2, &3]);
-}
-
-#[test]
-fn test_into_iterator_ref_mut() {
-    assert_eq!(ThinVec::new().iter_mut().collect::<Vec<&mut String>>(), Vec::<&mut String>::new());
-    assert_eq!(ThinVec::from(vec![1]).iter_mut().collect::<Vec<_>>(), vec![&mut 1]);
-    assert_eq!(ThinVec::from(vec![1, 2]).iter_mut().collect::<Vec<_>>(), vec![&mut 1, &mut 2]);
-    assert_eq!(
-        ThinVec::from(vec![1, 2, 3]).iter_mut().collect::<Vec<_>>(),
-        vec![&mut 1, &mut 2, &mut 3],
-    );
-}
diff --git a/compiler/rustc_middle/Cargo.toml b/compiler/rustc_middle/Cargo.toml
index 008d2c7091c..cca17a4eccd 100644
--- a/compiler/rustc_middle/Cargo.toml
+++ b/compiler/rustc_middle/Cargo.toml
@@ -7,34 +7,35 @@ edition = "2021"
 doctest = false
 
 [dependencies]
-rustc_arena = { path = "../rustc_arena" }
 bitflags = "1.2.1"
+chalk-ir = "0.80.0"
 either = "1.5.0"
 gsgdt = "0.1.2"
-tracing = "0.1"
-rustc-rayon = { version = "0.4.0", optional = true }
-rustc-rayon-core = { version = "0.4.0", optional = true }
 polonius-engine = "0.13.0"
+rand = "0.8.4"
+rand_xoshiro = "0.6.0"
 rustc_apfloat = { path = "../rustc_apfloat" }
+rustc_arena = { path = "../rustc_arena" }
+rustc_ast = { path = "../rustc_ast" }
 rustc_attr = { path = "../rustc_attr" }
-rustc_feature = { path = "../rustc_feature" }
-rustc_hir = { path = "../rustc_hir" }
-rustc_target = { path = "../rustc_target" }
-rustc_macros = { path = "../rustc_macros" }
 rustc_data_structures = { path = "../rustc_data_structures" }
-rustc_query_system = { path = "../rustc_query_system" }
 rustc_errors = { path = "../rustc_errors" }
+rustc_feature = { path = "../rustc_feature" }
 rustc_graphviz = { path = "../rustc_graphviz" }
+rustc_hir = { path = "../rustc_hir" }
 rustc_index = { path = "../rustc_index" }
+rustc_macros = { path = "../rustc_macros" }
+rustc_query_system = { path = "../rustc_query_system" }
+rustc-rayon-core = { version = "0.4.0", optional = true }
+rustc-rayon = { version = "0.4.0", optional = true }
 rustc_serialize = { path = "../rustc_serialize" }
-rustc_ast = { path = "../rustc_ast" }
-rustc_span = { path = "../rustc_span" }
-chalk-ir = "0.80.0"
-smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 rustc_session = { path = "../rustc_session" }
+rustc_span = { path = "../rustc_span" }
+rustc_target = { path = "../rustc_target" }
 rustc_type_ir = { path = "../rustc_type_ir" }
-rand = "0.8.4"
-rand_xoshiro = "0.6.0"
+smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
+thin-vec = "0.2.8"
+tracing = "0.1"
 
 [features]
 rustc_use_parallel_compiler = ["rustc-rayon", "rustc-rayon-core"]
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 0f34aa10a12..ba14f9f0203 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -1829,9 +1829,9 @@ pub mod tls {
     use crate::dep_graph::TaskDepsRef;
     use crate::ty::query;
     use rustc_data_structures::sync::{self, Lock};
-    use rustc_data_structures::thin_vec::ThinVec;
     use rustc_errors::Diagnostic;
     use std::mem;
+    use thin_vec::ThinVec;
 
     #[cfg(not(parallel_compiler))]
     use std::cell::Cell;
diff --git a/compiler/rustc_parse/src/parser/attr_wrapper.rs b/compiler/rustc_parse/src/parser/attr_wrapper.rs
index ed54af9f53f..d6a32d888bd 100644
--- a/compiler/rustc_parse/src/parser/attr_wrapper.rs
+++ b/compiler/rustc_parse/src/parser/attr_wrapper.rs
@@ -355,7 +355,7 @@ impl<'a> Parser<'a> {
             && matches!(self.capture_state.capturing, Capturing::Yes)
             && has_cfg_or_cfg_attr(final_attrs)
         {
-            let attr_data = AttributesData { attrs: final_attrs.to_vec().into(), tokens };
+            let attr_data = AttributesData { attrs: final_attrs.iter().cloned().collect(), tokens };
 
             // Replace the entire AST node that we just parsed, including attributes,
             // with a `FlatToken::AttrTarget`. If this AST node is inside an item
diff --git a/compiler/rustc_query_impl/Cargo.toml b/compiler/rustc_query_impl/Cargo.toml
index c37ae4f3253..e7f12caaf33 100644
--- a/compiler/rustc_query_impl/Cargo.toml
+++ b/compiler/rustc_query_impl/Cargo.toml
@@ -8,7 +8,6 @@ doctest = false
 
 [dependencies]
 measureme = "10.0.0"
-rustc-rayon-core = { version = "0.4.0", optional = true }
 rustc_ast = { path = "../rustc_ast" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }
@@ -17,10 +16,12 @@ rustc_index = { path = "../rustc_index" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_middle = { path = "../rustc_middle" }
 rustc_query_system = { path = "../rustc_query_system" }
+rustc-rayon-core = { version = "0.4.0", optional = true }
 rustc_serialize = { path = "../rustc_serialize" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
 rustc_target = { path = "../rustc_target" }
+thin-vec = "0.2.8"
 tracing = "0.1"
 
 [features]
diff --git a/compiler/rustc_query_impl/src/plumbing.rs b/compiler/rustc_query_impl/src/plumbing.rs
index eab627c5d4c..f38f29e14af 100644
--- a/compiler/rustc_query_impl/src/plumbing.rs
+++ b/compiler/rustc_query_impl/src/plumbing.rs
@@ -4,6 +4,9 @@
 
 use crate::keys::Key;
 use crate::{on_disk_cache, Queries};
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::sync::Lock;
+use rustc_errors::{Diagnostic, Handler};
 use rustc_middle::dep_graph::{self, DepKind, DepNodeIndex, SerializedDepNodeIndex};
 use rustc_middle::ty::tls::{self, ImplicitCtxt};
 use rustc_middle::ty::{self, TyCtxt};
@@ -12,14 +15,9 @@ use rustc_query_system::ich::StableHashingContext;
 use rustc_query_system::query::{
     QueryContext, QueryJobId, QueryMap, QuerySideEffects, QueryStackFrame,
 };
-
-use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
-use rustc_data_structures::sync::Lock;
-use rustc_data_structures::thin_vec::ThinVec;
-use rustc_errors::{Diagnostic, Handler};
-
 use std::any::Any;
 use std::num::NonZeroU64;
+use thin_vec::ThinVec;
 
 #[derive(Copy, Clone)]
 pub struct QueryCtxt<'tcx> {
diff --git a/compiler/rustc_query_system/Cargo.toml b/compiler/rustc_query_system/Cargo.toml
index b7787aeb8f7..bafc6b0a082 100644
--- a/compiler/rustc_query_system/Cargo.toml
+++ b/compiler/rustc_query_system/Cargo.toml
@@ -7,9 +7,8 @@ edition = "2021"
 doctest = false
 
 [dependencies]
+parking_lot = "0.11"
 rustc_arena = { path = "../rustc_arena" }
-tracing = "0.1"
-rustc-rayon-core = { version = "0.4.0", optional = true }
 rustc_ast = { path = "../rustc_ast" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }
@@ -17,12 +16,14 @@ rustc_feature = { path = "../rustc_feature" }
 rustc_hir = { path = "../rustc_hir" }
 rustc_index = { path = "../rustc_index" }
 rustc_macros = { path = "../rustc_macros" }
+rustc-rayon-core = { version = "0.4.0", optional = true }
 rustc_serialize = { path = "../rustc_serialize" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
 rustc_target = { path = "../rustc_target" }
-parking_lot = "0.11"
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
+thin-vec = "0.2.8"
+tracing = "0.1"
 
 [features]
 rustc_use_parallel_compiler = ["rustc-rayon-core"]
diff --git a/compiler/rustc_query_system/src/query/mod.rs b/compiler/rustc_query_system/src/query/mod.rs
index a1f2b081d43..51b4a2f35e2 100644
--- a/compiler/rustc_query_system/src/query/mod.rs
+++ b/compiler/rustc_query_system/src/query/mod.rs
@@ -15,12 +15,11 @@ mod config;
 pub use self::config::{QueryConfig, QueryDescription, QueryVTable};
 
 use crate::dep_graph::{DepContext, DepNodeIndex, HasDepContext, SerializedDepNodeIndex};
-
 use rustc_data_structures::sync::Lock;
-use rustc_data_structures::thin_vec::ThinVec;
 use rustc_errors::Diagnostic;
 use rustc_hir::def::DefKind;
 use rustc_span::Span;
+use thin_vec::ThinVec;
 
 /// Description of a frame in the query stack.
 ///
diff --git a/compiler/rustc_query_system/src/query/plumbing.rs b/compiler/rustc_query_system/src/query/plumbing.rs
index 6ae9147ff77..aa10d2d3442 100644
--- a/compiler/rustc_query_system/src/query/plumbing.rs
+++ b/compiler/rustc_query_system/src/query/plumbing.rs
@@ -14,7 +14,6 @@ use rustc_data_structures::profiling::TimingGuard;
 #[cfg(parallel_compiler)]
 use rustc_data_structures::sharded::Sharded;
 use rustc_data_structures::sync::Lock;
-use rustc_data_structures::thin_vec::ThinVec;
 use rustc_errors::{DiagnosticBuilder, ErrorGuaranteed, FatalError};
 use rustc_session::Session;
 use rustc_span::{Span, DUMMY_SP};
@@ -24,6 +23,7 @@ use std::fmt::Debug;
 use std::hash::Hash;
 use std::mem;
 use std::ptr;
+use thin_vec::ThinVec;
 
 pub struct QueryState<K> {
     #[cfg(parallel_compiler)]
diff --git a/compiler/rustc_serialize/Cargo.toml b/compiler/rustc_serialize/Cargo.toml
index dbc5c15195c..3b0b3144f2c 100644
--- a/compiler/rustc_serialize/Cargo.toml
+++ b/compiler/rustc_serialize/Cargo.toml
@@ -6,6 +6,7 @@ edition = "2021"
 [dependencies]
 indexmap = "1.9.1"
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
+thin-vec = "0.2.8"
 
 [dev-dependencies]
 rustc_macros = { path = "../rustc_macros" }
diff --git a/compiler/rustc_serialize/src/collection_impls.rs b/compiler/rustc_serialize/src/collection_impls.rs
index 5e53f0b104d..8f8c504117c 100644
--- a/compiler/rustc_serialize/src/collection_impls.rs
+++ b/compiler/rustc_serialize/src/collection_impls.rs
@@ -1,13 +1,12 @@
 //! Implementations of serialization for structures found in liballoc
 
-use std::hash::{BuildHasher, Hash};
-
 use crate::{Decodable, Decoder, Encodable, Encoder};
+use smallvec::{Array, SmallVec};
 use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet, LinkedList, VecDeque};
+use std::hash::{BuildHasher, Hash};
 use std::rc::Rc;
 use std::sync::Arc;
-
-use smallvec::{Array, SmallVec};
+use thin_vec::ThinVec;
 
 impl<S: Encoder, A: Array<Item: Encodable<S>>> Encodable<S> for SmallVec<A> {
     fn encode(&self, s: &mut S) {
@@ -23,6 +22,19 @@ impl<D: Decoder, A: Array<Item: Decodable<D>>> Decodable<D> for SmallVec<A> {
     }
 }
 
+impl<S: Encoder, T: Encodable<S>> Encodable<S> for ThinVec<T> {
+    fn encode(&self, s: &mut S) {
+        self.as_slice().encode(s);
+    }
+}
+
+impl<D: Decoder, T: Decodable<D>> Decodable<D> for ThinVec<T> {
+    fn decode(d: &mut D) -> ThinVec<T> {
+        let len = d.read_usize();
+        (0..len).map(|_| Decodable::decode(d)).collect()
+    }
+}
+
 impl<S: Encoder, T: Encodable<S>> Encodable<S> for LinkedList<T> {
     fn encode(&self, s: &mut S) {
         s.emit_usize(self.len());
diff --git a/src/librustdoc/Cargo.toml b/src/librustdoc/Cargo.toml
index cbdfea89efb..7bc35c7d551 100644
--- a/src/librustdoc/Cargo.toml
+++ b/src/librustdoc/Cargo.toml
@@ -10,18 +10,19 @@ path = "lib.rs"
 arrayvec = { version = "0.7", default-features = false }
 askama = { version = "0.11", default-features = false, features = ["config"] }
 atty = "0.2"
-pulldown-cmark = { version = "0.9.2", default-features = false }
+itertools = "0.10.1"
 minifier = "0.2.2"
-serde = { version = "1.0", features = ["derive"] }
+once_cell = "1.10.0"
+pulldown-cmark = { version = "0.9.2", default-features = false }
+regex = "1"
+rustdoc-json-types = { path = "../rustdoc-json-types" }
 serde_json = "1.0"
+serde = { version = "1.0", features = ["derive"] }
 smallvec = "1.8.1"
 tempfile = "3"
-itertools = "0.10.1"
-regex = "1"
-rustdoc-json-types = { path = "../rustdoc-json-types" }
+thin-vec = "0.2.8"
 tracing = "0.1"
 tracing-tree = "0.2.0"
-once_cell = "1.10.0"
 
 [dependencies.tracing-subscriber]
 version = "0.3.3"
diff --git a/src/librustdoc/clean/auto_trait.rs b/src/librustdoc/clean/auto_trait.rs
index 5441a7bd29e..0239b8da57e 100644
--- a/src/librustdoc/clean/auto_trait.rs
+++ b/src/librustdoc/clean/auto_trait.rs
@@ -123,7 +123,7 @@ where
             kind: Box::new(ImplItem(Box::new(Impl {
                 unsafety: hir::Unsafety::Normal,
                 generics: new_generics,
-                trait_: Some(clean_trait_ref_with_bindings(self.cx, trait_ref, &[])),
+                trait_: Some(clean_trait_ref_with_bindings(self.cx, trait_ref, ThinVec::new())),
                 for_: clean_middle_ty(ty, self.cx, None),
                 items: Vec::new(),
                 polarity,
diff --git a/src/librustdoc/clean/blanket_impl.rs b/src/librustdoc/clean/blanket_impl.rs
index da15c3c2b1f..cc734389e07 100644
--- a/src/librustdoc/clean/blanket_impl.rs
+++ b/src/librustdoc/clean/blanket_impl.rs
@@ -115,7 +115,7 @@ impl<'a, 'tcx> BlanketImplFinder<'a, 'tcx> {
                             ),
                             // FIXME(eddyb) compute both `trait_` and `for_` from
                             // the post-inference `trait_ref`, as it's more accurate.
-                            trait_: Some(clean_trait_ref_with_bindings(cx, trait_ref.0, &[])),
+                            trait_: Some(clean_trait_ref_with_bindings(cx, trait_ref.0, ThinVec::new())),
                             for_: clean_middle_ty(ty.0, cx, None),
                             items: cx.tcx
                                 .associated_items(impl_def_id)
diff --git a/src/librustdoc/clean/inline.rs b/src/librustdoc/clean/inline.rs
index f367edcbf5a..f000b79d310 100644
--- a/src/librustdoc/clean/inline.rs
+++ b/src/librustdoc/clean/inline.rs
@@ -3,9 +3,10 @@
 use std::iter::once;
 use std::sync::Arc;
 
+use thin_vec::ThinVec;
+
 use rustc_ast as ast;
 use rustc_data_structures::fx::FxHashSet;
-use rustc_data_structures::thin_vec::ThinVec;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::DefId;
@@ -461,7 +462,7 @@ pub(crate) fn build_impl(
         ),
     };
     let polarity = tcx.impl_polarity(did);
-    let trait_ = associated_trait.map(|t| clean_trait_ref_with_bindings(cx, t, &[]));
+    let trait_ = associated_trait.map(|t| clean_trait_ref_with_bindings(cx, t, ThinVec::new()));
     if trait_.as_ref().map(|t| t.def_id()) == tcx.lang_items().deref_trait() {
         super::build_deref_target_impls(cx, &trait_items, ret);
     }
diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index 420159b5a67..a27d1a16e3b 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -33,7 +33,8 @@ use std::collections::hash_map::Entry;
 use std::collections::BTreeMap;
 use std::default::Default;
 use std::hash::Hash;
-use std::{mem, vec};
+use std::mem;
+use thin_vec::ThinVec;
 
 use crate::core::{self, DocContext, ImplTraitParam};
 use crate::formats::item_type::ItemType;
@@ -125,7 +126,7 @@ fn clean_generic_bound<'tcx>(
                 bug!("clean: parenthesized `GenericBound::LangItemTrait`");
             };
 
-            let trait_ = clean_trait_ref_with_bindings(cx, trait_ref, &bindings);
+            let trait_ = clean_trait_ref_with_bindings(cx, trait_ref, bindings);
             GenericBound::TraitBound(
                 PolyTrait { trait_, generic_params: vec![] },
                 hir::TraitBoundModifier::None,
@@ -147,14 +148,14 @@ fn clean_generic_bound<'tcx>(
 pub(crate) fn clean_trait_ref_with_bindings<'tcx>(
     cx: &mut DocContext<'tcx>,
     trait_ref: ty::TraitRef<'tcx>,
-    bindings: &[TypeBinding],
+    bindings: ThinVec<TypeBinding>,
 ) -> Path {
     let kind = cx.tcx.def_kind(trait_ref.def_id).into();
     if !matches!(kind, ItemType::Trait | ItemType::TraitAlias) {
         span_bug!(cx.tcx.def_span(trait_ref.def_id), "`TraitRef` had unexpected kind {:?}", kind);
     }
     inline::record_extern_fqn(cx, trait_ref.def_id, kind);
-    let path = external_path(cx, trait_ref.def_id, true, bindings.to_vec(), trait_ref.substs);
+    let path = external_path(cx, trait_ref.def_id, true, bindings, trait_ref.substs);
 
     debug!("ty::TraitRef\n  subst: {:?}\n", trait_ref.substs);
 
@@ -164,7 +165,7 @@ pub(crate) fn clean_trait_ref_with_bindings<'tcx>(
 fn clean_poly_trait_ref_with_bindings<'tcx>(
     cx: &mut DocContext<'tcx>,
     poly_trait_ref: ty::PolyTraitRef<'tcx>,
-    bindings: &[TypeBinding],
+    bindings: ThinVec<TypeBinding>,
 ) -> GenericBound {
     let poly_trait_ref = poly_trait_ref.lift_to_tcx(cx.tcx).unwrap();
 
@@ -327,7 +328,7 @@ fn clean_poly_trait_predicate<'tcx>(
     let poly_trait_ref = pred.map_bound(|pred| pred.trait_ref);
     Some(WherePredicate::BoundPredicate {
         ty: clean_middle_ty(poly_trait_ref.skip_binder().self_ty(), cx, None),
-        bounds: vec![clean_poly_trait_ref_with_bindings(cx, poly_trait_ref, &[])],
+        bounds: vec![clean_poly_trait_ref_with_bindings(cx, poly_trait_ref, ThinVec::new())],
         bound_params: Vec::new(),
     })
 }
@@ -402,7 +403,7 @@ fn clean_projection<'tcx>(
     def_id: Option<DefId>,
 ) -> Type {
     let lifted = ty.lift_to_tcx(cx.tcx).unwrap();
-    let trait_ = clean_trait_ref_with_bindings(cx, lifted.trait_ref(cx.tcx), &[]);
+    let trait_ = clean_trait_ref_with_bindings(cx, lifted.trait_ref(cx.tcx), ThinVec::new());
     let self_type = clean_middle_ty(ty.self_ty(), cx, None);
     let self_def_id = if let Some(def_id) = def_id {
         cx.tcx.opt_parent(def_id).or(Some(def_id))
@@ -1588,12 +1589,12 @@ pub(crate) fn clean_middle_ty<'tcx>(
                 AdtKind::Enum => ItemType::Enum,
             };
             inline::record_extern_fqn(cx, did, kind);
-            let path = external_path(cx, did, false, vec![], substs);
+            let path = external_path(cx, did, false, ThinVec::new(), substs);
             Type::Path { path }
         }
         ty::Foreign(did) => {
             inline::record_extern_fqn(cx, did, ItemType::ForeignType);
-            let path = external_path(cx, did, false, vec![], InternalSubsts::empty());
+            let path = external_path(cx, did, false, ThinVec::new(), InternalSubsts::empty());
             Type::Path { path }
         }
         ty::Dynamic(obj, ref reg) => {
@@ -1617,7 +1618,7 @@ pub(crate) fn clean_middle_ty<'tcx>(
             let mut bounds = dids
                 .map(|did| {
                     let empty = cx.tcx.intern_substs(&[]);
-                    let path = external_path(cx, did, false, vec![], empty);
+                    let path = external_path(cx, did, false, ThinVec::new(), empty);
                     inline::record_extern_fqn(cx, did, ItemType::Trait);
                     PolyTrait { trait_: path, generic_params: Vec::new() }
                 })
@@ -1693,7 +1694,7 @@ pub(crate) fn clean_middle_ty<'tcx>(
                         }
                     }
 
-                    let bindings: Vec<_> = bounds
+                    let bindings: ThinVec<_> = bounds
                         .iter()
                         .filter_map(|bound| {
                             if let ty::PredicateKind::Projection(proj) = bound.kind().skip_binder()
@@ -1714,7 +1715,7 @@ pub(crate) fn clean_middle_ty<'tcx>(
                         })
                         .collect();
 
-                    Some(clean_poly_trait_ref_with_bindings(cx, trait_ref, &bindings))
+                    Some(clean_poly_trait_ref_with_bindings(cx, trait_ref, bindings))
                 })
                 .collect::<Vec<_>>();
             bounds.extend(regions);
@@ -1845,12 +1846,8 @@ fn clean_generic_args<'tcx>(
             })
             .collect::<Vec<_>>()
             .into();
-        let bindings = generic_args
-            .bindings
-            .iter()
-            .map(|x| clean_type_binding(x, cx))
-            .collect::<Vec<_>>()
-            .into();
+        let bindings =
+            generic_args.bindings.iter().map(|x| clean_type_binding(x, cx)).collect::<ThinVec<_>>();
         GenericArgs::AngleBracketed { args, bindings }
     }
 }
diff --git a/src/librustdoc/clean/types.rs b/src/librustdoc/clean/types.rs
index 909a47d07b1..2808b400a0b 100644
--- a/src/librustdoc/clean/types.rs
+++ b/src/librustdoc/clean/types.rs
@@ -8,6 +8,7 @@ use std::sync::OnceLock as OnceCell;
 use std::{cmp, fmt, iter};
 
 use arrayvec::ArrayVec;
+use thin_vec::ThinVec;
 
 use rustc_ast::attr;
 use rustc_ast::util::comments::beautify_doc_string;
@@ -15,7 +16,6 @@ use rustc_ast::{self as ast, AttrStyle};
 use rustc_attr::{ConstStability, Deprecation, Stability, StabilityLevel};
 use rustc_const_eval::const_eval::is_unstable_const_fn;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
-use rustc_data_structures::thin_vec::ThinVec;
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, DefKind, Res};
 use rustc_hir::def_id::{CrateNum, DefId, LOCAL_CRATE};
@@ -1303,7 +1303,7 @@ impl GenericBound {
     pub(crate) fn maybe_sized(cx: &mut DocContext<'_>) -> GenericBound {
         let did = cx.tcx.require_lang_item(LangItem::Sized, None);
         let empty = cx.tcx.intern_substs(&[]);
-        let path = external_path(cx, did, false, vec![], empty);
+        let path = external_path(cx, did, false, ThinVec::new(), empty);
         inline::record_extern_fqn(cx, did, ItemType::Trait);
         GenericBound::TraitBound(
             PolyTrait { trait_: path, generic_params: Vec::new() },
diff --git a/src/librustdoc/clean/utils.rs b/src/librustdoc/clean/utils.rs
index 718cbbd2b83..ac9ab339616 100644
--- a/src/librustdoc/clean/utils.rs
+++ b/src/librustdoc/clean/utils.rs
@@ -12,7 +12,6 @@ use crate::visit_lib::LibEmbargoVisitor;
 
 use rustc_ast as ast;
 use rustc_ast::tokenstream::TokenTree;
-use rustc_data_structures::thin_vec::ThinVec;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
@@ -23,6 +22,7 @@ use rustc_middle::ty::{self, DefIdTree, TyCtxt};
 use rustc_span::symbol::{kw, sym, Symbol};
 use std::fmt::Write as _;
 use std::mem;
+use thin_vec::ThinVec;
 
 #[cfg(test)]
 mod tests;
@@ -102,7 +102,7 @@ fn external_generic_args<'tcx>(
     cx: &mut DocContext<'tcx>,
     did: DefId,
     has_self: bool,
-    bindings: Vec<TypeBinding>,
+    bindings: ThinVec<TypeBinding>,
     substs: SubstsRef<'tcx>,
 ) -> GenericArgs {
     let args = substs_to_args(cx, substs, has_self);
@@ -112,7 +112,7 @@ fn external_generic_args<'tcx>(
             // The trait's first substitution is the one after self, if there is one.
             match substs.iter().nth(if has_self { 1 } else { 0 }).unwrap().expect_ty().kind() {
                 ty::Tuple(tys) => tys.iter().map(|t| clean_middle_ty(t, cx, None)).collect::<Vec<_>>().into(),
-                _ => return GenericArgs::AngleBracketed { args: args.into(), bindings: bindings.into() },
+                _ => return GenericArgs::AngleBracketed { args: args.into(), bindings },
             };
         let output = None;
         // FIXME(#20299) return type comes from a projection now
@@ -130,7 +130,7 @@ pub(super) fn external_path<'tcx>(
     cx: &mut DocContext<'tcx>,
     did: DefId,
     has_self: bool,
-    bindings: Vec<TypeBinding>,
+    bindings: ThinVec<TypeBinding>,
     substs: SubstsRef<'tcx>,
 ) -> Path {
     let def_kind = cx.tcx.def_kind(did);
diff --git a/src/tools/tidy/src/deps.rs b/src/tools/tidy/src/deps.rs
index 5770bb7bc33..b124490f747 100644
--- a/src/tools/tidy/src/deps.rs
+++ b/src/tools/tidy/src/deps.rs
@@ -223,6 +223,7 @@ const PERMITTED_DEPENDENCIES: &[&str] = &[
     "time",
     "tinystr",
     "tinyvec",
+    "thin-vec",
     "tracing",
     "tracing-attributes",
     "tracing-core",