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.

Author: nnethercote

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",
@@ -4878,6 +4887,12 @@ version = "0.15.0"
 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"

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"

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:

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,

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"

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 = {

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"

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![

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),

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),