Optimize empty token streams

@@ -7,7 +7,7 @@ use std::borrow::Cow; use std::hash::Hash; use std::ops::Range;-use std::sync::Arc;+use std::sync::{Arc, LazyLock}; use std::{cmp, fmt, iter, mem};  use rustc_data_structures::stable_hash::{StableHash, StableHashCtxt, StableHasher};@@ -419,6 +419,11 @@ fn make_attr_token_stream(     AttrTokenStream::new(stack_top.inner) } +/// A shared empty attr token stream, used to avoid an unnecessary `Arc` allocation for every empty+/// token stream.+static EMPTY_ATTR_TOKEN_STREAM: LazyLock<AttrTokenStream> =+    LazyLock::new(|| AttrTokenStream(Arc::new(Vec::new())));+ /// Like `TokenTree`, but for `AttrTokenStream`. #[derive(Clone, Debug, Encodable, Decodable)] pub enum AttrTokenTree {@@ -431,8 +436,12 @@ pub enum AttrTokenTree { }  impl AttrTokenStream {-    pub fn new(tokens: Vec<AttrTokenTree>) -> AttrTokenStream {-        AttrTokenStream(Arc::new(tokens))+    pub fn new(tts: Vec<AttrTokenTree>) -> AttrTokenStream {+        if tts.is_empty() {+            EMPTY_ATTR_TOKEN_STREAM.clone()+        } else {+            AttrTokenStream(Arc::new(tts))+        }     }      /// Converts this `AttrTokenStream` to a plain `Vec<TokenTree>`. During@@ -620,13 +629,18 @@ pub enum Spacing {     JointHidden, } +/// A shared empty token stream, used to avoid an unnecessary `Arc` allocation for every empty+/// token stream.+static EMPTY_TOKEN_STREAM: LazyLock<TokenStream> =+    LazyLock::new(|| TokenStream(Arc::new(Vec::new())));+ /// A `TokenStream` is an abstract sequence of tokens, organized into [`TokenTree`]s.-#[derive(Clone, Debug, Default, PartialEq, Eq, Hash, Encodable, Decodable)]+#[derive(Clone, Debug, PartialEq, Eq, Hash, Encodable, Decodable)] pub struct TokenStream(Arc<Vec<TokenTree>>);  impl TokenStream {     pub fn new(tts: Vec<TokenTree>) -> TokenStream {-        TokenStream(Arc::new(tts))+        if tts.is_empty() { TokenStream::default() } else { TokenStream(Arc::new(tts)) }     }      pub fn is_empty(&self) -> bool {@@ -840,6 +854,12 @@ impl TokenStream {     } } +impl Default for TokenStream {+    fn default() -> TokenStream {+        EMPTY_TOKEN_STREAM.clone()+    }+}+ impl FromIterator<TokenTree> for TokenStream {     fn from_iter<I: IntoIterator<Item = TokenTree>>(iter: I) -> Self {         TokenStream::new(iter.into_iter().collect::<Vec<TokenTree>>())