Try to reduce codegen complexity of TokenStream's FromIterator and Extend impls

This is an experimental patch to try to reduce the codegen complexity of
TokenStream's FromIterator and Extend implementations for downstream
crates, by moving the core logic into a helper type. This might help
improve build performance of crates which depend on proc_macro as
iterators are used less, and the compiler may take less time to do
things like attempt specializations or other iterator optimizations.

The change intentionally sacrifices some optimization opportunities,
such as using the specializations for collecting iterators derived from
Vec::into_iter() into Vec.

This is one of the simpler potential approaches to reducing the amount
of code generated in crates depending on proc_macro, so it seems worth
trying before other more-involved changes.

Author: mystor

library/proc_macro/src/lib.rs

@@ -230,14 +230,89 @@ impl From<TokenTree> for TokenStream {
     }
 }
 
+/// Non-generic helper for implementing `FromIterator<TokenTree>` and
+/// `Extend<TokenTree>` with less monomorphization in calling crates.
+struct ExtendStreamWithTreesHelper {
+    trees: Vec<
+        bridge::TokenTree<
+            bridge::client::TokenStream,
+            bridge::client::Span,
+            bridge::client::Symbol,
+        >,
+    >,
+}
+
+impl ExtendStreamWithTreesHelper {
+    fn new(capacity: usize) -> Self {
+        ExtendStreamWithTreesHelper { trees: Vec::with_capacity(capacity) }
+    }
+
+    fn push(&mut self, tree: TokenTree) {
+        self.trees.push(tree_to_bridge_tree(tree));
+    }
+
+    fn build(self) -> TokenStream {
+        if self.trees.is_empty() {
+            TokenStream(None)
+        } else {
+            TokenStream(Some(bridge::client::TokenStream::concat_trees(None, self.trees)))
+        }
+    }
+
+    fn extend(self, stream: &mut TokenStream) {
+        if self.trees.is_empty() {
+            return;
+        }
+        stream.0 = Some(bridge::client::TokenStream::concat_trees(stream.0.take(), self.trees))
+    }
+}
+
+/// Non-generic helper for implementing `FromIterator<TokenStream>` and
+/// `Extend<TokenStream>` with less monomorphization in calling crates.
+struct ExtendStreamWithStreamsHelper {
+    streams: Vec<bridge::client::TokenStream>,
+}
+
+impl ExtendStreamWithStreamsHelper {
+    fn new(capacity: usize) -> Self {
+        ExtendStreamWithStreamsHelper { streams: Vec::with_capacity(capacity) }
+    }
+
+    fn push(&mut self, stream: TokenStream) {
+        if let Some(stream) = stream.0 {
+            self.streams.push(stream);
+        }
+    }
+
+    fn build(mut self) -> TokenStream {
+        if self.streams.len() <= 1 {
+            TokenStream(self.streams.pop())
+        } else {
+            TokenStream(Some(bridge::client::TokenStream::concat_streams(None, self.streams)))
+        }
+    }
+
+    fn extend(mut self, stream: &mut TokenStream) {
+        if self.streams.is_empty() {
+            return;
+        }
+        let base = stream.0.take();
+        if base.is_none() && self.streams.len() == 1 {
+            stream.0 = self.streams.pop();
+        } else {
+            stream.0 = Some(bridge::client::TokenStream::concat_streams(base, self.streams));
+        }
+    }
+}
+
 /// Collects a number of token trees into a single stream.
 #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
 impl iter::FromIterator<TokenTree> for TokenStream {
     fn from_iter<I: IntoIterator<Item = TokenTree>>(trees: I) -> Self {
-        TokenStream(Some(bridge::client::TokenStream::concat_trees(
-            None,
-            trees.into_iter().map(tree_to_bridge_tree).collect(),
-        )))
+        let iter = trees.into_iter();
+        let mut builder = ExtendStreamWithTreesHelper::new(iter.size_hint().0);
+        iter.for_each(|tree| builder.push(tree));
+        builder.build()
     }
 }
 
@@ -246,30 +321,30 @@ impl iter::FromIterator<TokenTree> for TokenStream {
 #[stable(feature = "proc_macro_lib", since = "1.15.0")]
 impl iter::FromIterator<TokenStream> for TokenStream {
     fn from_iter<I: IntoIterator<Item = TokenStream>>(streams: I) -> Self {
-        TokenStream(Some(bridge::client::TokenStream::concat_streams(
-            None,
-            streams.into_iter().filter_map(|stream| stream.0).collect(),
-        )))
+        let iter = streams.into_iter();
+        let mut builder = ExtendStreamWithStreamsHelper::new(iter.size_hint().0);
+        iter.for_each(|stream| builder.push(stream));
+        builder.build()
     }
 }
 
 #[stable(feature = "token_stream_extend", since = "1.30.0")]
 impl Extend<TokenTree> for TokenStream {
     fn extend<I: IntoIterator<Item = TokenTree>>(&mut self, trees: I) {
-        *self = TokenStream(Some(bridge::client::TokenStream::concat_trees(
-            self.0.take(),
-            trees.into_iter().map(|tree| tree_to_bridge_tree(tree)).collect(),
-        )));
+        let iter = trees.into_iter();
+        let mut builder = ExtendStreamWithTreesHelper::new(iter.size_hint().0);
+        iter.for_each(|tree| builder.push(tree));
+        builder.extend(self);
     }
 }
 
 #[stable(feature = "token_stream_extend", since = "1.30.0")]
 impl Extend<TokenStream> for TokenStream {
     fn extend<I: IntoIterator<Item = TokenStream>>(&mut self, streams: I) {
-        *self = TokenStream(Some(bridge::client::TokenStream::concat_streams(
-            self.0.take(),
-            streams.into_iter().filter_map(|stream| stream.0).collect(),
-        )));
+        let iter = streams.into_iter();
+        let mut builder = ExtendStreamWithStreamsHelper::new(iter.size_hint().0);
+        iter.for_each(|stream| builder.push(stream));
+        builder.extend(self);
     }
 }