Merge #1638

1638: Avoid cloning a TtToken in SubtreeTokenSource::mk_token r=matklad a=lnicola

According to `perf record`, this function is the hottest one while running `ra_cli analysis-stats`:

Before:

```
6.05%  ra_cli  ra_cli        <ra_mbe::subtree_source::SubtreeTokenSource as ra_parser::TokenSource>::lookahead_nth
5.56%  ra_cli  ra_cli        <rowan::green::GreenNode as core::hash::Hash>::hash
4.16%  ra_cli  libc-2.29.so  _int_malloc
3.93%  ra_cli  ra_cli        ra_mbe::subtree_source::SubtreeTokenSource::get

Database loaded, 255 roots, 231.676882ms
Crates in this dir: 27
Total modules found: 282
Total declarations: 9642
Total functions: 3287
Total expressions: 64616
Expressions of unknown type: 9111 (14%)
Expressions of partially unknown type: 3417 (5%)
Analysis: 24.012797561s, 0b allocated 0b resident
target/release/ra_cli analysis-stats  24.60s user 0.56s system 100% cpu 25.125 total
```

After:

```
5.85%  ra_cli  ra_cli        <rowan::green::GreenNode as core::hash::Hash>::hash
4.55%  ra_cli  libc-2.29.so  _int_malloc
4.31%  ra_cli  ra_cli        ra_parser::parser::Parser::nth
3.61%  ra_cli  ra_cli        <ra_syntax::parsing::text_token_source::TextTokenSource as ra_parser::TokenSource>::lookahead_nth
3.54%  ra_cli  ra_cli        ra_syntax::syntax_node::SyntaxTreeBuilder::finish_node
3.46%  ra_cli  libc-2.29.so  _int_free
3.12%  ra_cli  libc-2.29.so  malloc
2.76%  ra_cli  ra_cli        ra_parser::event::process
2.68%  ra_cli  ra_cli        alloc::sync::Arc<T>::drop_slow
2.50%  ra_cli  ra_cli        ra_mbe::subtree_source::SubtreeTokenSource::get
2.31%  ra_cli  ra_cli        <smol_str::SmolStr as core::hash::Hash>::hash
2.04%  ra_cli  libc-2.29.so  __memmove_avx_unaligned_erms
1.92%  ra_cli  ra_cli        <ra_mbe::subtree_source::SubtreeTokenSource as ra_parser::TokenSource>::lookahead_nth

Database loaded, 255 roots, 236.176803ms
Crates in this dir: 27
Total modules found: 282
Total declarations: 9642
Total functions: 3287
Total expressions: 64620
Expressions of unknown type: 9107 (14%)
Expressions of partially unknown type: 3425 (5%)
Analysis: 22.562328486s, 0b allocated 0b resident
target/release/ra_cli analysis-stats  23.12s user 0.57s system 100% cpu 23.659 total
```

r? @edwin0cheng, @matklad 

Co-authored-by: Laurențiu Nicola <[email protected]>

1 files changed, 32 insertions, 30 deletions

diff --git a/crates/ra_mbe/src/subtree_source.rs b/crates/ra_mbe/src/subtree_source.rs
index 6603ff34d..9d6d0133f 100644
--- a/crates/ra_mbe/src/subtree_source.rs
+++ b/crates/ra_mbe/src/subtree_source.rs
@@ -1,6 +1,6 @@
 use ra_parser::{Token, TokenSource};
 use ra_syntax::{classify_literal, SmolStr, SyntaxKind, SyntaxKind::*, T};
-use std::cell::{Cell, RefCell};
+use std::cell::{Cell, Ref, RefCell};
 use tt::buffer::{Cursor, TokenBuffer};
 
 #[derive(Debug, Clone, Eq, PartialEq)]
@@ -20,8 +20,8 @@ impl<'a> SubtreeTokenSource<'a> {
     // Helper function used in test
     #[cfg(test)]
     pub fn text(&self) -> SmolStr {
-        match self.get(self.curr.1) {
-            Some(tt) => tt.text,
+        match *self.get(self.curr.1) {
+            Some(ref tt) => tt.text.clone(),
             _ => SmolStr::new(""),
         }
     }
@@ -41,44 +41,46 @@ impl<'a> SubtreeTokenSource<'a> {
     }
 
     fn mk_token(&self, pos: usize) -> Token {
-        match self.get(pos) {
-            Some(tt) => Token { kind: tt.kind, is_jointed_to_next: tt.is_joint_to_next },
+        match *self.get(pos) {
+            Some(ref tt) => Token { kind: tt.kind, is_jointed_to_next: tt.is_joint_to_next },
             None => Token { kind: EOF, is_jointed_to_next: false },
         }
     }
 
-    fn get(&self, pos: usize) -> Option<TtToken> {
-        let mut cached = self.cached.borrow_mut();
-        if pos < cached.len() {
-            return cached[pos].clone();
+    fn get(&self, pos: usize) -> Ref<Option<TtToken>> {
+        if pos < self.cached.borrow().len() {
+            return Ref::map(self.cached.borrow(), |c| &c[pos]);
         }
 
-        while pos >= cached.len() {
-            let cursor = self.cached_cursor.get();
-            if cursor.eof() {
-                cached.push(None);
-                continue;
-            }
-
-            match cursor.token_tree() {
-                Some(tt::TokenTree::Leaf(leaf)) => {
-                    cached.push(Some(convert_leaf(&leaf)));
-                    self.cached_cursor.set(cursor.bump());
-                }
-                Some(tt::TokenTree::Subtree(subtree)) => {
-                    self.cached_cursor.set(cursor.subtree().unwrap());
-                    cached.push(Some(convert_delim(subtree.delimiter, false)));
+        {
+            let mut cached = self.cached.borrow_mut();
+            while pos >= cached.len() {
+                let cursor = self.cached_cursor.get();
+                if cursor.eof() {
+                    cached.push(None);
+                    continue;
                 }
-                None => {
-                    if let Some(subtree) = cursor.end() {
-                        cached.push(Some(convert_delim(subtree.delimiter, true)));
+
+                match cursor.token_tree() {
+                    Some(tt::TokenTree::Leaf(leaf)) => {
+                        cached.push(Some(convert_leaf(&leaf)));
                         self.cached_cursor.set(cursor.bump());
                     }
+                    Some(tt::TokenTree::Subtree(subtree)) => {
+                        self.cached_cursor.set(cursor.subtree().unwrap());
+                        cached.push(Some(convert_delim(subtree.delimiter, false)));
+                    }
+                    None => {
+                        if let Some(subtree) = cursor.end() {
+                            cached.push(Some(convert_delim(subtree.delimiter, true)));
+                            self.cached_cursor.set(cursor.bump());
+                        }
+                    }
                 }
             }
         }
 
-        cached[pos].clone()
+        Ref::map(self.cached.borrow(), |c| &c[pos])
     }
 }
 
@@ -103,8 +105,8 @@ impl<'a> TokenSource for SubtreeTokenSource<'a> {
 
     /// Is the current token a specified keyword?
     fn is_keyword(&self, kw: &str) -> bool {
-        match self.get(self.curr.1) {
-            Some(t) => t.text == *kw,
+        match *self.get(self.curr.1) {
+            Some(ref t) => t.text == *kw,
             _ => false,
         }
     }