Merge #1195

1195: Add cached for SubtreeSource r=matklad a=edwin0cheng

I just did some simple profiling, the most slowest part of the mbe related code is the tree traversal, so i add a very simple cache in here is the result:

``` target\release\ra_cli analysis-stats```

*Before
```
Database loaded, 21 roots, 1.2731481s
Crates in this dir: 26
Total modules found: 282
Total declarations: 4783
Total functions: 3126
Total expressions: 59452
Expressions of unknown type: 8999 (15%)
Expressions of partially unknown type: 2672 (4%)
Analysis: 17.9596334s
```

```
*After
Database loaded, 21 roots, 1.7425901s
Crates in this dir: 26
Total modules found: 282
Total declarations: 4783
Total functions: 3126
Total expressions: 59452
Expressions of unknown type: 8999 (15%)
Expressions of partially unknown type: 2672 (4%)
Analysis: 13.4007118s
```

Save 4 seconds :)


Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>

crates/ra_mbe/src/subtree_source.rs

@@ -219,17 +219,33 @@ pub(crate) trait Querier {
 #[derive(Debug)]
 pub(crate) struct WalkerOwner<'a> {
     walker: RefCell<SubTreeWalker<'a>>,
+    cached: RefCell<Vec<Option<TtToken>>>,
 }
 
 impl<'a> WalkerOwner<'a> {
     fn new<I: Into<TokenSeq<'a>>>(ts: I) -> Self {
-        WalkerOwner { walker: RefCell::new(SubTreeWalker::new(ts.into())) }
+        WalkerOwner {
+            walker: RefCell::new(SubTreeWalker::new(ts.into())),
+            cached: RefCell::new(Vec::with_capacity(10)),
+        }
     }
 
     fn get<'b>(&self, pos: usize) -> Option<TtToken> {
-        self.set_pos(pos);
-        let walker = self.walker.borrow();
-        walker.current().cloned()
+        let mut cached = self.cached.borrow_mut();
+        if pos < cached.len() {
+            return cached[pos].clone();
+        }
+
+        while pos >= cached.len() {
+            let len = cached.len();
+            cached.push({
+                self.set_pos(len);
+                let walker = self.walker.borrow();
+                walker.current().cloned()
+            });
+        }
+
+        return cached[pos].clone();
     }
 
     fn set_pos(&self, pos: usize) {