Change empty replace range condition.

The new condition is equivalent in practice, but it's much more obvious
that it would result in an empty range, because the condition lines up
with the contents of the iterator.

Author: nnethercote

compiler/rustc_parse/src/parser/attr_wrapper.rs

@@ -295,21 +295,22 @@ impl<'a> Parser<'a> {
 
         let num_calls = end_pos - start_pos;
 
-        // If we have no attributes, then we will never need to
-        // use any replace ranges.
-        let replace_ranges: Box<[ReplaceRange]> = if ret.attrs().is_empty() && !self.capture_cfg {
-            Box::new([])
-        } else {
-            // Grab any replace ranges that occur *inside* the current AST node.
-            // We will perform the actual replacement when we convert the `LazyAttrTokenStream`
-            // to an `AttrTokenStream`.
-            self.capture_state.replace_ranges[replace_ranges_start..replace_ranges_end]
-                .iter()
-                .cloned()
-                .chain(inner_attr_replace_ranges.iter().cloned())
-                .map(|(range, data)| ((range.start - start_pos)..(range.end - start_pos), data))
-                .collect()
-        };
+        // This is hot enough for `deep-vector` that checking the conditions for an empty iterator
+        // is measurably faster than actually executing the iterator.
+        let replace_ranges: Box<[ReplaceRange]> =
+            if replace_ranges_start == replace_ranges_end && inner_attr_replace_ranges.is_empty() {
+                Box::new([])
+            } else {
+                // Grab any replace ranges that occur *inside* the current AST node.
+                // We will perform the actual replacement when we convert the `LazyAttrTokenStream`
+                // to an `AttrTokenStream`.
+                self.capture_state.replace_ranges[replace_ranges_start..replace_ranges_end]
+                    .iter()
+                    .cloned()
+                    .chain(inner_attr_replace_ranges.iter().cloned())
+                    .map(|(range, data)| ((range.start - start_pos)..(range.end - start_pos), data))
+                    .collect()
+            };
 
         let tokens = LazyAttrTokenStream::new(LazyAttrTokenStreamImpl {
             start_token,