From 8a390bae06dbb5686b581797b8f46cb0353dc255 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Wed, 10 Jul 2024 14:41:39 +1000
Subject: [PATCH] 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.
---
 .../rustc_parse/src/parser/attr_wrapper.rs    | 31 ++++++++++---------
 1 file changed, 16 insertions(+), 15 deletions(-)

diff --git a/compiler/rustc_parse/src/parser/attr_wrapper.rs b/compiler/rustc_parse/src/parser/attr_wrapper.rs
index 67d2424bb48..e90568f177c 100644
--- a/compiler/rustc_parse/src/parser/attr_wrapper.rs
+++ b/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,