perf: only compute start offset for overlong lines

CONTRIBUTING.md

@@ -419,6 +419,13 @@ Summary
   159.43 ± 2.48 times faster than 'pycodestyle crates/ruff/resources/test/cpython'
 ```
 
+To benchmark a subset of rules, e.g. `LineTooLong` and `DocLineTooLong`:
+
+```shell
+cargo build --release && hyperfine --warmup 10 \
+  "./target/release/ruff ./crates/ruff/resources/test/cpython/ --no-cache -e --select W505,E501"
+```
+
 You can run `poetry install` from `./scripts/benchmarks` to create a working environment for the
 above. All reported benchmarks were computed using the versions specified by
 `./scripts/benchmarks/pyproject.toml` on Python 3.11.

crates/ruff/src/rules/pycodestyle/helpers.rs

@@ -53,16 +53,8 @@ pub(super) fn is_overlong(
     task_tags: &[String],
     tab_size: TabSize,
 ) -> Option<Overlong> {
-    let mut start_offset = line.start();
     let mut width = LineWidth::new(tab_size);
-
-    for c in line.chars() {
-        if width < limit {
-            start_offset += c.text_len();
-        }
-        width = width.add_char(c);
-    }
-
+    width = width.add_str(line.as_str());
     if width <= limit {
         return None;
     }
@@ -91,6 +83,17 @@ pub(super) fn is_overlong(
         }
     }
 
+    // Obtain the start offset of the part of te line that exceeds the limit
+    let mut start_offset = line.start();
+    let mut start_width = LineWidth::new(tab_size);
+    for c in line.chars() {
+        if start_width < limit {
+            start_offset += c.text_len();
+            start_width = start_width.add_char(c);
+        } else {
+            break;
+        }
+    }
     Some(Overlong {
         range: TextRange::new(start_offset, line.end()),
         width: width.get(),