From 7a0af0973c975b3003c2c31ed08a097349f4c569 Mon Sep 17 00:00:00 2001
From: Mara Bos <m-ou.se@m-ou.se>
Date: Sun, 14 May 2023 16:24:11 +0200
Subject: [PATCH] Simplify find_width_of_character_at_span.

(cherry picked from commit 6289c57dc0ee8ebbe9e20fad808f85aed0afeceb)
---
 compiler/rustc_span/src/lib.rs        |  1 +
 compiler/rustc_span/src/source_map.rs | 31 ++++++---------------------
 2 files changed, 8 insertions(+), 24 deletions(-)

diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index aa8859ed1a358..28a8d8fc1ecc6 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -20,6 +20,7 @@
 #![feature(min_specialization)]
 #![feature(rustc_attrs)]
 #![feature(let_chains)]
+#![feature(round_char_boundary)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
 
diff --git a/compiler/rustc_span/src/source_map.rs b/compiler/rustc_span/src/source_map.rs
index 88e3674f89943..56573814e9039 100644
--- a/compiler/rustc_span/src/source_map.rs
+++ b/compiler/rustc_span/src/source_map.rs
@@ -1019,36 +1019,19 @@ impl SourceMap {
 
         let src = local_begin.sf.external_src.borrow();
 
-        // We need to extend the snippet to the end of the src rather than to end_index so when
-        // searching forwards for boundaries we've got somewhere to search.
-        let snippet = if let Some(ref src) = local_begin.sf.src {
-            &src[start_index..]
+        let snippet = if let Some(src) = &local_begin.sf.src {
+            src
         } else if let Some(src) = src.get_source() {
-            &src[start_index..]
+            src
         } else {
             return 1;
         };
-        debug!("snippet=`{:?}`", snippet);
 
-        let mut target = if forwards { end_index + 1 } else { end_index - 1 };
-        debug!("initial target=`{:?}`", target);
-
-        while !snippet.is_char_boundary(target - start_index) && target < source_len {
-            target = if forwards {
-                target + 1
-            } else {
-                match target.checked_sub(1) {
-                    Some(target) => target,
-                    None => {
-                        break;
-                    }
-                }
-            };
-            debug!("target=`{:?}`", target);
+        if forwards {
+            (snippet.ceil_char_boundary(end_index + 1) - end_index) as u32
+        } else {
+            (end_index - snippet.floor_char_boundary(end_index - 1)) as u32
         }
-        debug!("final target=`{:?}`", target);
-
-        if forwards { (target - end_index) as u32 } else { (end_index - target) as u32 }
     }
 
     pub fn get_source_file(&self, filename: &FileName) -> Option<Lrc<SourceFile>> {