Update substring match for substitutions

Author: faculerena

compiler/rustc_errors/src/lib.rs

@@ -306,20 +306,36 @@ impl TrimmedSubstitutionPart {
 /// `BB` is. Return the length of the prefix, the "trimmed" suggestion, and the length
 /// of the suffix.
 fn as_substr<'a>(original: &'a str, suggestion: &'a str) -> Option<(usize, &'a str, usize)> {
-    let common_prefix = original
+    // Find common suffix by iterating backward through characters.
+    // We search from the end first to prefer complete suffix matches,
+    // which fixes prefix finding in cases like "sync" -> "std::sync"
+    // where we want to recognize "sync" as the full suffix.
+    let suffix: usize = original
         .chars()
-        .zip(suggestion.chars())
+        .rev()
+        .zip(suggestion.chars().rev())
         .take_while(|(c1, c2)| c1 == c2)
         .map(|(c, _)| c.len_utf8())
         .sum();
-    let original = &original[common_prefix..];
-    let suggestion = &suggestion[common_prefix..];
-    if suggestion.ends_with(original) {
-        let common_suffix = original.len();
-        Some((common_prefix, &suggestion[..suggestion.len() - original.len()], common_suffix))
-    } else {
-        None
+
+    // After removing the common suffix, find any common prefix
+    let prefix: usize = original[..original.len() - suffix]
+        .chars()
+        .zip(suggestion[..suggestion.len() - suffix].chars())
+        .take_while(|(c1, c2)| c1 == c2)
+        .map(|(c, _)| c.len_utf8())
+        .sum();
+
+    // Check that the original is fully covered by prefix + suffix (nothing in the middle)
+    if prefix + suffix != original.len() {
+        return None;
     }
+
+    // Extract and return the sandwiched part from the suggestion
+    let sandwiched = &suggestion[prefix..suggestion.len() - suffix];
+
+    // Only succeed if suggestion is actually different from original
+    (sandwiched.len() > 0 || original.is_empty()).then_some((prefix, sandwiched, suffix))
 }
 
 impl CodeSuggestion {
@@ -425,7 +441,7 @@ impl CodeSuggestion {
                     return None;
                 }
 
-                let mut highlights = vec![];
+                let mut highlights: Vec<Vec<SubstitutionHighlight>> = vec![];
                 // To build up the result, we do this for each span:
                 // - push the line segment trailing the previous span
                 //   (at the beginning a "phantom" span pointing at the start of the line)