Use underline suggestions for purely 'additive' replacements

compiler/rustc_errors/src/lib.rs

@@ -230,6 +230,17 @@ impl SubstitutionPart {
         !self.snippet.is_empty() && self.replaces_meaningful_content(sm)
     }
 
+    /// Whether this is a replacement that overwrites source with a snippet
+    /// in a way that isn't a superset of the original string. For example,
+    /// replacing "abc" with "abcde" is not destructive, but replacing it
+    /// it with "abx" is, since the "c" character is lost.
+    pub fn is_destructive_replacement(&self, sm: &SourceMap) -> bool {
+        self.is_replacement(sm)
+            && !sm
+                .span_to_snippet(self.span)
+                .is_ok_and(|snippet| self.snippet.trim_start().starts_with(snippet.trim_start()))
+    }
+
     fn replaces_meaningful_content(&self, sm: &SourceMap) -> bool {
         sm.span_to_snippet(self.span)
             .map_or(!self.span.is_empty(), |snippet| !snippet.trim().is_empty())