The end of a span can be *before* the first char in a line

Author: oli-obk

src/librustc_errors/snippet.rs

@@ -70,7 +70,7 @@ impl MultilineAnnotation {
 
     pub fn as_end(&self) -> Annotation {
         Annotation {
-            start_col: self.end_col - 1,
+            start_col: self.end_col.saturating_sub(1),
             end_col: self.end_col,
             is_primary: self.is_primary,
             label: self.label.clone(),