Replace overflowing sub with checked_sub.

In the case when a no-break-space \u{a0} exists before any non
white space characters an error is generated that points into
the white space. When the terminal width is also too small a
subtraction is triggered here that would overflow. That
subtraction implicitly assumed that no errors occur in the
white space. This commit replaces that subtraction with an
explicitly checked one to avoid this overflow.

Author: uwaces

compiler/rustc_errors/src/emitter.rs

@@ -140,7 +140,14 @@ impl Margin {
 
         if self.computed_right - self.computed_left > self.column_width {
             // Trimming only whitespace isn't enough, let's get craftier.
-            if self.label_right - self.whitespace_left <= self.column_width {
+
+            // Note: self.label_right - self.whitespace_left will overflow in the case that the label
+            //       is addressing whitespace. This happens when a spurious no-breaking-space (\u{a0})
+            //       is present in the whitespace.
+            if let Some(non_whitespace_label_width) =
+                self.label_right.checked_sub(self.whitespace_left)
+                && non_whitespace_label_width <= self.column_width
+            {
                 // Attempt to fit the code window only trimming whitespace.
                 self.computed_left = self.whitespace_left;
                 self.computed_right = self.computed_left + self.column_width;