Make span_close() and span_open() more accurate after set_span() call.

When `Group::set_span()` is called the spans for `Group::span_open()` and `Group::span_close() ` methods just become the whole span instead of the spans the delimiters. This ensure this have at least a more useable value after a call to `Group::set_span()`.

Author: Keith-Cancel

library/proc_macro/src/lib.rs

@@ -880,7 +880,20 @@ impl Group {
     /// tokens at the level of the `Group`.
     #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
     pub fn set_span(&mut self, span: Span) {
-        self.0.span = bridge::DelimSpan::from_single(span.0);
+        // This is an invisible delimiter so they have no width, so just use start and end.
+        if matches!(self.0.delimiter, Delimiter::None) {
+            self.0.span =
+                bridge::DelimSpan { open: span.0.start(), close: span.0.end(), entire: span.0 };
+            return;
+        }
+
+        let rng = span.0.byte_range();
+        let beg = (rng.end - rng.start).saturating_sub(1);
+        self.0.span = bridge::DelimSpan {
+            open: span.0.subspan(Bound::Included(0), Bound::Excluded(1)).unwrap_or(span.0.start()),
+            close: span.0.subspan(Bound::Included(beg), Bound::Unbounded).unwrap_or(span.0.end()),
+            entire: span.0,
+        };
     }
 }