Improve handling of spans around macro result parse errors

Fixes #81543

After we expand a macro, we try to parse the resulting tokens as a AST
node. This commit makes several improvements to how we handle spans when
an error occurs:

* Only ovewrite the original `Span` if it's a dummy span. This preserves
  a more-specific span if one is available.
* Use `self.prev_token` instead of `self.token` when emitting an error
  message after encountering EOF, since an EOF token always has a dummy
  span
* Make `SourceMap::next_point` leave dummy spans unused. A dummy span
  does not have a logical 'next point', since it's a zero-length span.
  Re-using the span span preserves its 'dummy-ness' for other checks

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index f2fcce5c226..5512e849c45 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -1104,7 +1104,7 @@ impl<'a> Parser<'a> {
         let (prev_sp, sp) = match (&self.token.kind, self.subparser_name) {
             // Point at the end of the macro call when reaching end of macro arguments.
             (token::Eof, Some(_)) => {
-                let sp = self.sess.source_map().next_point(self.token.span);
+                let sp = self.sess.source_map().next_point(self.prev_token.span);
                 (sp, sp)
             }
             // We don't want to point at the following span after DUMMY_SP.
@@ -1721,7 +1721,7 @@ impl<'a> Parser<'a> {
     pub(super) fn expected_expression_found(&self) -> DiagnosticBuilder<'a> {
         let (span, msg) = match (&self.token.kind, self.subparser_name) {
             (&token::Eof, Some(origin)) => {
-                let sp = self.sess.source_map().next_point(self.token.span);
+                let sp = self.sess.source_map().next_point(self.prev_token.span);
                 (sp, format!("expected expression, found end of {}", origin))
             }
             _ => (