Avoid possible infinite loop when next_point reaching the end of file

compiler/rustc_span/src/source_map.rs

@@ -855,7 +855,8 @@ impl SourceMap {
     /// Returns a new span representing the next character after the end-point of this span.
     /// Special cases:
     /// - if span is a dummy one, returns the same span
-    /// - if next_point reached the end of source, return span with lo = hi
+    /// - if next_point reached the end of source, return a span exceeding the end of source,
+    ///   which means sm.span_to_snippet(next_point) will get `Err`
     /// - respect multi-byte characters
     pub fn next_point(&self, sp: Span) -> Span {
         if sp.is_dummy() {
@@ -864,9 +865,6 @@ impl SourceMap {
         let start_of_next_point = sp.hi().0;
 
         let width = self.find_width_of_character_at_span(sp, true);
-        if width == 0 {
-            return Span::new(sp.hi(), sp.hi(), sp.ctxt(), None);
-        }
         // If the width is 1, then the next span should only contain the next char besides current ending.
         // However, in the case of a multibyte character, where the width != 1, the next span should
         // span multiple bytes to include the whole character.
@@ -938,7 +936,7 @@ impl SourceMap {
         // Ensure indexes are also not malformed.
         if start_index > end_index || end_index > source_len - 1 {
             debug!("find_width_of_character_at_span: source indexes are malformed");
-            return 0;
+            return 1;
         }
 
         let src = local_begin.sf.external_src.borrow();

compiler/rustc_span/src/source_map/tests.rs

@@ -511,16 +511,17 @@ fn test_next_point() {
     assert_eq!(span.lo().0, 4);
     assert_eq!(span.hi().0, 5);
 
-    // A non-empty span at the last byte should advance to create an empty
-    // span pointing at the end of the file.
+    // Reaching to the end of file, return a span that will get error with `span_to_snippet`
     let span = Span::with_root_ctxt(BytePos(4), BytePos(5));
     let span = sm.next_point(span);
     assert_eq!(span.lo().0, 5);
-    assert_eq!(span.hi().0, 5);
+    assert_eq!(span.hi().0, 6);
+    assert!(sm.span_to_snippet(span).is_err());
 
-    // Empty span pointing just past the last byte.
+    // Reaching to the end of file, return a span that will get error with `span_to_snippet`
     let span = Span::with_root_ctxt(BytePos(5), BytePos(5));
     let span = sm.next_point(span);
     assert_eq!(span.lo().0, 5);
-    assert_eq!(span.hi().0, 5);
+    assert_eq!(span.hi().0, 6);
+    assert!(sm.span_to_snippet(span).is_err());
 }

src/test/ui/parser/issue-103451.rs

@@ -0,0 +1,5 @@
+// error-pattern: this file contains an unclosed delimiter
+// error-pattern: expected value, found struct `R`
+struct R { }
+struct S {
+    x: [u8; R

src/test/ui/parser/issue-103451.stderr

@@ -0,0 +1,32 @@
+error: this file contains an unclosed delimiter
+  --> $DIR/issue-103451.rs:5:15
+   |
+LL | struct S {
+   |          - unclosed delimiter
+LL |     x: [u8; R
+   |        -      ^
+   |        |
+   |        unclosed delimiter
+
+error: this file contains an unclosed delimiter
+  --> $DIR/issue-103451.rs:5:15
+   |
+LL | struct S {
+   |          - unclosed delimiter
+LL |     x: [u8; R
+   |        -      ^
+   |        |
+   |        unclosed delimiter
+
+error[E0423]: expected value, found struct `R`
+  --> $DIR/issue-103451.rs:5:13
+   |
+LL | struct R { }
+   | ------------ `R` defined here
+LL | struct S {
+LL |     x: [u8; R
+   |             ^ help: use struct literal syntax instead: `R {}`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0423`.