fix lexing of comments with many \r

src/libsyntax/parse/lexer/mod.rs

@@ -226,7 +226,7 @@ impl<'a> StringReader<'a> {
                     loop {
                         idx = match string[idx..].find('\r') {
                             None => break,
-                            Some(it) => it + 1
+                            Some(it) => idx + it + 1
                         };
                         if string[idx..].chars().next() != Some('\n') {
                             self.err_span_(start + BytePos(idx as u32 - 1),

src/test/ui/parser/several-carriage-returns-in-doc-comment.rs

@@ -0,0 +1,10 @@
+// Issue #62863
+// ignore-tidy-cr
+
+// Note: if you see ^M in this file, that's how your editor renders literal `\r`
+
+/// This doc comment contains three isolated `\r` symbols
+//~^ ERROR bare CR not allowed in doc-comment
+//~| ERROR bare CR not allowed in doc-comment
+//~| ERROR bare CR not allowed in doc-comment
+fn main() {}

src/test/ui/parser/several-carriage-returns-in-doc-comment.stderr

@@ -0,0 +1,20 @@
+error: bare CR not allowed in doc-comment
+  --> $DIR/several-carriage-returns-in-doc-comment.rs:6:12
+   |
+LL | /// This doc comment contains three isolated `\r` symbols
+   |            ^
+
+error: bare CR not allowed in doc-comment
+  --> $DIR/several-carriage-returns-in-doc-comment.rs:6:32
+   |
+LL | /// This doc comment contains three isolated `\r` symbols
+   |                               ^
+
+error: bare CR not allowed in doc-comment
+  --> $DIR/several-carriage-returns-in-doc-comment.rs:6:52
+   |
+LL | /// This doc comment contains three isolated `\r` symbols
+   |                                                  ^
+
+error: aborting due to 3 previous errors
+