Fix a crash/mislex when more than one frontmatter closing possibility is considered

compiler/rustc_lexer/src/lib.rs

@@ -599,14 +599,16 @@ impl Cursor<'_> {
             if potential_closing.is_none() {
                 // a less fortunate recovery if all else fails which finds any dashes preceded by whitespace
                 // on a standalone line. Might be wrong.
+                let mut base_index = 0;
                 while let Some(closing) = rest.find("---") {
                     let preceding_chars_start = rest[..closing].rfind("\n").map_or(0, |i| i + 1);
                     if rest[preceding_chars_start..closing].chars().all(is_horizontal_whitespace) {
                         // candidate found
-                        potential_closing = Some(closing);
+                        potential_closing = Some(closing + base_index);
                         break;
                     } else {
                         rest = &rest[closing + 3..];
+                        base_index += closing + 3;
                     }
                 }
             }

tests/ui/frontmatter/unclosed-6.rs

@@ -0,0 +1,12 @@
+---
+//~^ ERROR unclosed frontmatter
+🦀---
+ ---
+
+// This test checks the location of the --- recovered by the parser is not
+// incorrectly tracked during the less fortunate recovery case and multiple
+// candidates are found, as seen with #146847
+
+#![feature(frontmatter)]
+
+fn main() {}

tests/ui/frontmatter/unclosed-6.stderr

@@ -0,0 +1,19 @@
+error: unclosed frontmatter
+  --> $DIR/unclosed-6.rs:1:1
+   |
+LL | / ---
+LL | |
+LL | | 🦀---
+LL | |  ---
+...  |
+LL | |
+   | |_^
+   |
+note: frontmatter opening here was not closed
+  --> $DIR/unclosed-6.rs:1:1
+   |
+LL | ---
+   | ^^^
+
+error: aborting due to 1 previous error
+