Always emit trailing slash error

Author: estebank

src/libsyntax/parse/unescape_error_reporting.rs

@@ -191,7 +191,7 @@ pub(crate) fn emit_unescape_error(
             handler.span_err(span, "empty character literal")
         }
         EscapeError::LoneSlash => {
-            panic!("lexer accepted unterminated literal with trailing slash")
+            handler.span_err(span, "invalid trailing slash in literal")
         }
     }
 }

src/test/ui/parser/issue-62913.rs

@@ -0,0 +1,3 @@
+"\u\\"
+//~^ ERROR incorrect unicode escape sequence
+//~| ERROR invalid trailing slash in literal

src/test/ui/parser/issue-62913.stderr

@@ -0,0 +1,16 @@
+error: incorrect unicode escape sequence
+  --> $DIR/issue-62913.rs:1:2
+   |
+LL | "\u\"
+   |  ^^^ incorrect unicode escape sequence
+   |
+   = help: format of unicode escape sequences is `\u{...}`
+
+error: invalid trailing slash in literal
+  --> $DIR/issue-62913.rs:1:5
+   |
+LL | "\u\"
+   |     ^
+
+error: aborting due to 2 previous errors
+