According to #14767 and the grammar right above this documentation, nested comments are supported.
line_comment : "//" non_eol * ;
~~~~
-Comments in Rust code follow the general C++ style of line and block-comment forms,
-with no nesting of block-comment delimiters.
+Comments in Rust code follow the general C++ style of line and block-comment forms.
+Nested block comments are supported.
Line comments beginning with exactly _three_ slashes (`///`), and block
comments beginning with exactly one repeated asterisk in the block-open