These #[...] and #![...] are not comments; they're attributes (somewhat similar to Java's @Something annotations). Comments in Rust start with // or /<star> like in C or C++. There's only the special case that "doc comments" (starting with /// or /<star><star> or //! or /<star>!) are automatically converted into #[doc="..."] or #![doc="..."] attributes.

The difference is that attributes are always meaningful to the compiler (and can only be used in specific places); other than doc comments (which turn into attributes), comments never are meaningful to the compiler.

(edit: HN's comment markup eats the * character, I replaced them with <star> above)