Lowercase (i8, u64) or anonymity (`[]`, &, *) means you're dealing with something fundamental to the language. Not just with special compiler/language support (like Result) but way below that, something the language doesn't itself express but has intrinsic knowledge of.
Because one is a pretty bog-standard struct: https://doc.rust-lang.org/src/alloc/string.rs.html#294-296 while the other is a primitive type: https://doc.rust-lang.org/std/#primitives
Lowercase (i8, u64) or anonymity (`[]`, &, *) means you're dealing with something fundamental to the language. Not just with special compiler/language support (like Result) but way below that, something the language doesn't itself express but has intrinsic knowledge of.