Some expect String while others expect &str.
Once you understand why, it’s easy to intuit which one should be used where, but it was a day 1 speed bump for me.
Like, if you have fn f(s: &str), you can do:
let x: String = String::from(something);
f(&x);
To explain: both String and &str is essentially a pointer plus metadata, but &String is a double pointer (a pointer to a pointer). Rust knows how to convert double pointers to pointers, using the Deref trait. This implicit conversion is applied only in a handful of places, and only if it doesn't lead to any ambiguity (if f received a generic parameter it wouldn't work for example).
If you wanted to convert String to &str explictly, you would need to write &*x, like this (see in the playground here [2]):
let x: String = String::from(something);
f(&*x);
[0] https://doc.rust-lang.org/book/ch15-02-deref.html#implicit-d...
[1] https://play.rust-lang.org/?version=stable&mode=debug&editio...
[2] https://play.rust-lang.org/?version=stable&mode=debug&editio...*
[3] https://doc.rust-lang.org/alloc/string/struct.String.html#im...