Rust String Parameter Types

Type	Owned alloc?	Accepts	Use when
&str	No	string slices only	you only read, no conversion needed
impl AsRef<str>	No	&str, String, Cow<str>	read-only, zero-cost generic
impl Into<String>	Yes, on call	&str, String	you need to store/own it
impl ToString	Yes, always	anything with Display	avoid — always allocates
Cow<'a, str>	Sometimes	borrowed or owned	you want to delay allocation
String	Yes	String only	explicit, just take ownership

Practical rules:

• Just reading → &str or impl AsRef<str>
• Need to own/store → impl Into<String> (call .into() inside)
• Avoid impl ToString — it always allocates, use impl Display if you need formatting
• Cow<str> is niche — useful when you sometimes need to mutate/own and sometimes don’t

Most common correct pattern:

// read only
fn foo(s: impl AsRef<str>) {
    let s = s.as_ref();
}
 
// store it
fn foo(s: impl Into<String>) {
    let s = s.into();
}