Add impl From<&String> for String

[jsgf]

If a function takes impl Into<String> it would be nice to pass it a &String (say, from pattern matching) without additional friction.

#59825 implements this.

[Stargateur]

that could hide unnecessary clone

[jsgf]

This is functionally identical to the existing From<&str> for String impl.

The clone would be necessary - if the receiving function needs to own the String, and all you have is a &String, then someone has to do a clone. If it didn't need to own it then it could take &str (or impl AsRef<str>). And if you had a String to pass in, then it would hit the no-op impl From<T> for T blanket impl.

[hellow554]

Do you have an example why this is useful? &String coerces to &str so there is no need to implement this (also https://stackoverflow.com/questions/40006219)

[jsgf]

@hellow554 > &String coerces to &str

No it doesn't, but it Derefs to it. The specific case I had was:

fn somefunction(s: impl Into<String>) {
    println!("a String: {}", s.into())
}

match &someenum {
SomeCase { string_field } => somefunction(string_field), // string_field: &String
...
}

Currently the nicest way to handle this is somefunction(string_field.as_str()), since there's a From<&str> for String.

[hellow554]

Okay, that's true. But you will have the same issue with Vec, &Vec and &[_]. What about that?

[jsgf]

To quote me: #59825 (comment)