A more convenient concatenation with a string lite

问题:

In the nightly Rust it is no longer possible to designate a string literal as String with a "~" character.

In C++, for example, I'm using user-defined literals to concatenate string literals without the crust of mentioning std::string every time:

inline std::string operator"" _s (const char* str, size_t size) {return std::string (str, size);}
foo ("Hello, "_s + "world!");

Is there a similar feature existing or planned in Rust to make string literal concatenation less painful than String::from_str ("Hello, ") + "world!"?

回答1:

If you literally (hah) have string literals, you can use the concat! macro:

let lit = concat!("Hello, ", "world!")

You can natively split strings over several lines:

let lit = "Hello, \
           World";

The \ consumes all following whitespace, including the leading spaces on the next line; omitting the \ will include the string data "verbatim", with newlines and leading spaces etc.

You can add a &str to a String:

let s = "foo".to_string() + "bar" + "baz";

You could use push_str iteratively:

let mut s = "foo".to_string();
s.push_str("bar");
s.push_str("baz");

You could use SliceConcatExt::concat:

let s = ["foo", "bar", "baz"].concat();

If all else fails, you can define a macro to do exactly what you want.

See also:

• How to concatenate static strings in Rust
• What is the syntax for a multiline string literal?
• How do I concatenate strings?

回答2:

You can use the format! macro. It is more readable, more translation-friendly, more efficient, and more powerful (you can concatenate more than just strings, just like C++'s ostringstream). It is also completely type-safe.

format!("Hello, {}", "world!")

You can also use named arguments to improve readability.

format!("hello, {who}", who = "world")

The full formatting syntax is described in std::fmt.

Rust does not have user-defined literals. I think adding such a feature is backward-compatible, so maybe this feature will be added after Rust 1.0.

回答3:

You can concatenate str literals without using String with the concat! macro:

let input = concat!("Hello", ' ', "world");

To make it a string, specify the destination type and use into:

let input: String = concat!("Hello", ' ', "world").into();

Full program:

fn main() {
    let input: String = concat!("Hello", ' ', "world").into();
    println!("{}", input);  // Output: Hello world
}