Macro concat_str 

macro_rules! concat_str {
    (
        @pre {$($pre:stmt,)*},
        @strname $strname:ident,
        @len $($len:expr)?, 
        @appends {$($appends:expr,)*},
        $head:literal $(, $($tail:tt)*)?
    ) => { ... };
    (
        @pre {$($pre:stmt,)*},
        @strname $strname:ident,
        @len $($len:expr)?, 
        @appends {$($appends:expr,)*},
        $head:ident $(, $($tail:tt)*)?
    ) => { ... };
    (
        @pre {$($pre:stmt,)*},
        @strname $strname:ident,
        @len $($len:expr)?, 
        @appends {$($appends:expr,)*},
        $head:expr $(, $($tail:tt)*)?
    ) => { ... };
    (
        @pre {$($pre:stmt,)*},
        @strname $strname:ident,
        @len $len:expr,
        @appends {$($appends:expr,)*},
    ) => { ... };
    ($($tt:tt)+) => { ... };
}

Concatenates a sequence of string-like arguments into a new String.

This macro offers high-performance string concatenation by calculating the total length of all arguments upfront and performing only a single memory allocation. It serves as a more efficient alternative to repeated use of the + operator or format! when all arguments are string-based.

Arguments

The macro accepts a comma-separated list of arguments that can be converted to &str:

• String literals (e.g., "hello").
• &String variables (e.g., &my_string).
• &str slices (e.g., my_slice).
• Expressions that evaluate to a string type (e.g., &user.name or &num.to_string()).

Panics

The macro itself does not introduce panics. However, expressions passed to it might panic, for example, due to indexing out of bounds.

Examples

Basic usage:

let s = concat_str!("the quick ", "brown ", "fox");
assert_eq!(s, "the quick brown fox");

Mixing literals and variables:

let color = "brown";
let animal = String::from("fox");
let sentence = concat_str!("the quick ", color, " ", &animal, " jumps over...");
assert_eq!(sentence, "the quick brown fox jumps over...");

Using expressions:

let num = 42;
let text = concat_str!("The answer is ", &num.to_string(), ".");
assert_eq!(text, "The answer is 42.");