[FIXED] What is the syntax: `instance.method::<SomeThing>()`?

Issue

I read the below syntax from byteorder:

rdr.read_u16::<BigEndian>()

I can't find any documentation which explains the syntax instance.method::<SomeThing>()

Solution

This construct is called turbofish. If you search for this statement, you will discover its definition and its usage.

Although the first edition of The Rust Programming Language is outdated, I feel that this particular section is better than in the second book.

Quoting the second edition:

path::<...>, method::<...>
Specifies parameters to generic type, function, or method in an expression; often referred to as turbofish (e.g., "42".parse::<i32>())

You can use it in any kind of situation where the compiler is not able to deduce the type parameter, e.g.

fn main () {
    let a = (0..255).sum();
    let b = (0..255).sum::<u32>();
    let c: u32 = (0..255).sum();
}

a does not work because it cannot deduce the variable type.
b does work because we specify the type parameter directly with the turbofish syntax.
c does work because we specify the type of c directly.

Answered By - hellow

Answer Checked By - Willingham (PHPFixing Volunteer)