lang::callback_shorthand

A callback written without |v|: a std free function named in value position stands for the closure that calls it, as in xs.map(math::abs).

A callback slot takes a closure. When that closure would do nothing but call one std function on its parameter, name the function instead.

A std function in value position

Naming a std free function where a callback is expected takes the closure that calls it:

use std::{encoding::base64, math}

fn main() {
    println!("{:?}", #[1.0, -2.0].map(math::abs))
    println!("{:?}", #["ab", "cd"].map(base64::encode))
}

Each of those is the closure |v| math::abs(v) / |v| base64::encode(v). The parameter count comes from the function's own signature, so a std item with no fixed parameter list has no closure to stand for and reports GT0015. A macro is not a function at all: fmt::format reports GR0018 and is written format!(..), inside a closure of your own:

fn main() {
    println!("{:?}", #[1, 2].map(|v| format!("<{}>", v)))
}

Everything else is a written closure

There is no projection shorthand. A method call, a field read, an index, and a tuple projection are each written as the closure they are:

struct Person { name: String }

fn main() {
    println!("{:?}", #[" a ", " b "].map(|v| v.trim()))
    println!("{:?}", #[(1, 2), (3, 4)].map(|t| t.0))

    let people = #[Person { name: "ada" }, Person { name: "grace" }]
    println!("{:?}", people.map(|p| p.name))
}

$ is not a callback shorthand, and is not part of the language at all: xs.map($.abs) reports GP0027. See |> for the closure a pipe step is written as.