Coercions

Type coercions are implicit operations that change the type of a value. They happen automatically at certain locations when the expected type doesn't match the actual type of an expression.

The locations where coercions can happen are called "coercion sites" and listed in this Reference section. The allowed coercions are then listed here.

In this step, we desugar these coercions into explicit conversions. The outcome is either an as-cast ($expr as $ty) or a reborrow (e.g. &*$expr).

For example:

#![allow(unused)]
fn main() {
fn foo(s: &str) { .. }
let x: String = ...;
let string_ref: &String = &x;
foo(string_ref);

// becomes:
foo(&**string_ref);
}

#![allow(unused)]
fn main() {
let x = 42u32;
let dyn_x: &dyn Debug = &x;
let meta = core::ptr::metadata(dyn_x);

// becomes:
let dyn_x: &dyn Debug = &x as &dyn Debug;
let meta = core::ptr::metadata(dyn_x as *const dyn Debug);
}

After this step, expressions have the type expected of them.