Ergonomic string literal interpolation in macro definitions.
Replaces metavariables ($foo) and arbitrary expressions in string literals (including doc comments) and concatenates them with surrounding text fragments with core::concat!.
```rust
macro_rules! mymacro {
($count:expr, $name:expr, fn $func:ident()) => {
/// Returns "$$ $name, ${stringify!($count)} to beam up".
fn $func() -> &'static str {
"$$ $name, ${stringify!($count)} to beam up"
}
};
}
// The above definition expands to:
//
// macro_rules! mymacro {
// ($count:expr, $name:expr, fn $func:ident()) => {
// #[doc = concat!("Returns \"$ ", $name, ", ",
// stringify!($count), " to beam up\".")]
// fn func() -> &'static str {
// concat!("$ ", $name, ", ",
// stringify!($count), " to beam up")
// }
// };
// }
//
mymacro!(3, "Scotty", fn func());
assert_eq!(func(), "$ Scotty, 3 to beam up"); ```