Why does this rust HashMap macro no longer work? - rust

Question

I previously used:
#[macro_export]
macro_rules! map(
{ T:ident, $($key:expr => $value:expr),+ } => {
{
let mut m = $T::new();
$(
m.insert($key, $value);
)+
m
}
};
)
To create objects, like this:
let mut hm = map! {HashMap, "a string" => 21, "another string" => 33};
However, this no longer seems to work. The compiler reports:
- Failed:
macros.rs:136:24: 136:31 error: no rules expected the token `HashMap`
macros.rs:136 let mut hm = map! {HashMap, "a string" => 21, "another string" => 33};
^~~~~~~
What's changed with macro definitions that makes this no longer work?
The basic example below works fine:
macro_rules! foo(
{$T:ident} => { $T; };
)
struct Blah;
#[test]
fn test_map_create() {
let mut bar = foo!{Blah};
}
So this seem to be some change to how the {T:ident, $(...), +} expansion is processed?
What's going on here?

Answer 1

You’re lacking the $ symbol before T.

Answer 2

How about this play.rust-lang.org?
// nightly rust
#![feature(type_name_of_val)]
use std::collections::{BTreeMap, HashMap};
macro_rules! map {
($T:ty; $( $key:literal : $val:expr ),* ,) => {
{
// $T is the full HashMap<String, i32>, <$T> is just HashMap
let mut m: $T = <$T>::new();
$(
m.insert($key.into(), $val);
)*
m
}
};
// handle no tailing comma
($T:ty; $( $key:literal : $val:expr ),*) => {
map!{$T; $( $key : $val ,)*}
}
}
fn main() {
let hm = map! {HashMap<String, i32>;
"a": 1,
"b": 2,
"c": 3,
};
let bm = map! {BTreeMap<String, i32>;
"1": 1,
"2": 2,
"3": 3
};
println!("typeof hm = {}", std::any::type_name_of_val(&hm));
println!("typeof bm = {}", std::any::type_name_of_val(&bm));
dbg!(hm, bm);
}
Additional, use {}, macro_rules map {} means it will return an item