Implement hash_map macro

[stifskere]

Implementation of #144032

Implements the hash_map macro under std/src/macros.rs.

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[NyxCode]

Wouldn't using one of HashMap::with_capacity(), HashMap::reserve(), FromIterator<(K, V)>, Extend<(K, V)> or From<[(K, V); N]> be better, avoiding unnecessary allocations?

[stifskere]

Wouldn't using one of HashMap::with_capacity(), HashMap::reserve(), FromIterator<(K, V)>, Extend<(K, V)> or From<[(K, V); N]> be better, avoiding unnecessary allocations?

I was thinking about HashMap::with_capacity() but I couldn't see how I could convert a repetition into a number.

From... would essentially be the same, just reallocation, so for clarity I just expanded into insert.

I could see that 0 $(+ 1)* could be evaluated constantly by the compiler, thus we could get a size, but I'm not sure if it's the best way.
It could not be the best way, because that expansion doesn't at all include a reference to the repetition declaration, so it can't be. There is no logical way of getting the size with a macro_rules unless with more complexity we make a recursive macro, which would limit the hash_map size to 254 without extra compiler parameters.

I will add a question about the size in the tracking issue.

[NyxCode]

All the options i mentioned should be equivalent to with_capacity as far as I can tell, though I don't know if the optimizer can look through all options equally well (e.g. wrt. stack usage).
Both these should work, for example:

let len = 1 $(+ {stringify!($k); 1})*;
let mut map = HashMap::with_capacity(len);
$( map.insert($k, $v); )*

HashMap::from([$(($k, $v)),*])