std: fix memory leak in ArrayHashMap

[GethDW]

No description provided.

[andrewrk]

Instead let's swap the order so that the ensureCapacity happens first, and then no cleanup code is needed.

[Jarred-Sumner]

I wonder if this could be a single memory allocation combining the index header and the array of entries

[andrewrk]

The fields for ArrayHashMap are:

        /// It is permitted to access this field directly.
        entries: DataList = .{},

        /// When entries length is less than `linear_scan_max`, this remains `null`.
        /// Once entries length grows big enough, this field is allocated. There is
        /// an IndexHeader followed by an array of Index(I) structs, where I is defined
        /// by how many total indexes there are.
        index_header: ?*IndexHeader = null,

Having the header be a separate allocation is an optimization for maps that are below a certain size threshold.

You are welcome to experiment with changing the memory layout here - if you can demonstrate a perf enhancement without breaking the API, that's an easy merge.

[andrewrk]

Thanks!

[andrewrk]

I'd like to add a test to this before merging, especially since there seems to be some interest in reworking the implementation. Should be pretty straightforward with std.testing.checkAllAllocationFailures.

[andrewrk]

Thanks!