Auto merge of #29454 - stepancheg:vec-reserve, r=bluss

Before this patch `reserve` function allocated twice as requested
amount elements (not twice as capacity).  It leaded to unnecessary
excessive memory usage in scenarios like this:

```
let mut v = Vec::new();
v.push(17);
v.extend(0..10);
println!("{}", v.capacity());
```

`Vec` allocated 22 elements, while it could allocate just 11.

`reserve` function must have a property of keeping `push` operation
cost (which calls `reserve`) `O(1)`. To achieve this `reserve` must
exponentialy grow its capacity when it does reallocation.

There's better strategy to implement `reserve`:

```
let new_capacity = max(current_capacity * 2, requested_capacity);
```

This strategy still guarantees that capacity grows at `O(1)` with
`reserve`, and fixes the issue with `extend`.

Patch imlpements this strategy.

0 files changed, 0 insertions, 0 deletions