> The claim that using reserve_exact "throws away the amortized growth promise" is wrong
Well, in some sense this depends on what exactly you think you're amortizing over. If you `Vec::reserve` with size N, fill to N, and then append a single element you get the usual amortized O(1) growth of an append (or at least you can, the docs for `Vec::reserve` say it may reserve additional space, not that it must). But if you `Vec::reserve_exact` with size N, fill to N, and then append a single element you are guaranteeing that that first append triggers a potentially O(N) resize.
From that point on in either case you would get the usual amortized growth of course.
> if you `Vec::reserve_exact` with size N, fill to N, and then append a single element you are guaranteeing that that first append triggers a potentially O(N) resize.
The documentation does not guarantee this, because memory allocators can't typically allocate arbitrarily-sized blocks of memory, instead rounding to the nearest supported size. For example, for small allocations glibc malloc allocates in multiples of 8 bytes, with a minimum size of 24 bytes. So if you make a 35-byte allocation, there will be 5 bytes of wasted space at the end which you could theoretically use to store more elements without reallocating if your collection grows.
If you're using the system allocator, Rust can't take advantage of this, because the C malloc/free APIs don't provide any (portable) way for the allocator to inform the application about this excess capacity. But Rust's (currently unstable) API for custom allocators does expose this information, and the documentation is written to allow Vec to take advantage of this information if available. If you reserve_exact space for 35 u8's, and your allocator rounds to 40 bytes (and informs the Vec of this the allocator API), then the vector is allowed to set its capacity to 40, meaning the next append would not trigger a resize.
On current stable Rust, this is all just theoretical and Vec works as you describe -- but the documentation specifically does not promise this because the situation is expected to change in the future.
Sure, but this isn't a job for reserve_exact. Vec::reserve is like a framing hammer made for speed and momentum. You might dent a little extra wood (overallocate), but you drive lots of nails fast and keep amortized O(1) growth.
reserve_exact is a jeweler's hammer so great when you know the exact setting and won't touch it again, precise fit, zer slack etc.
Now, try to frame a house with jeweler's hammer, tapping in tiny increments and resizing every few boards and you will crawl into quadratic time.
Who is using jeweler's hammer to frame their house?
So what I'm arguing is if you don't misuse reserve_exact, then as you said you still get amortized growth. The OP's example misuses the tool and then blames it for not behaving differently on that first append.
Well, in some sense this depends on what exactly you think you're amortizing over. If you `Vec::reserve` with size N, fill to N, and then append a single element you get the usual amortized O(1) growth of an append (or at least you can, the docs for `Vec::reserve` say it may reserve additional space, not that it must). But if you `Vec::reserve_exact` with size N, fill to N, and then append a single element you are guaranteeing that that first append triggers a potentially O(N) resize.
From that point on in either case you would get the usual amortized growth of course.