Functions like isMinGW() and isGnuLibC() have a good reason to exist: They look
at multiple components of the target. But functions like isWasm(), isDarwin(),
isGnu(), etc only exist to save 4-8 characters. I don't think this is a good
enough reason to keep them, especially given that:
* It's not immediately obvious to a reader whether target.isDarwin() means the
same thing as target.os.tag.isDarwin() precisely because isMinGW() and similar
functions *do* look at multiple components.
* It's not clear where we would draw the line. The logical conclusion before
this commit would be to also wrap Arch.isX86(), Os.Tag.isSolarish(),
Abi.isOpenHarmony(), etc... this obviously quickly gets out of hand.
* It's nice to just have a single correct way of doing something.
This allocator has no purpose since it cannot truly fulfill the role of
page allocation, and std.heap.wasm_allocator is better both in terms of
performance and code size.
This commit redefines `std.heap.page_allocator` to be less strict:
"On operating systems that support memory mapping, this allocator makes
a syscall directly for every allocation and free. Otherwise, it falls
back to the preferred singleton for the target. Thread-safe."
This now matches how it was actually being implemented, and matches its
use sites - which are mainly as the backing allocator for
`std.heap.ArenaAllocator`.
Most of this migration was performed automatically with `zig fmt`. There
were a few exceptions which I had to manually fix:
* `@alignCast` and `@addrSpaceCast` cannot be automatically rewritten
* `@truncate`'s fixup is incorrect for vectors
* Test cases are not formatted, and their error locations change
Now it can refuse to resize when it would disturb the metadata tracking
strategy, resulting in smaller code size, a simpler implementation, and
less fragmentation.
The previous version had a fatal flaw: it did ensureCapacity(1) on the
freelist when allocating, but I neglected to consider that you could
free() twice in a row. Silly!
This strategy allocates an intrusive freelist node with every
allocation, big or small. It also does not have the problems with resize
because in this case we can push the upper areas of freed stuff into the
corresponding freelist.
