HACL* HMAC does not properly use vectorized BLAKE-2

Bug report

Bug description:

First of all, it doesn't really make sense to support HMAC-BLAKE-2 because BLAKE-2 is already a keyed hash function so it already has the properties expected by HMAC. Since HACL* provides it (I don't remember if I actually asked them to, so if I did, it's my fault), I decided to expose it anyway.

HACL* provides SIMD-accelerated implementation for BLAKE-2 which is exposed if the HACL_CAN_COMPILE_VEC128 and HACL_CAN_COMPILE_VEC256 macros are defined. CPython, however, needs to do a bit more work to be able to know whether we can define those macros or not. The work is achieved in two steps:

• In configure, depending on the host capabilities, the macros are defined.
• When compiling, depending on the host capabilities once again, the macros are undefined.

The generic headers are afterwards included, e.g.:

#define HACL_CAN_COMPILE_VEC128 _Py_HACL_CAN_COMPILE_VEC128
#define HACL_CAN_COMPILE_VEC256 _Py_HACL_CAN_COMPILE_VEC256

#include "_hacl/Hacl_HMAC.h"
#include "_hacl/Hacl_Streaming_HMAC.h"  // Hacl_Agile_Hash_* identifiers
#include "_hacl/Hacl_Streaming_Types.h" // Hacl_Streaming_Types_error_code

Now, there is a slight issue. The conditional code is actually inside the .c file and not the .h. However, that .c file is an HACL* file that is compiled separately and doesn't have any defines. It also unconditionally includes Hacl_Hash_Blake2s_Simd128.h and Hacl_Hash_Blake2s_Simd256.h files. So I'm not entirely sure about what to do :(

Am I incorrectly using the files @msprotz?

CPython versions tested on:

CPython main branch, 3.14, 3.15

Operating systems tested on:

No response

Linked PRs

• gh-146118

Start by reading configure and the HACL* HMAC integration around Hacl_HMAC.h, Hacl_Streaming_HMAC.h, Hacl_Streaming_Types.h, and the separately compiled HACL* .c file. Trace how HACL_CAN_COMPILE_VEC128 and HACL_CAN_COMPILE_VEC256 are defined or undefined, then verify that HMAC-BLAKE-2 uses the supported vectorized implementation on capable hosts without breaking other builds.