mi_heap_visit_blocks callback is missing explicit calling convention

[seanmiddleditch]

The callback mi_block_visit_fun is missing a calling convention specification (e.g. __cdecl or the like), which can cause linkage problems when mixing a mimalloc library compiled with one ABI but linked into a target that uses a different default ABI (e.g. using the /Gv switch that can be common in performance-oriented codebases).

Its standard practice in Windows code to always tag a calling convention on callback function declarations to avoid this issue. Windows headers use the CALLBACK macro for this purpose though specifying a new macro for mimalloc's purpose would suffice too so the code remains portable.

The idea is that this function can be declared in mimalloc.h using this specifier and then any user code that is defining a callback for mi_heap_visit_blocks would use the same specifier macro for the callback declaration, guaranteeing compatibility even if the default calling conventions differ.

[daanx]

Thanks for spotting this. I will add this.

[daanx]

Fixed in the latest dev branch