[Animats]

That's for local variables. Microsoft and Linus Torvalds didn't like it, because it's a way to suddenly cause unexpected stack growth of arbitrary size. That feature was made optional in C++11, and Microsoft never implemented it.

[Arnavion]

FWIW Microsoft does have SAL annotations to do the same thing. For example fread's prototype is

    size_t fread(
        _Out_writes_bytes_(_ElementSize*_Count) void * _DstBuf,
        _In_ size_t _ElementSize,
        _In_ size_t _Count,
        _Inout_ FILE * _File
    );

https://docs.microsoft.com/en-us/visualstudio/code-quality/a...

[MaulingMonkey]

C++ compilers also have references to arrays which can be abused in some cases:

    template < size_t len > int read(int fd, char (&buf)[len]); // array size will be infered
    int read(int fd, char (&buf)[1024]); // array size must be exactly 1024