I'm not even talking about variably sized arrays, just creating a statically sized one and passing it into functions that take dynamically-sized one. For instance, a read function that fills an existing buffer doesn't care if the buffer is on the heap or on the stack, it only cares that it doesn't overrun the bounds.

alloca-style variable arrays is a whole other can of worms of danger and complexity.

What do you mean? Variable-length arrays are OK in C.

Not in C11, they were removed due to security issues and became an optional language feature.

oops -- you're right.

https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html

Legal in C99, available in C++ on gcc/clang.

Optional in C11, not accepted in ANSI C++17.

This is what I really like about Ada. Variable-sized arrays and structs on the stack is easy, safe, and efficient in Ada.