Is there a language that makes this explicit, allocates the variables on the stack via compiler enforced notation?
Heap allocation is what requires requesting memory from an allocator.
I think only in the sense that some GCd language have value (stack) types as a separate hierarchy from heap types? E.g. structs in C# or Swift are stack-allocated (and value-identity, and copied) whereas classes are heap-allocated.
Adding that for java is one of the goals of Project Valhalla I believe.
T vs T*.
It would be kind of neat if you could have an annotation on the variable instead that didn't change the type.
You could in C++ make a reference T& which is almost that - references behave identically to the real thing. But I think freeing the memory backing a reference is probably quite questionable?
Well, variables cannot be forced to stack specifically. They are placed in the "local scope". And that would usually be either stack or CPU registers - thinking in stack only is a somewhat flawed mental model.
Both C# and F# complicate this by supporting closures, iterator and async methods which capture variables placing them in a state machine box / display class instead which would be located on the heap, unless stack-allocated by escape analysis (unlikely because these usually cross method boundaries).
However, .NET has `ref structs` (or, in F#, [<Struct; IsByRefLike>] types) which are subject to lifetime analysis and can never be placed on the heap directly or otherwise.