Add Compiler Error when returning pointer to stack variable

[mbarkhau]

I'm new to zig and low level programming in general. Naturally when I encountered the code for setting up an allocater I tried to encapsulate it.

    var direct = std.heap.DirectAllocator.init();
    defer direct.deinit();
    var arena = std.heap.ArenaAllocator.init(&direct.allocator);
    defer arena.deinit();

I tried to (in retrospect naively) create a struct to take care of the repetition in my tests:

const MyStruct = struct {
    
    _alloc1: *std.heap.DirectAllocator,
    _alloc2: *std.heap.ArenaAllocator,

    pub fn init() !MyStruct {
        var direct = std.heap.DirectAllocator.init();
        var arena = std.heap.ArenaAllocator.init(&direct.allocator);
        return MyStruct{
            ._alloc1 = &direct,
            ._alloc2 = &arena,
        };
    }
    pub fn deinit(self: MyStruct) void {
        self._alloc2.deinit();
        self._alloc1.deinit();
    }
};

When using this I get a segfault though.

In #zig @shawnl was helpful enough to explain the issue. The allocators are created on the stack of the init function and so the pointers to them that are set on the struct are invalid once the init function returns.

I am opening this issue because @shawnl claims there may be a way to figure out when such pointers are created and survive beyond the lifetime of the stack variables and for the compiler to raise an error.

[andrewrk]

Related: #2301

[matu3ba]

Closely related: #5725

[Parzival-3141]

Is #2301 still related to this issue? It seems to me that #5725 and this issue are proposing compile-time checks resulting in compile-errors, while #2301 is for runtime safety checks.