std.fmt.formatType fails when a field is a function

[RossComputerGuy]

Zig Version

0.11.0-dev.1969+d525ecb52

Steps to Reproduce and Observed Behavior

const std = @import("std");

const ExampleStruct = struct {
  method: fn () void,
};

fn method_impl() void {
}

pub fn main() void {
  const s = ExampleStruct {
    .method = method_impl,
  };

  std.debug.print("{}\n", .{ s });
}

Try using this example to print the structure. I believe the expected functionality should be similar to C which is to print the memory address of the method. Instead we get this error:

/nix/store/b9gg248ddpqp5pv26xmz937v2b6vwkc9-zig-0.11.0-dev.1969+d525ecb52/lib/std/fmt.zig:708:73: error: expected pointer, found 'fn() void'
            return format(writer, "{s}@{x}", .{ @typeName(T), @ptrToInt(value) });
                                                                        ^~~~~
referenced by:
    formatType__anon_6727: /nix/store/b9gg248ddpqp5pv26xmz937v2b6vwkc9-zig-0.11.0-dev.1969+d525ecb52/lib/std/fmt.zig:596:21
    format__anon_6715: /nix/store/b9gg248ddpqp5pv26xmz937v2b6vwkc9-zig-0.11.0-dev.1969+d525ecb52/lib/std/fmt.zig:184:13
    remaining reference traces hidden; use '-freference-trace' to see all reference traces

Expected Behavior

Zig should either print the memory address or name of the method instead of failing to compile.

[Vexu]

fn () void is a function body type, not a function pointer. It doesn't exist at runtime so it doesn't have an address and the formatting code doesn't have access to the name of the function.

[RossComputerGuy]

Why does *const fn () void work but not fn () void? I though all functions have an address or else they couldn't be executed by the CPU?

[Vexu]

Because *const fn() void is a function pointer so it can also exist at runtime.

[RossComputerGuy]

Now I'm confused, how does adding *const change the function in a way that makes the address accessible? As far as I know, you cannot execute code unless it is mapped into memory. That means it would have an address, so no matter if it's got *const or not, it exists in memory.

[Vexu]

fn () void is a function body type, it does not exist at runtime. *const fn () void is a function pointer which has an address at runtime.

[RossComputerGuy]

Well, that explains it a bit more but my methods are called at runtime and not at comptime. Adding *const did fix the issue but it also required a few other things in this to make it so it wouldn't error out on saying some structs depend on each other:

• https://github.com/ExpidusOS/neutron/commit/dd6b4fe71ac5bf6570521a92485b39a751e5a79d
• https://github.com/ExpidusOS/neutron/commit/33f58941078f7f5f72153e2f070200b33e461b3a
• https://github.com/ExpidusOS/neutron/commit/3a244cd8b51e885c94e629de75dcf074d2c8fc75

[RossComputerGuy]

I've figured out how to fix it in my code, maybe it should throw an error differently there like:

Expected constant pointer function, got a function body type.