On Fri, Apr 16, 2021 at 8:10 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> How well would ? operator fit that pattern? _If_ it's just a syntax sugar
> along the lines of "if argument matches Err(_), return Err(_)", the types
> shouldn't be an issue, but that might need some fun with releasing resources,
> etc. If it's something more elaborate... details, please.
Yes, it is just syntax sugar -- it doesn't introduce any power to the language.
It was introduced because it is a very common pattern when using the
`Result` and `Option` enums. In fact, before it existed, it was just a
simple macro that you could also implement yourself.
For instance, given `Foo` and `Bar` types that need RAII cleanup of
some kind (let's say `kill_foo()` and `kill_bar()`):
fn foo() -> KernelResult<Foo> {
if black_box() {
return Err(EINVAL);
}
// something that gets you a `Foo`
let foo = ...;
Ok(foo)
}
fn bar() -> KernelResult<Bar> {
let p = foo()?;
// something that gets you a `Bar`, possibly using the `p`
let bar = ...;
Ok(bar)
}
This reduces to (full example at https://godbolt.org/z/hjTxd3oP1):
bar:
push rbx
mov ebx, 1
call qword ptr [rip + black_box@GOTPCREL]
test al, al
jne .LBB2_2
call qword ptr [rip + kill_foo@GOTPCREL]
xor ebx, ebx
.LBB2_2:
mov eax, ebx
mov edx, -1234
pop rbx
ret
You can see `bar()` calls `black_box()`. If it failed, it returns the
EINVAL. Otherwise, it cleans up the `foo` automatically and returns
the successful `bar`.
Cheers,
Miguel
