Idea: add can_panic macro

[rw]

I find that when I use no_panic in a crate, I end up learning more about which of my functions can panic.

For me, this typically occurs when I forget that allocations can fail, so my function can't be annotated with no_panic.

In my workflow, after I think through this reasoning, I want to annotate the function with information that informs the reader that I did try to annotate with no_panic, but that my attempt failed because the function may actually panic.

To this end, I suggest a no-op annotation called can_panic as a complement to no_panic.

[dtolnay]

I would prefer not to build this into this crate. A "no-op annotation" is what comments are for (whether that be a doc comment or an ordinary comment).

Standard library documentation has a convention of using a # Panics section in rustdoc to indicate all the panicking conditions. https://rust-lang.github.io/api-guidelines/documentation.html#c-failure