Add new function_casts_as_integer lint

[GuillaumeGomez]

The function_casts_as_integer lint detects cases where users cast a function pointer into an integer.

warn-by-default

Example

fn foo() {}
let x = foo as usize;

warning: casting a function into an integer implicitly
  --> $DIR/function_casts_as_integer.rs:9:17
   |
LL |     let x = foo as usize;
   |                 ^^^^^^^^
   |
help: add `fn() as usize`
   |
LL |     let x = foo as fn() as usize;
   |                 +++++++

Explanation

You should never cast a function directly into an integer but go through a cast as fn first to make it obvious what's going on. It also allows to prevent confusion with (associated) constants.

Related to #81686 and https://stackoverflow.com/questions/68701177/whats-the-meaning-of-casting-a-rust-enum-variant-to-a-numeric-data-type

r? @Urgau

[Noratrieb]

Suggested change

	lint_function_casts_as_integer = casting a function into an integer implicitly
	.cast_as_fn = add `{$cast_from_ty} as {$cast_to_ty}`
	lint_function_casts_as_integer = direct cast of function item into an integer
	.cast_as_fn = first cast to a function pointer `{$cast_from_ty}`

how about this wording? it's not really implicit, it may just be unexpected

[GuillaumeGomez]

Sounds good (better) to me!

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

[flip1995]

Clippy has a few lints for fn to integer casts. But they are all restriction or style lints in Clippy. Adding a warn-by-default lint about this to rustc might be a bit aggressive 🤔

[GuillaumeGomez]

I know, I implemented one myself. 😉 I think it highlights the fact that this is a big issue and that the compiler should warn about it and eventually even forbid this fn to integer cast (you need to cast to an fn pointer first).

But in any case, it's up to the lang team.

[rustbot]

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

[bors]

☔ The latest upstream changes (presumably #141668) made this pull request unmergeable. Please resolve the merge conflicts.

[GuillaumeGomez]

CI finally passed. \o/