Auto merge of #7705 - michaelsproul:fn_to_numeric_cast_any, r=camsteffen

Restriction lint for function pointer casts

The existing lints for function pointer casts cover the cases where a cast is non-portable or would result in truncation, however there's currently no way to forbid numeric function pointer casts entirely.

I've added a new lint `fn_to_numeric_cast_any`, which allows one to ban _all_ numeric function pointer casts, including to `usize`. This is useful if you're writing high-level Rust and want to opt-out of potentially surprising behaviour, avoiding silent bugs from forgotten parentheses, e.g.

```rust
fn foo() -> u32 {
    10
}

fn main() {
    let _ = foo as usize; // oops, forgot to call foo and got a random address instead!
}
```

~~I'm open to suggestions for the naming of the lint, because `fn_to_numeric_cast_any` is a bit clunky. Ideally I'd call this lint `fn_to_numeric_cast`, but that name is already taken for the more specific lint~~. We've stuck with `fn_to_numeric_cast_any` to avoid renaming the existing lint, or choosing a different name that's too generic (like `fn_cast`).

I'm also open to changing the suggestion behaviour, as adding parentheses is only one of many possible ways to fix the lint.

changelog: add ``[`fn_to_numeric_cast_any`]`` restriction lint

1 files changed, 1 insertions, 0 deletions

diff --git a/clippy_lints/src/lib.register_lints.rs b/clippy_lints/src/lib.register_lints.rs
index 2ba2b3da55c..3c6d8103b6d 100644
--- a/clippy_lints/src/lib.register_lints.rs
+++ b/clippy_lints/src/lib.register_lints.rs
@@ -67,6 +67,7 @@ store.register_lints(&[
     casts::CAST_SIGN_LOSS,
     casts::CHAR_LIT_AS_U8,
     casts::FN_TO_NUMERIC_CAST,
+    casts::FN_TO_NUMERIC_CAST_ANY,
     casts::FN_TO_NUMERIC_CAST_WITH_TRUNCATION,
     casts::PTR_AS_PTR,
     casts::UNNECESSARY_CAST,