Rollup merge of #141470 - GuillaumeGomez:function_casts_as_integer, r=urgau

Add new `function_casts_as_integer` lint

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

*warn-by-default*

### Example

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

r? ````@urgau````

Author: Zalathar

src/shims/native_lib/trace/parent.rs

@@ -500,7 +500,8 @@ fn handle_segfault(
     capstone_disassemble(&instr, addr, cs, acc_events).expect("Failed to disassemble instruction");
 
     // Move the instr ptr into the deprotection code.
-    #[expect(clippy::as_conversions)]
+    #[allow(unknown_lints)]
+    #[expect(clippy::as_conversions, function_casts_as_integer)]
     new_regs.set_ip(mempr_off as usize);
     // Don't mess up the stack by accident!
     new_regs.set_sp(stack_ptr);
@@ -552,7 +553,8 @@ fn handle_segfault(
     new_regs = regs_bak;
 
     // Reprotect everything and continue.
-    #[expect(clippy::as_conversions)]
+    #[allow(unknown_lints)]
+    #[expect(clippy::as_conversions, function_casts_as_integer)]
     new_regs.set_ip(mempr_on as usize);
     new_regs.set_sp(stack_ptr);
     ptrace::setregs(pid, new_regs).unwrap();

tests/pass/backtrace/backtrace-api-v1.rs

@@ -1,5 +1,7 @@
 //@normalize-stderr-test: "::<.*>" -> ""
 
+#![allow(function_casts_as_integer)]
+
 #[inline(never)]
 fn func_a() -> Box<[*mut ()]> {
     func_b::<u8>()

tests/pass/backtrace/backtrace-api-v1.stdout

@@ -1,5 +1,5 @@
-tests/pass/backtrace/backtrace-api-v1.rs:27:9 (func_d)
-tests/pass/backtrace/backtrace-api-v1.rs:14:9 (func_c)
-tests/pass/backtrace/backtrace-api-v1.rs:9:5 (func_b::<u8>)
-tests/pass/backtrace/backtrace-api-v1.rs:5:5 (func_a)
-tests/pass/backtrace/backtrace-api-v1.rs:34:18 (main)
+tests/pass/backtrace/backtrace-api-v1.rs:29:9 (func_d)
+tests/pass/backtrace/backtrace-api-v1.rs:16:9 (func_c)
+tests/pass/backtrace/backtrace-api-v1.rs:11:5 (func_b::<u8>)
+tests/pass/backtrace/backtrace-api-v1.rs:7:5 (func_a)
+tests/pass/backtrace/backtrace-api-v1.rs:36:18 (main)