Warn on pointer cast with type inference

What it does

Lint should warn when casts using type inference are being used on pointers types.

Recently we had a bug on a FFI boundary where a *const T was being cast with self.inner() as *mut _ when calling a C function with signature sys::foo(T**), but inner function is defined as fn inner() -> *const T.
This lead to passing a pointer as pointer to pointer, and the application crashing at runtime.

Categories (optional)

• Kind: clippy::correctness, clippy::pedantic

Original code might be buggy. Intent is to have a mechanism that warns or errors when such a code is found on particular scenarios, specifically for our case in code involving the FFI boundary.

Drawbacks

Might get annoying for most use cases, thats why it was suggested also as pedantic linter.

Example

sys::foo(self.inner() as *mut _);

Could be written as:

sys::foo(self.inner() as *mut T);

In this case code would fail to compile, but provides us the chance to catch and fix the error at compile time.

The issue names no implementation file, test, or entry point. Start by locating Clippy's existing cast-related lint registration and tests, then determine how pointer casts using type inference are represented. Done means the shown pointer cast produces a warning or error with an appropriate lint category, while explicit pointer types remain usable.