improper_ctypes lint should check types referenced in C ABI function definitions

[tomjakubowski]

For example, this compiles with no warnings or errors:

#![feature(lang_items)]
#![crate_type="staticlib"]
#![allow(missing_copy_implementations)]
#![deny(improper_ctypes)]
#![no_std]

pub struct Blah {
    pub x: i32
}

#[no_mangle]
pub extern "C" fn yikes(b: Blah) -> i32 {
    b.x
}

#[lang = "copy"] trait Copy {}
#[lang = "sized"] trait Sized {}
#[lang = "stack_exhausted"] extern fn stack_exhausted() {}
#[lang = "eh_personality"] extern fn eh_personality() {}
#[lang = "panic_fmt"] fn panic_fmt() -> ! { loop {} }

Even though Blah lacks the #[repr(C)] attribute.

[tomjakubowski]

To clarify: foreign functions (fn inside an extern block) are checked. Regular functions with a C ABI are not.

[DanielKeep]

This is still an issue. It just came up in a Stack Overflow question, with something roughly equivalent to this:

#[no_mangle]
pub extern fn print(s: String) {
    println!("{}", s);
}

This produces no warnings. For context, the user was trying to bind to this function from C#, which went about as well as you'd expect.

[arielb1]

@DanielKeep

Its even worse here - this is an extern "Rust" function, and these have their own ABI.

[TimNN]

@arielb1 https://github.com/rust-lang/rust/blob/master/src/doc/trpl/ffi.md#calling-rust-code-from-c seems to imply that a pub extern fn something() {} uses the C calling convention by default.

Edit: and the reference seems to agree: https://github.com/rust-lang/rust/blob/master/src/doc/reference.md#extern-functions