Fix to make incorrect return type into error

[MikuroXina]

This commit introduces a compile time error for incorrect return types.

For example, the following will now fail to compile:

#[swift_bridge::bridge]
mod ffi {
    extern "Rust" {
        fn get_reference() -> SomeType;
    }
}

fn get_reference() -> &'static SomeType {
    &SomeType
}

Before this commit the above example would compile and calling get_reference would cause undefined behavior.

[chinedufn]

Thanks for your first contribution!

Left one minor request then after that we can merge this.

[chinedufn]

@MikuroXina I merged your PR as is, thanks for the contribution!

I added a commit message to illustrate the change. We try to explain every commit using a thorough example.

I've also updated your PR body with an example.

[chinedufn]

Sorry for pointing you in the wrong direction.

I made a change to improve the compile time error message here 1b33e95

Here's the new message:

swift-bridge/crates/swift-bridge-macro/tests/ui/incorrect-return-type.stderr

Lines 1 to 33 in 1b33e95

	error[E0308]: mismatched types
	--> tests/ui/incorrect-return-type.rs:6:1
	|
	6 | #[swift_bridge::bridge]
	| ^^^^^^^^^^^^^^^^^^^^^^^ expected struct `SomeType`, found `&SomeType`
	...
	9 | type SomeType;
	| ______________-
	10 | |
	11 | | #[swift_bridge(rust_name = "some_function")]
	12 | | fn fn1() -> SomeType;
	| |_____________________________- expected due to this
	|
	= note: this error originates in the attribute macro `swift_bridge::bridge` (in Nightly builds, run with -Z macro-backtrace for more info)
	error[E0308]: mismatched types
	--> tests/ui/incorrect-return-type.rs:6:1
	|
	6 | #[swift_bridge::bridge]
	| ^^^^^^^^^^^^^^^^^^^^^^^ expected struct `SomeType`, found enum `Option`
	...
	9 | type SomeType;
	| ______________-
	10 | |
	11 | | #[swift_bridge(rust_name = "some_function")]
	12 | | fn fn1() -> SomeType;
	13 | | #[swift_bridge(rust_name = "another_function")]
	14 | | fn fn2() -> SomeType;
	| |_____________________________- expected due to this
	|
	= note: expected struct `SomeType`
	found enum `Option<SomeType>`
	= note: this error originates in the attribute macro `swift_bridge::bridge` (in Nightly builds, run with -Z macro-backtrace for more info)

[chinedufn]

Closed #12