Make Rust ABI stable enough to provide plugins functionality

[iddm]

Each language has it's own rules of creating a plugins.
In java we can make a project which may open a .jar at runtime to extend the program functionality. In python we could do almost the same thing using native tools. In C++ with Qt we can create a QPlugin/QLibrary which does the same thing. In C we may simply use dlopen.

Now creating an extensible application in rust is problematic:

1. Traits are not stable.
2. extern "Rust" is not safe yet too if you switch between compiler versions.

Even if I did not provide much items above, creating a plugin in Rust is very hard and needs a lot of unsafe blocks, checks, hacks and simulations (for example, trait-object simulation). Also everything needs to be C-compliant. As a language user I see that language attempts to be the most simple and safe as possible at the same time. It should have an ability to extend the program's functionality during runtime without using C convention - that would help to avoid unsafe blocks.

Original question was discussed here:
https://users.rust-lang.org/t/how-to-write-plugins-in-rust/

Here and here is some use of hacky-like code to emulate plugin functionality. I believe it works but this is absolutely weird - coding in Rust for Rust with C convention. We must provide a way to make normal, human-oriented way to extend Rust programs in Rust language without any narrowing to C.

P.S. I did not make any RFC yet.

[steveklabnik]

This is a subset of #600

[DemiMarie]

One option would be to at least have a way to detect compiler/library mismatches, so that plugin loading can fail.

[nrc]

Closing since this is covered by #600.