Language items are entirely unsafe

[emberian]

The compiler never checks that the language item it receives is of the correct type or signature. This leads to craziness like:

#[no_std];

#[main]
#[lang="start"]
fn start() { }

[2:21:07]/tmp> rustc foo.rs
task 'rustc' has overflowed its stack
zsh: illegal hardware instruction (core dumped) rustc foo.rs

or, more benignly:

#[no_std];

#[lang="start"]
fn start() { }

#[main]
fn main() { }

which just plain asserts:

rustc: /build/rust-git/src/rust/src/llvm/lib/IR/Instructions.cpp:276: void llvm::CallInst::init(llvm::Value*, llvm::ArrayRef<llvm::Value*>, const llvm::Twine&): Assertion `(Args.size() == FTy->getNumParams() || (FTy->isVarArg() && Args.size() > FTy->getNumParams())) && "Calling a function with bad signature!"' failed.

or, more crazily:

#[no_std];

#[lang="start"]
mod a { }

#[main]
fn main() { }

which "just" ICEs:

error: internal compiler error: node_id_to_type: no type for node `mod a (id=2)`
This message reflects a bug in the Rust compiler. 
We would appreciate a bug report: https://github.com/mozilla/rust/wiki/HOWTO-submit-a-Rust-bug-report
task 'rustc' failed at 'explicit failure', /build/rust-git/src/rust/src/libsyntax/diagnostic.rs:102
task '<main>' failed at 'explicit failure', /build/rust-git/src/rust/src/librustc/lib.rs:395

Language items are the interface between the compiler and the language. They need to be well-defined and verified for correctness.

[huonw]

Possibly a dup of #9307.

[alexcrichton]

Closing in favor of #9307 as that's probably the best solution to go about doing this (although these are some good test cases).