Implement the Fn trait for bare fn pointers in the compiler

[nikomatsakis]

Implement the Fn trait for bare fn pointers in the compiler rather
than doing it using hard-coded impls. This means that it works also
for more complex fn types involving bound regions.

[nikomatsakis]

r? @pcwalton

[japaric]

Could it be that this fixes #19126? 😍 I'll give it a go!

[nikomatsakis]

@japaric that was the idea :)

[reem]

Very happy to see this being fixed :)

[japaric]

@nikomatsakis I always get this LLVM assertion:

• when bootstrapping rust, specifically during stage0/libcollections compilation
• when using a compiler that includes this PR as the stage0 compiler
• on a branch that includes this PR and unboxes the closures in the Option methods
• but only if I use the -C codegen_units flag:

rustc: /root/rust/src/llvm/lib/IR/Constants.cpp:1609: static llvm::Constant* llvm::ConstantExpr::getPointerCast(llvm::Constant*, llvm::Type*): Assertion `S->getType()->isPtrOrPtrVectorTy() && "Invalid cast"' failed.

I'll investigate it later, and open an issue with some minimal reproduction script. (But my guess, is that is something cross-crate related)

[nikomatsakis]

@japaric hmm, ok. I'll add some cross-crate tests. It also occurs to me I forgot to add the cache that I meant to add, so right now it generates fresh adapters as needed.

[nikomatsakis]

The requirement for the -C codegen-units flag is interesting though...

[pcwalton]

Since you cloned the sig above, why not just move inputs and output out?

[nikomatsakis]

good point, holdover from an older version where I needed the clone for something

[japaric]

Could someone make a snapshot containing this change? This is needed to land the migration to unboxed closures patch.

cc @aturon @alexcrichton

[alexcrichton]

One should be on the way!