implicitly construct/dereference function pointers

[jameysharp]

In C, a function call looks like this:

int putchar(int);
putchar('A');

Indirecting through a function pointer looks like this:

int (*f)(int) = &putchar;
(*f)('B');

But it can also look like this:

int (*g)(int) = putchar;
g('C');

Notably, C automatically dereferences a function pointer if it's called, and it automatically takes the address of a function if it's used where a function pointer is expected.

In Rust, function items have a type such as fn(i32) -> i32, which is a kind of pointer. So first we need to translate C types like int (*)(int) to that Rust type, not to *const fn(i32) -> i32 or something.

Then we need two special cases in expression evaluation:

• Taking the address of an expression that has function type should be a no-op.
• Dereferencing an expression that has function type should be a no-op.

Here's an example Rust program which calls the C standard library's qsort function, passing it a callback which is implemented in Rust: https://is.gd/QLJxHh