MIR-borrowck: add permisson checks to `fn access_lvalue`

[pnkfelix]

As noted in a comment by arielb1, the fn access_lvalue needs to check somewhere that the accessor actually has sufficient permissions for the operation being requested, aka no mutable borrows of immutable references or moves through a non-Box reference.

[pnkfelix]

To elaborate, here are some examples of the kinds of scenarios we are worried about:

&mut of imm ref

For mutable borrows of immutable references, here is a case that mir-borrowck today does not properly catch (and, I think, none of the PR's in flight will fix):

fn main() {
    let mut x = &3;
    let rw = &mut *x;
    *rw = 4;
    println!("x: {:?}", x);
}

Compiling this with -Z borrowck-mir on today's master yields this single error from the AST-borrowck alone:

error[E0596]: cannot borrow immutable borrowed content `*x` as mutable
 --> check-access-lvalue.rs:3:19
  |
3 |     let rw = &mut *x;
  |                   ^^ cannot borrow as mutable

error: aborting due to previous error

Move through non-Box ref

... hmm... I am having trouble constructing an example of this where AST-borrowck and MIR-borrowck differ in behavior. (Which is strange because I thought there was significant portions of box-support that remained to be implemented. I'll have to look more deeply into this.)

[pnkfelix]

@zilbuz also notes this example (though I think it might a bit different thing to check, since this AFAICT is about checking that the local _a has a let mut declaration...)

fn main() {
    let a = 1;
    let _a = &a;
    let __a = &mut _a;
}

[nikomatsakis]

Here is some pseudocode for the various permissions I think we might want to check:

https://gist.github.com/nikomatsakis/90102114e8ba5792d4e28a6960942ae6

In each case, there is a function that, given an lvalue, tells you whether it can be mutated or whether it can be moved from. The function returns Err(lv) if the action is illegal, in which case lv is the lvalue we can "blame" for that.

[arielb1]

The permission checks still let you assign/mutate through an immutable reference:

fn main() {
    let a = &5;
    *a = 6; // no MIR error !!!
}

e.g. the test compile-fail/borrowck/borrowck-borrow-mut-base-ptr-in-aliasable-loc.rs