Add mutablity checks and left hand size assignee checker

[philberty]

In order to assign to a name we must ensure the LHS is a valid expression
to assign to. This leads onto actually checking if this is a mutable
declaration or not.

Once these checks pass the name resolver we can in GIMPLE create immutable
types for these declarations to help with optimization.

Fixes #77

[bjorn3]

This doesn't handle interior mutability, right? If you have multiple immutable references to an UnsafeCell you can derive a mutable reference to the contents of this UnsafeCell from any of these immutable references for as long as only one such mutable reference exists at any point. libcore implements Freeze for all types that don't contain an UnsafeCell to help the compiler with handling interior mutability.

let foo = &UnsafeCell::new(Vec::new());
let bar = &*foo;
unsafe { (*foo.get()).push(1); }
unsafe { (*bar.get()).push(2); }
unsafe { assert_eq!((*foo.get()).len(), 2); }

is not UB.

[philberty]

This doesn't handle interior mutability, right? If you have multiple immutable references to an UnsafeCell you can derive a mutable reference to the contents of this UnsafeCell from any of these immutable references for as long as only one such mutable reference exists at any point. libcore implements Freeze for all types that don't contain an UnsafeCell to help the compiler with handling interior mutability.

let foo = &UnsafeCell::new(Vec::new());
let bar = &*foo;
unsafe { (*foo.get()).push(1); }
unsafe { (*bar.get()).push(2); }
unsafe { assert_eq!((*foo.get()).len(), 2); }

is not UB.

Yes, this PR and issue are about basic mutability rules for builtin types apart from pointers and unsafe etc. I was going to tackle unsafe in the 3rd data structure milestone and start basic pointers in the next one starting next week. Is it ok if I use your comment to raise a new issue for interior mutability?

Thanks again @bjorn3

[bjorn3]

Is it ok if I use your comment to raise a new issue for interior mutability?

Sure