Procs do not capture mutability

[nickdesaulniers]

As an extension of the work from #10617 & #11626 , a proc shouldn't have to shadow a variable to get the mutability. This shouldn't be necessary.

[nickdesaulniers]

When I pull the shadowing out of the test, I'm seeing:
cannot assign to immutable captured outer variable in a heap closure which I think comes from loan checking within the borrow checker. Not certain how that string is built up. cc @nikomatsakis @alexcrichton

[nikomatsakis]

Ultimately the cause of this error is the categorization created by mem_categorization, which categorizes copied upvars as immutable.

[nickdesaulniers]

@nikomatsakis , ok trying to understand this. I'm guessing I'm getting warmer? Not sure what a "one shot stack closure" is.

[nikomatsakis]

@nickdesaulniers the relevant line is here. A "one shot stack closure" is often called a "once closure" -- not currently really supported, though some code exists -- it's a closure that only executes once, but which is stored on the stack. In any case, a proc type is (somewhat hackishly) represented as a closure with a OwnedSigil right now in the code.

[nickdesaulniers]

@nikomatsakis , I had changed the mutbl member to be upvar_cmt.mutbl.inherit() (moving the definition of upvar_cmt out above the conditional). Should I instead make the var_is_refd true for OwnedSigil cases?

[alexcrichton]

As @huonw noted in #12260, this code seems like it should work

fn main() {
    let mut x = 1;
    proc() { x = 2 };
}

proc-mut.rs:3:14: 3:15 error: cannot assign to immutable captured outer variable in a heap closure
proc-mut.rs:3     proc() { x = 2 };
                           ^