Make some borrow checker errors more user friendly

[ktt3ja]

A mutable and immutable borrow place some restrictions on what you can
with the variable until the borrow ends. This commit attempts to convey
to the user what those restrictions are. Also, if the original borrow is
a mutable borrow, the error message has been changed (more specifically,
i. "cannot borrow x as immutable because it is also borrowed as
mutable" and ii. "cannot borrow x as mutable more than once" have
been changed to "cannot borrow x because it is already borrowed as
mutable").

In addition, this adds a (custom) span note to communicate where the
original borrow ends.

fn main() {
    match true {
        true => {
            let mut x = 1;
            let y = &x;
            let z = &mut x;
        }
        false => ()
    }
}

test.rs:6:21: 6:27 error: cannot borrow `x` as mutable because it is already borrowed as immutable
test.rs:6             let z = &mut x;
                              ^~~~~~
test.rs:5:21: 5:23 note: previous borrow of `x` occurs here; the immutable borrow prevents subsequent moves or mutable borrows of `x` until the borrow ends
test.rs:5             let y = &x;
                              ^~
test.rs:7:10: 7:10 note: previous borrow ends here
test.rs:3         true => {
test.rs:4             let mut x = 1;
test.rs:5             let y = &x;
test.rs:6             let z = &mut x;
test.rs:7         }
                  ^

fn foo3(t0: &mut &mut int) {
    let t1 = &mut *t0;
    let p: &int = &**t0;
}

fn main() {}

test.rs:3:19: 3:24 error: cannot borrow `**t0` because it is already borrowed as mutable
test.rs:3     let p: &int = &**t0;
                            ^~~~~
test.rs:2:14: 2:22 note: previous borrow of `**t0` as mutable occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `**t0` until the borrow ends
test.rs:2     let t1 = &mut *t0;
                       ^~~~~~~~
test.rs:4:2: 4:2 note: previous borrow ends here
test.rs:1 fn foo3(t0: &mut &mut int) {
test.rs:2     let t1 = &mut *t0;
test.rs:3     let p: &int = &**t0;
test.rs:4 }
          ^

For the "previous borrow ends here" note, if the span is too long (has too many lines), then only the first and last lines are printed, and the middle is replaced with dot dot dot:

fn foo3(t0: &mut &mut int) {
    let t1 = &mut *t0;
    let p: &int = &**t0;



}

fn main() {}

test.rs:3:19: 3:24 error: cannot borrow `**t0` because it is already borrowed as mutable
test.rs:3     let p: &int = &**t0;
                            ^~~~~
test.rs:2:14: 2:22 note: previous borrow of `**t0` as mutable occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `**t0` until the borrow ends
test.rs:2     let t1 = &mut *t0;
                       ^~~~~~~~
test.rs:7:2: 7:2 note: previous borrow ends here
test.rs:1 fn foo3(t0: &mut &mut int) {
...
test.rs:7 }
          ^

(Sidenote: the span_end_note currently also has issue #11715)

[ktt3ja]

cc @nikomatsakis: I would like to check that my change to the error messages is not wrong or misleading.

[ktt3ja]

@pcwalton: It passes the existing tests now, but it looks like I can't make new test cases for this since compiletest/runtest expects a note at the beginning of the span :( For example, I would have to put:

fn foo3(t0: &mut &mut int) { //~ NOTE previous borrow ends here
    let t1 = &mut *t0;
    let p: &int = &**t0;
}

even though custom_span_note would point the arrow at the last bracket. (EDIT: I just thought of a way to circumvent it, so I'll try that now)

[ktt3ja]

@pcwalton: r?