MIR borrowck: print lvalues in error messages in the same way that the AST borrowck

[zilbuz]

Fix #44974

• Print fields with .name rather than .<num>
• Autoderef values if followed by a field or an index
• Output [..] when borrowing inside a slice

[rust-highfive]

r? @pnkfelix

(rust_highfive has picked a reviewer for you, use r? to override)

[zilbuz]

I'd like some feedback on this PR. I can't think of any example that trigger the messages marked debug!() in my diff of borrow_check.rs. Can someone give me some example ?

[pnkfelix]

@zilbuz I'm about to hit the sack, but if you use the following input code, I think you should see some examples of some of the cases you are wondering about:

#![feature(slice_patterns)]

fn main() {
    #[derive(Debug)]
    enum E<X> { A(X), B { x: X } }

    #[derive(Debug)]
    struct S<'a, X: 'a, Y>{ x: &'a [X], y: (Y, Y), };

    let e = E::A(3);

    match e {
        E::A(ref ax) => println!("e.ax: {:?}", ax),
        E::B { x: ref bx } => println!("e.bx: {:?}", bx),
    }

    let s = S { x: &[1, 2, 3,], y: (999, 998) };

    match s {
        S  {
            x: &[ref x0, ref xn..],
            y: (ref y0, ref y1)
        } => {
            println!("s.x0: {:?} s.xn: {:?}", x0, xn);
            println!("s.y0: {:?}, s.y1: {:?}", y0, y1);
        }

        _ => panic!("other case"),
    }
}

[zilbuz]

r? @pnkfelix

The only commit untested is e32e81c. I couldn't find a way to access a field through a subslice. I'd have to be able to do something like [1..3].x or xn @ [F { x }..] in a match maybe...

[pnkfelix]

oh interesting...

(I think this represents a bug in MIR-borrowck. But fixing that is orthogonal to this PR in any case.)

[pnkfelix]

(likewise for all the other cases below that are paths based off of static variables)

[pnkfelix]

Filed as #45129

[pnkfelix]

@bors r+

thanks for all the work here @zilbuz !

[bors]

📌 Commit e32e81c has been approved by pnkfelix

[bors]

⌛️ Testing commit e32e81c with merge fd85391...

[bors]

💔 Test failed - status-travis

[kennytm]

@bors retry

• Android SDK HTTPS issue (fixed in #45193)

[bors]

⌛️ Testing commit e32e81c with merge d274114...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: pnkfelix
Pushing d274114 to master...