`a[b]`, the sugary version of `*a.index(&b)`, seems to require type inference hints?

[japaric]

STR

// Sorry for the long example, didn't have time to make it shorter!
struct Mat<T> {
    data: Vec<T>,
    shape: (uint, uint),
}

impl<T> Mat<T> {
    fn new(data: Vec<T>, shape@(nrows, ncols): (uint, uint)) -> Mat<T> {
        assert!(data.len() == nrows * ncols);

        Mat {
            data: data,
            shape: shape,
        }
    }

    fn row<'a>(&'a self, row: uint) -> Row<&'a Mat<T>> {
        let (nrows, _) = self.shape;

        assert!(row < nrows);

        Row {
            mat: self,
            row: row,
        }
    }
}

impl<T> Index<(uint, uint), T> for Mat<T> {
    fn index<'a>(&'a self, &(row, col): &(uint, uint)) -> &'a T {
        let (nrows, ncols) = self.shape;

        assert!(row < nrows, col < ncols);

        &self.data[row * ncols + col]
    }
}

impl<'a, T> Index<(uint, uint), T> for &'a Mat<T> {
    fn index<'b>(&'b self, index: &(uint, uint)) -> &'b T {
        (*self).index(index)
    }
}

struct Row<M> {
    mat: M,
    row: uint,
}

impl<T, M: Index<(uint, uint), T>> Index<uint, T> for Row<M> {
    fn index<'a>(&'a self, col: &uint) -> &'a T {
        &self.mat[(self.row, *col)]
    }
}

fn main() {
    let m = Mat::new(vec!(1u, 2, 3, 4, 5, 6), (2, 3));
    let r = m.row(1);

    // GOOD
    println!("{}", r.index(&2) == &6);

    // BAD the sugary version doesn't work
    println!("{}", r[2] == 6);

    // BAD type annotating the index/rhs doesn't help...
    println!("{}", r[2u] == 6u);

    // GOOD it works OK in reverse...
    println!("{}", 6 == r[2]);

    // BAD indirection has no positive effect
    let e = r[2];
    println!("{}", e == 6);

    // GOOD unless it's type annotated
    let e: uint = r[2];
    println!("{}", e == 6);
}

All the lines marked with BAD yield these errors:

r[2] == 6: cannot determine a type for this bounded type parameter: unconstrained type
r[2u] == 6u: the type of this value must be known in this context
e == 6: the type of this value must be known in this context

Version

$ rustc --version
rustc 0.12.0-pre (459ffc2adc74f5e8b64a76f5670edb419b9f65da 2014-07-17 01:16:19 +0000)

[alexcrichton]

cc @pcwalton, this looks suspicious

[pnkfelix]

cc me