Attach different lifetimes to the tile block struct and its data.

[DiamondLovesYou]

So the tile block reference doesn't need to live as long as the data it "holds".

[coveralls]

Coverage remained the same at 70.922% when pulling 5e6ce710a066eca3a47944c603decddec423a73e on legionella-rs:blk-ctx-lifetimes into dc34123f5316c66547d3cfbfb86855991db1a1ec on xiph:master.

[tdaede]

Do you have a future change in mind that this enables?

[rom1v]

In theory, it releases some unnecessary constraints. In practice, the lifetime parameters are exposed to the type, so every usage of the type needs to pass all of them (at least as '_).

So in the general case, instead of:

struct MyView<'a> {
    pub w: &'a W,
    pub x: &'a X,
    pub y: &'a Y,
    pub z: &'a Z,
}

You would get:

struct MyView<'a, 'b, 'c, 'd> {
    pub w: &'a W,
    pub x: &'b X,
    pub y: &'c Y,
    pub z: &'d Z,
}

And passing that type to a function:

fn f(v: &MyView<'_, '_, '_, '_>) { … }

Just for information, I initially implemented it with 2 lifetimes (with less constraints though) in tiling.78 (70bd1e5571a5069a7bba827a7530fcc73b216f53). I decided not to, because I considered it was not worth it, and change it in tiling.79 (2883c68120f27e810f66a367335fae556bf87c7b).

But of course, if there is a benefit, we could change it.