This allows to directly map from a DefPathHash to the crate it
originates from, without constructing side tables to do that mapping.
It also allows to reliably and cheaply check for DefPathHash collisions.
The discussion seems to have resolved that this lint is a bit "noisy" in
that applying it in all places would result in a reduction in
readability.
A few of the trivial functions (like `Path::new`) are fine to leave
outside of closures.
The general rule seems to be that anything that is obviously an
allocation (`Box`, `Vec`, `vec![]`) should be in a closure, even if it
is a 0-sized allocation.
