New lint: slow or unsafe vector initialization

[ishitatsuyuki]

Cross post of rust-lang/rust#54628. See the thread for more details. The content below is a dumb summary.

Bad example

// resize
let mut vec1 = Vec::with_capacity(len);
vec1.resize(len, 0);
// extend
let mut vec2 = Vec::with_capacity(len);
vec2.extend(repeat(0).take(len))

Also some unsafe approaches exist, but it's TBD.

Good example

// vec! macro
let mut vec3 = vec![0; len];

[mcarton]

Bit of context for people interested on that one: https://medium.com/@shnatsel/how-ive-found-vulnerability-in-a-popular-rust-crate-and-you-can-too-3db081a67fb