I think inlining is function inlining, which is an optimisation that lots of compilers do. The change here is to move (some of) that earlier in the compilation process (in the "middle end" of the Rust compiler rather than in the LLVM backend), which can make things more efficient by reducing the total amount of work that needs to be done.

That makes sense, thanks!

For inlining in general, I think the wikipedia article is good https://en.wikipedia.org/wiki/Inline_expansion.

The reason it's so important is that inlining synergises with other optimizations, its often run before or after other passes like constant propagation. Consider something like:

  fn add(x : usize, y : usize) -> usize { x + y }

  fn main() {
    add(5, 1)
  }

Without inlining we can't easily optimize the code of `main`. However, if we inline `add` into main, constant propagation will then directly see `5 + 1` and optimize that into `6`.

I understand, thank you!