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

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.