What does 'inline' mean?

[Dukc]

On Monday, 8 June 2020 at 08:47:36 UTC, Stefan Koch wrote:
> Is that a valid concern?
> Will _any_ project out there break because of that?

It's easy to imagine. If one has a very simple mathematical 
function taking, say int and float and returning a float, and 
that function is called in a very hot loop, performance can drop 
noticeably if inlining suddently fails. Especially if the 
mathematical function calls another function instead of using the 
compiler primitives directly.