What does 'inline' mean?

[welkam]

On Saturday, 13 June 2020 at 01:58:55 UTC, Manu wrote:
> On Sat, Jun 13, 2020 at 11:31 AM Walter Bright via 
> Digitalmars-d < digitalmars-d at puremagic.com> wrote:
>> For example, I had to keep saying why? why? why?
>
> I've answered your "why"s so many times.
> <...>
> #2 why? - users want to add a hint to override the optimisers
> judgement

Oh cmon, Manu. This does not answer why. This is what statement 
that has "why?" before it.
WHAT:
Users want to add a hint to override the optimisers judgement
WHY:
Inlining has a big effect on programs performance and compiler`s 
heuristics based inlining algorithm doesn't always make the right 
decision. Manually going through the code base and marking select 
functions with a tag that tells inliner whether to inline that 
function or not can improve performance by several percent. In 
areas where high performance is critical and you have gain a lot 
by extracting even 1% of performance a tag that controls inliner 
is absolutely necessary. There fore D needs a way to control 
which function gets inlined and which is not otherwise D wont be 
even considered for writing code in high performance computing 
applications.

Now do that for other 2.
P.s. Remember (X) and (Y)?