Re: Did `inline' used to do something else?

[Joost Kremers <joostkremers@fastmail.fm>]

On Sat, Nov 18 2017, Eric Abrahamsen wrote:
> The function `inline' is an alias for `progn'.
>
> Gnus is full of code where inline is called with a single 
> argument, ie a
> no-op, which makes me think `inline' used to do something else? 
> I've
> tried searching git logs, with no results -- did this function 
> used to
> actually inline code?

I looked at the source to see where it is defined and found the 
following in `byte-run.el':

    ;; Redefined in byte-optimize.el.
    ;; This is not documented--it's not clear that we should 
    promote it.
    (fset 'inline 'progn)

At first, I couldn't find the referenced file `byte-optimize.el', 
so I went back in time, but this `fset' call plus comment was 
already present in what seems to be the first version of 
`byte-run.el' from 25 years ago. Then I noticed that there is a 
file called `byte-opt.el' and indeed, this is the file being 
referred to. It contains the following:

    ;;; byte-compile optimizers to support inlining

    (put 'inline 'byte-optimizer 'byte-optimize-inline-handler)

Followed by a definition of `byte-optimize-inline-handler'. 
There's no mention of the symbol property `byte-optimizer', and I 
know next to nothing about byte compilation, but its name is 
suggestive, of course. (In fact, all `defsubst' does is to define 
a function and to put this property in the function symbol's plist 
with the value `byte-compile-inline-expand'.)

So, long story short, even though `inline' is presented as an 
alias for `progn', it's handled differently by the byte compiler, 
presumably to deal with performance bottlenecks.

-- 
Joost Kremers
Life has its moments