bug#65251: 30.0.50; Duration in compilation buffer

[Mattias Engdegård]

> In the *compilation* buffer, we see timestamps when the compilation
> started and finished.  It would be nice to also see how long the
> compilation command took.

Not a bad idea.
 
> +;; The time when the compilation started.
> +(defvar compilation--start-time nil)

What about using defvar-local? Then...

> +        (setq-local compilation--start-time (current-time))

can use plain setq. And if you use (float-time) here, then... 

> +     (let* ((secs (float-time (time-since compilation--start-time))))

...this becomes a simple subtraction: (- (float-time) compilation--start-time)

> +       (cond ((< secs 1) (format "%.0fms" (* secs 1000)))
> +          ((< secs 10) (format "%.2fs" secs))
> +          ((< secs 60) (format "%.1fs" secs))
> +          (t (format-seconds "%hh%mm%z%ss" secs)))))

First of all, proper style is to separate the number and unit by a space.
The 'ms' case isn't very important -- 745 ms is no more readable than 0.745 s, 
probably less so.
The last case is also less than readable. Something like 3:45:58 would be 
better.

The reader would also like to know what this new time indication means. What 
about

   ..., duration 34.5 s

or

   ..., 34.5 s elapsed

?