bug#61093: Indented file names confuse compilation buffer

[Mattias Engdegård]

> I think it might be ok to ignore leading whitespace, because file
> names do not start very often with whitespace.

The story goes like this: a tool uses a modification of the GNU message format 
and its users then expect Emacs to conform to that variant.

The problem with doing that is that each little tweak makes the compilation 
message rules less robust and more likely to collide with one another and 
become slower. There are about 60 regexps now, most of which are used by very 
few people, and we keep adding. Build logs can become quite long so performance 
is not unimportant.

Maybe it's safe to accept and ignore not arbitrary leading whitespace but a 
single tab, which your tool seems to emit. Or you could ask those making it to 
cease emitting the tab.

You could also put your own rule in compilation-regexp-alist. It might look 
like this:

;; Message pattern for ancillary locations (notes) from the Go compiler
(let ((rule
       `(go-note
         ,(rx bol "\t"
              (group                    ; 1: hyperlink
               (group                    ; 2: file
                (not (in " \t\n:"))
                (* (not (in "\t\n"))))
               ":"
               (group (+ digit))         ; 3: line
               ":"
               (group (+ digit))         ; 4: column
               ":")
              " "
              (+ nonl))                ; message
          2 3 4 0 1)))
  (setq compilation-error-regexp-alist-alist
        (remq (assq 'go-note compilation-error-regexp-alist-alist)
              compilation-error-regexp-alist-alist))
  (push rule compilation-error-regexp-alist-alist)
  (setq compilation-error-regexp-alist
        (remq 'go-note compilation-error-regexp-alist))
  (push 'go-note compilation-error-regexp-alist))