The daemon's buffer draws errors, warnings and notes in compilation's faces and the program's own output in a face of its own

TODO.org

@@ -2079,12 +2079,6 @@ per-phase; making it per-form would need a resync point inside a body.
 CLOSED: [2026-09-25]
 A file with no =main= starts on a stub =main= that returns and parks; =load-file= (=C-c C-k=, already its key — the inspector stays on =C-c C-i=) keeps what compiles and lists the rest. Rules out =flan dev= with no file at all, and =--two-process= on a file with no =main=.
 
-** NEXT The daemon buffer is navigable but not coloured
-Decided 2026-09-25: errors, warnings and notes take compilation-mode's faces, and the program's own output takes a face of its own so it reads apart from the compiler's.
-=*flan*= is all plain text. =compilation-minor-mode= is on (=emacs/flan.el:822=)
-so =next-error= works, but a minor mode installs no font-lock. Open: whether the
-program's output should look different from the compiler's.
-
 ** DONE compilation-mode steps over the notes
 CLOSED: [2026-09-25]
 The daemon buffer and the diagnostics buffer set =compilation-skip-threshold= to

emacs/flan.el

@@ -412,6 +412,25 @@ nil keeps everything."
         (let ((inhibit-read-only t))
           (delete-region (point-min) (line-beginning-position)))))))
 
+(defface flan-output-face '((t :inherit font-lock-string-face))
+  "Face for the running program's own output in the daemon's buffer.
+It sets that output apart from what the compiler and the daemon say, whose
+errors, warnings and notes take `compilation-mode''s faces."
+  :group 'flan)
+
+(defun flan--daemon-buffer-setup ()
+  "Make the current buffer the daemon's log: navigable and coloured.
+The daemon writes a diagnostic as `file:line:col: message', the shape
+`compilation-minor-mode' already reads, so it only has to be switched on.
+The minor mode adds its font-lock rules but turns nothing on, and a process
+buffer is in `fundamental-mode', which global font-lock skips; so font-lock
+is switched on here, first, or the rules would never be drawn."
+  (font-lock-mode 1)
+  ;; A log, not source: a quote the program printed opens no string.
+  (setq-local font-lock-keywords-only t)
+  (compilation-minor-mode 1)
+  (flan--navigable-notes))
+
 (defun flan--append-output (text)
   "Append TEXT, the running program's own output, where it can be read.
 Two places.  The daemon's log always gets it, so output lands somewhere
@@ -423,7 +442,13 @@ open, above its prompt, which is where whoever is typing there is looking."
             (inhibit-read-only t))
         (save-excursion
           (goto-char (point-max))
-          (insert text))
+          ;; Marked as it is inserted, because nothing in the text says whose
+          ;; it is.  A line the program prints in the diagnostic shape is
+          ;; still read as one by `compilation-minor-mode', and takes its
+          ;; face.  `face' for a buffer with font-lock off, `font-lock-face'
+          ;; so fontification does not strip it.
+          (insert (propertize text 'face 'flan-output-face
+                              'font-lock-face 'flan-output-face)))
         (flan--trim-lines)
         ;; Follow the tail only for someone who was already at it; a reader
         ;; scrolled back is reading something.
@@ -893,8 +918,7 @@ It builds the program first, which for a cold project is most of this."
       ;; program runs, and `compilation-mode' would claim it as the output of
       ;; one finished command — killing the process on a `recompile', among
       ;; other things it has no business doing to a live session.
-      (compilation-minor-mode 1)
-      (flan--navigable-notes))
+      (flan--daemon-buffer-setup))
     (make-process
      :name "flan-daemon" :buffer buf
      :command args

emacs/test-flan.el

@@ -533,6 +533,37 @@ already rely on it — so nothing here is a stand-in for the real thing."
   ;; The session is not poisoned by that: a good form still lands.
   (flan--eval "(defn step [] i64 (set ticks (+ ticks 100)) ticks)" "form")
 
+  ;; The daemon's buffer is coloured: the compiler's errors, warnings and
+  ;; notes take compilation's faces and the program's own output takes
+  ;; `flan-output-face'.  Checked on `font-lock-face', which is what
+  ;; fontification writes; batch Emacs cannot turn `font-lock-mode' on, so
+  ;; nothing here aliases it to `face'.
+  (let ((flan-daemon-buffer " *flan-colour-test*"))
+    (with-current-buffer (get-buffer-create flan-daemon-buffer)
+      (insert "flan dev: built x.flan in 3ms\n"
+              "/tmp/a.flan:2:8: expected i32, found string\n"
+              "/tmp/a.flan:3:1: warning: w\n"
+              "/tmp/a.flan:4:1: note: n\n")
+      (flan--daemon-buffer-setup)
+      (flan--append-output "said hi\n")
+      (font-lock-ensure)
+      (let ((face-on (lambda (text)
+                       (goto-char (point-min))
+                       (search-forward text)
+                       (get-text-property (match-beginning 0) 'font-lock-face))))
+        (test-flan--check
+         "the daemon's buffer draws an error, a warning and a note in compilation's faces"
+         (and (memq 'compilation-error (ensure-list (funcall face-on "a.flan:2")))
+              (memq 'compilation-warning (ensure-list (funcall face-on "a.flan:3")))
+              (memq 'compilation-info (ensure-list (funcall face-on "a.flan:4")))))
+        (test-flan--check
+         "the program's output takes its own face"
+         (eq (funcall face-on "said") 'flan-output-face))
+        (test-flan--check
+         "the daemon's own line is left plain"
+         (null (funcall face-on "flan dev:")))))
+    (kill-buffer flan-daemon-buffer))
+
   ;; The program's own output arrives on replies and lands in the daemon's
   ;; buffer — no REPL is open yet, and the log is the fallback that makes a
   ;; println never depend on one.