fix: suggest setting --prefix on empty install

When a user has not set --prefix they will get a confusing error message
about mandir. The suggestion here is to also set --mandir leading them
down a rabbit hole of specifying a complete install layout. What they
probably want to be doing is speciying --prefix so we change the error
message here to suggest that instead.

fix #7814

Signed-off-by: Ali Caglayan <alizter@gmail.com>

bin/install_uninstall.ml

@@ -45,9 +45,13 @@ let get_dirs context ~prefix_from_command_line ~from_command_line =
     match v with
     | Some v -> v
     | None ->
+      (* We suggest that the user sets --prefix first rather than the specific
+         missing option, since this is the most common case. *)
       User_error.raise
         [ Pp.textf "The %s installation directory is unknown." name ]
-        ~hints:[ Pp.textf "It could be specified with --%s" name ]
+        ~hints:
+          [ Pp.textf "It can be specified with --prefix or by setting --%s" name
+          ]
   in
   { Roots.lib_root = must_be_defined "libdir" roots.lib_root
   ; libexec_root = must_be_defined "libexecdir" roots.libexec_root

test/blackbox-tests/test-cases/install/install-no-prefix.t

@@ -16,7 +16,8 @@ Here we specify the behaviour of dune install when no install prefix is given.
 
 We should suggest to the user here that they should use --prefix.
 
+  $ unset OPAM_SWITCH_PREFIX
   $ dune install
   Error: The mandir installation directory is unknown.
-  Hint: It could be specified with --mandir
+  Hint: It can be specified with --prefix or by setting --mandir
   [1]