Ask the object, not the text, whether the daemon built with -g

test_session.ml asserts that a debug session emits the metadata, which is
the unpassed-argument defect itself. It cannot see the other half: it is
Build.shared that turns the flag into -g and -O0 on the module, and a
daemon that dropped Build.debug from its opts would still emit perfect IR
and then compile it away — llvm.dbg.declare describes an alloca and
mem2reg deletes the alloca, so the symptom would be a module that looks
right in every text assertion and has no locals in the debugger.

So this drives a real `flan dev --debug`, sends one redefinition, and runs
llvm-dwarfdump over the .so the daemon actually wrote. The line table is
the needle because it is what a breakpoint in a .flan buffer resolves
against, and it names the file the form was typed in rather than anything
on disk. Skipped where there is no llvm-dwarfdump.

test/test_dev.ml

@@ -697,6 +697,87 @@ let () =
     end;
     List.iter (fun f -> try Sys.remove f with Sys_error _ -> ()) [ ssock; sout ];
 
+    (* --debug, and the half the IR cannot show.
+
+       [test_session.ml] asserts that a debug session *emits* the metadata,
+       which is the unpassed-argument defect itself. It cannot see the other
+       half: [Build.shared] is what turns the flag into [-g] and [-O0] on the
+       module, and a daemon that dropped [Build.debug] from its opts would
+       still emit perfect IR and then compile it away — [llvm.dbg.declare]
+       describes an alloca and mem2reg deletes the alloca. So this goes to the
+       .so the daemon actually wrote and asks the object, not the text.
+
+       The line table is the needle because it is what a breakpoint in a .flan
+       buffer resolves against, and it names the file the form was typed in
+       rather than any file on disk. *)
+    if Sys.command "command -v llvm-dwarfdump > /dev/null 2>&1" = 0 then begin
+      let gsock = tmp "dbg.sock" and gout = tmp "dbg.out" in
+      (try Sys.remove gsock with Sys_error _ -> ());
+      let gfd =
+        Unix.openfile gout [ Unix.O_WRONLY; Unix.O_CREAT; Unix.O_TRUNC ] 0o600
+      in
+      let gpid =
+        Unix.create_process flan
+          [| flan; "dev"; "programs/dev-repl.flan"; "-s"; gsock; "--debug" |]
+          Unix.stdin gfd Unix.stderr
+      in
+      Unix.close gfd;
+      if not (await (fun () -> Sys.file_exists gsock)) then begin
+        fail "the --debug daemon never listened";
+        (try Unix.kill gpid Sys.sigkill with Unix.Unix_error _ -> ())
+      end
+      else begin
+        let c = connect gsock in
+        let r =
+          request c
+            "(:op \"eval\" :code \"(defn step [] i64 (let [n (i64 3)] (set ticks (+ ticks n)) ticks))\" :file \"/tmp/dbg.flan\")"
+        in
+        if status r <> "ok" then
+          fail "a --debug daemon refused an ordinary redefinition: %s"
+            (Option.value ~default:"" (Wire.string_field r "message"))
+        else begin
+          (* The daemon builds into /tmp/flan-dev-<pid>, one module per eval,
+             and never reuses a name — dlopen caches by path. The first is
+             m1.so. *)
+          let so =
+            Filename.concat
+              (Filename.concat (Filename.get_temp_dir_name ())
+                 (Printf.sprintf "flan-dev-%d" gpid))
+              "m1.so"
+          in
+          if not (Sys.file_exists so) then
+            fail "the --debug daemon left no module at %s" so
+          else begin
+            let dump = tmp "dbg.dwarf" in
+            let code =
+              Sys.command
+                (Printf.sprintf "llvm-dwarfdump --debug-line %s > %s 2>&1"
+                   (Filename.quote so) (Filename.quote dump))
+            in
+            let text =
+              if code <> 0 then ""
+              else In_channel.with_open_bin dump In_channel.input_all
+            in
+            (try Sys.remove dump with Sys_error _ -> ());
+            let has hay needle =
+              let n = String.length needle and h = String.length hay in
+              let rec go i =
+                i + n <= h && (String.sub hay i n = needle || go (i + 1))
+              in
+              n > 0 && go 0
+            in
+            if not (has text "dbg.flan") then
+              fail
+                "a --debug daemon's module carries no line table for the form's \
+                 file, so a line breakpoint would stay pending across C-c C-c"
+          end
+        end;
+        (try Unix.kill gpid Sys.sigkill with Unix.Unix_error _ -> ());
+        (try ignore (Unix.waitpid [] gpid) with Unix.Unix_error _ -> ())
+      end;
+      List.iter (fun f -> try Sys.remove f with Sys_error _ -> ()) [ gsock; gout ]
+    end;
+
     List.iter (fun f -> try Sys.remove f with Sys_error _ -> ())
       [ sock; out; bsock; bout ];
     if !failures = 0 then print_endline "dev: all tests passed"