flan build and flan run of a file with no main say so and show one, rather than failing at the link

bin/main.ml

@@ -736,6 +736,8 @@ let () =
           if List.mem warn_memory_flag rest then
             print_memory_warnings ~file:path p)
       in
+      Flan.Build.need_main ~file:path ~doing:"flan build has nothing to link"
+        f.program;
       ignore (Flan.Build.executable
                 ~opts:{ Flan.Build.default with checks; dev; debug; sanitize;
                                                 target; x86;
@@ -915,6 +917,8 @@ let () =
           (Printf.sprintf "flan-run-%d" (Unix.getpid ()))
       in
       let f = Flan.Front.linked ~all:true path in
+      Flan.Build.need_main ~file:path ~doing:"flan run has nothing to run"
+        f.program;
       ignore (Flan.Build.executable
                 ~opts:{ Flan.Build.default with checks; debug; sanitize;
                                                 x86;

lib/build.ml

@@ -744,6 +744,22 @@ let compile_c ~opts ?tflags ?(warn = []) ~src ~name () =
   end;
   obj
 
+(* A program with no [main] builds every function and then fails at the link,
+   as an undefined reference from the C startup code — a message about crt1.o
+   for a mistake in the .flan file. Asked here, by the commands that make an
+   executable, rather than inside [executable], whose callers include hosts
+   and tests that supply their own [main]. *)
+let need_main ~file ~doing (p : Tast.program) =
+  if not (List.exists (fun (f : Tast.fn) -> f.Tast.name = "main") p.Tast.fns)
+  then
+    failwith
+      (Printf.sprintf
+         "%s has no main, so %s. A program starts at a function named main, \
+          for example:\n\n\
+         \    (defn main [] i32\n\
+         \      0)"
+         file doing)
+
 (* [csrcs] and [lflags] come from the imported packages (see [Load]): the C
    shim a package binds through, and the arguments needed to link the library
    it binds to.

lib/dev.ml

@@ -4520,17 +4520,8 @@ let make_session_dir ~file dir =
    would find that out at the link — as a missing symbol, or as the merged
    build's rename finding nothing to rename. *)
 let need_main ~file (session : Session.t) =
-  if not
+  Build.need_main ~file ~doing:"flan dev has nothing to run"
-      (List.exists (fun (f : Tast.fn) -> f.Tast.name = "main")
+    session.Session.host
-         session.Session.host.Tast.fns)
-  then
-    failwith
-      (Printf.sprintf
-         "%s has no main, so flan dev has nothing to run. A program starts at \
-          a function named main, for example:\n\n\
-         \    (defn main [] i32\n\
-         \      0)"
-         file)
 
 (* [debug] is off by default, which keeps [flan dev] exactly what it was: a
    -O2 host and -O2 modules. It is opt-in rather than always-on because a debug

test/test_acceptance.ml

@@ -6472,6 +6472,18 @@ level "1"
     cli_case "--debug and an explicit -O are refused together"
       "build ../calc-me.flan --debug -O2 -o /dev/null" ~code:2
       ~says:[ "--debug"; "-O2"; "Drop one of the two" ];
+    (* A file with no main is refused by name before the link, which would
+       otherwise report an undefined reference from crt1.o. *)
+    let nomain = Filename.concat scratch "no-main.flan" in
+    Out_channel.with_open_bin nomain (fun oc ->
+      output_string oc "(defn f [] i32 0)\n");
+    cli_case "build of a file with no main names main"
+      (Printf.sprintf "build %s -o /dev/null" (Filename.quote nomain)) ~code:1
+      ~says:[ "has no main"; "(defn main [] i32" ];
+    cli_case "run of a file with no main names main"
+      (Printf.sprintf "run %s" (Filename.quote nomain)) ~code:1
+      ~says:[ "has no main"; "(defn main [] i32" ];
+    Sys.remove nomain;
 
     (* Every row above that went through the pool has been forked; nothing
        after this point may look at [failures] until every one of them has