Three edges of Reach's walk that nothing called

An index expression inside a place, a place under addr, and a
restart-case clause body are each the only route to a function in
reach-walk.flan. Drop any one of the three from the walk and the
function is not emitted, so the program stops linking rather than
answering wrong; each mutation was planted and watched fail here. The
addr case goes through a deref place on purpose, so the index case
cannot stand in for it.

test/programs/reach-walk.flan

@@ -0,0 +1,53 @@
+;;;; Three edges of Reach's walk, each the only route to one function.
+;;;;
+;;;; A release build is pruned: a function nothing reachable from main or from
+;;;; a global initialiser calls is not emitted at all. So an edge the walk
+;;;; forgets does not produce a wrong answer — it produces a program that does
+;;;; not link, with a message about a symbol nobody wrote. Each of the three
+;;;; functions below is called from exactly one place, and that place is an
+;;;; edge no other program in the corpus exercises:
+;;;;
+;;;;   index-of    the index expression of a place, (set (at a (f)) v)
+;;;;   through     a place under (addr ...), here a (deref ...) so that it is
+;;;;               the addr edge and not the index one again
+;;;;   placeholder a restart-case clause body, which is reached by a transfer
+;;;;               and never by a call the walk can see from the body
+;;;;
+;;;; Nothing here is about the values; the values are how the test notices that
+;;;; the program was built and ran at all.
+
+(defvar cells [4 i32])
+(defvar slot i32)
+
+(defstruct Nope [id i32])
+
+(defn index-of [] i32 2)
+
+(defn through [] (Ptr i32) (addr slot))
+
+(defn placeholder [] i32 42)
+
+;;; Signals with nothing to return, so the clause body is the only way past.
+(defn missing [] i32
+  (error (Nope {:id 1})))
+
+(defn pick [] i32
+  (restart-case (missing)
+    (use-placeholder [] (placeholder))))
+
+(defn main [] i32
+  ;; The index of a place is an expression, and it can call.
+  (set (at cells (index-of)) 10)
+  (print (at cells 2)) (println "")
+
+  ;; (addr (deref p)) is p, so this is the addr edge over a place whose own
+  ;; walk is Pderef rather than Pindex — the index case above cannot stand in
+  ;; for it.
+  (let [q (addr (deref (through)))]
+    (set (deref q) 20))
+  (print slot) (println "")
+
+  ;; The clause body, reached only because the handler transfers into it.
+  (handler-bind [(Nope [c] (invoke-restart 'use-placeholder))]
+    (print (pick)) (println ""))
+  0)

test/test_acceptance.ml

@@ -661,6 +661,32 @@ let () =
     outputs "a package reached along two routes" "programs/pkg-shared.flan"
       "ok\n";
 
+    (* Reach's walk, edge by edge. Pruning is what makes the link follow the
+       program, and the cost of getting it wrong is not a wrong answer: a
+       function the walk fails to reach is not emitted, and the build dies in
+       the linker naming a symbol nobody wrote. reach-walk.flan calls three
+       functions from three places that are each the only route to them — the
+       index expression of a place, a place under [addr], and a restart-case
+       clause body — so a walk that forgets any one of the three fails to build
+       here. Caught rather than raised, because a build that dies takes the
+       rest of the table with it. *)
+    (match compile "programs/reach-walk.flan" with
+     | exception Failure m ->
+       incr failures;
+       Printf.printf
+         "FAIL  Reach's walk: it did not build, which is what a pruned function looks like\n%s\n"
+         m
+     | exe ->
+       let code, text = run exe None in
+       let want = "10\n20\n42\n" in
+       if text <> want || code <> 0 then begin
+         incr failures;
+         Printf.printf
+           "FAIL  Reach's walk\n        got: %S (exit %d)\n     wanted: %S\n"
+           text code want
+       end;
+       (try Sys.remove exe with Sys_error _ -> ()));
+
     (* The refusals. Each is a thing that would otherwise fail later and
        elsewhere — as a name the checker says is unknown, or as a collision
        nobody wrote — so what is asserted is the *reason*, at the form that