Make a missing GC root slightly more obvious to the checker

Finding the missing GC root is really hard for the checker. The correct
error message is on the store to newargs, complaining that we are rooting
a value that may have been GC'ed. However, `newargs` gets moved around
and re-assigned all over that place in such a way that the analyzer has
a hard time tracking it (I've seen it be successful if that function
is cut down a bit, so it might also be a heuristics problem). By
introducing a fake use that is easier for the analyzer to reason about,
hopefully we can catch regressions to this particular (very hairy)
part of the code in the future.

Author: Keno

src/builtins.c

@@ -607,10 +607,12 @@ JL_CALLABLE(jl_f__apply)
                 jl_value_t *state = jl_fieldref(next, 1);
                 roots[stackalloc] = state;
                 _grow_to(&roots[0], &newargs, &arg_heap, &n_alloc, n + precount + 1, extra);
+                JL_GC_ASSERT_LIVE(value);
                 newargs[n++] = value;
                 if (arg_heap)
                     jl_gc_wb(arg_heap, value);
                 roots[stackalloc + 1] = NULL;
+                JL_GC_ASSERT_LIVE(state);
                 args[1] = state;
                 next = jl_apply_generic(jl_iterate_func, args, 2);
             }

src/julia_internal.h

@@ -1062,6 +1062,16 @@ extern arraylist_t partial_inst;
 #define JL_GCC_IGNORE_STOP
 #endif // _COMPILER_GCC_
 
+#ifdef __clang_analyzer__
+  // Not a safepoint (so it dosn't free other values), but an artificial use.
+  // Usually this is unnecessary because the analyzer can see all real uses,
+  // but sometimes real uses are harder for the analyzer to see, or it may
+  // give up before it sees it, so this can be helpful to be explicit.
+  void JL_GC_ASSERT_LIVE(jl_value_t *v) JL_NOTSAFEPOINT;
+#else
+  #define JL_GC_ASSERT_LIVE(x) (void)(x)
+#endif
+
 #ifdef __cplusplus
 }
 #endif

src/support/analyzer_annotations.h

@@ -45,5 +45,6 @@ extern "C" {
 #define JL_ROOTED_VALUE_COLLECTION
 #define JL_GC_PROMISE_ROOTED(x) (void)(x)
 #define jl_may_leak(x) (void)(x)
+#define JL_GC_ASSERT_LIVE(x)
 
 #endif