only warn about Vararg wrapping once

This quiets down the long message in #38136 to be once-per-process when
running with `--depwarn=yes` instead of spamming log files continually.
We probably should have considered doing this a long time ago, but
hopefully better now than never.

Author: vtjnash

src/jltypes.c

@@ -910,7 +910,9 @@ JL_DLLEXPORT jl_value_t *jl_type_unionall(jl_tvar_t *v, jl_value_t *body)
         if (jl_options.depwarn) {
             if (jl_options.depwarn == JL_OPTIONS_DEPWARN_ERROR)
                 jl_error("Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).\nYou may need to write `f(x::Vararg{T})` rather than `f(x::Vararg{<:T})` or `f(x::Vararg{T}) where T` instead of `f(x::Vararg{T} where T)`.");
-            jl_printf(JL_STDERR, "WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).\nYou may need to write `f(x::Vararg{T})` rather than `f(x::Vararg{<:T})` or `f(x::Vararg{T}) where T` instead of `f(x::Vararg{T} where T)`.\nTo make this warning an error, and hence obtain a stack trace, use `julia --depwarn=error`.\n");
+            static _Atomic(int) warnedonce;
+            if (jl_atomic_load_relaxed(&warnedonce) == 0 && jl_atomic_exchange_relaxed(&warnedonce, 1) == 0)
+                jl_printf(JL_STDERR, "WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).\nYou may need to write `f(x::Vararg{T})` rather than `f(x::Vararg{<:T})` or `f(x::Vararg{T}) where T` instead of `f(x::Vararg{T} where T)`.\nTo make this warning an error, and hence obtain a stack trace, use `julia --depwarn=error`.\n");
         }
         jl_vararg_t *vm = (jl_vararg_t*)body;
         int T_has_tv = vm->T && jl_has_typevar(vm->T, v);