Root globals in toplevel exprs (#54433)

This fixes #54422, the code here assumes that top level exprs are always
rooted, but I don't see that referenced anywhere else, or guaranteed, so
conservatively always root objects that show up in code.

src/codegen.cpp

@@ -5457,8 +5457,9 @@ static jl_cgval_t emit_expr(jl_codectx_t &ctx, jl_value_t *expr, ssize_t ssaidx_
         jl_value_t *val = expr;
         if (jl_is_quotenode(expr))
             val = jl_fieldref_noalloc(expr, 0);
-        if (jl_is_method(ctx.linfo->def.method)) // toplevel exprs are already rooted
-            val = jl_ensure_rooted(ctx, val);
+        // Toplevel exprs are rooted but because codegen assumes this is constant, it removes the write barriers for this code.
+        // This means we have to globally root the value here. (The other option would be to change how we optimize toplevel code)
+        val = jl_ensure_rooted(ctx, val);
         return mark_julia_const(ctx, val);
     }
 

test/gc.jl

@@ -27,3 +27,17 @@ end
     run_gctest("gc/objarray.jl")
     run_gctest("gc/chunks.jl")
 end
+
+#testset doesn't work here because this needs to run in top level
+#Check that we ensure objects in toplevel exprs are rooted
+global dims54422 = [] # allocate the Binding
+GC.gc(); GC.gc(); # force the binding to be old
+GC.enable(false); # prevent new objects from being old
+@eval begin
+    Base.Experimental.@force_compile # use the compiler
+    dims54422 = $([])
+    nothing
+end
+GC.enable(true); GC.gc(false) # incremental collection
+@test typeof(dims54422) == Vector{Any}
+@test isempty(dims54422)