REPL: hide any prints to stdio during complete_line (#55959)

stdlib/REPL/src/LineEdit.jl

@@ -366,7 +366,14 @@ end
 # Prompt Completions & Hints
 function complete_line(s::MIState)
     set_action!(s, :complete_line)
-    if complete_line(state(s), s.key_repeats, s.active_module)
+    # suppress stderr/stdout prints during completion computation
+    # i.e. ambiguous qualification warnings that are printed to stderr
+    # TODO: remove this suppression once such warnings are better handled
+    # TODO: but before that change Pipe to devnull once devnull redirects work for JL_STDERR etc.
+    completions_exist = redirect_stdio(;stderr=Pipe(), stdout=Pipe()) do
+        complete_line(state(s), s.key_repeats, s.active_module)
+    end
+    if completions_exist
         return refresh_line(s)
     else
         beep(s)
@@ -384,7 +391,13 @@ function check_for_hint(s::MIState)
     end
 
     completions, partial, should_complete = try
-        complete_line(st.p.complete, st, s.active_module; hint = true)::Tuple{Vector{String},String,Bool}
+        # suppress stderr/stdout prints during completion computation
+        # i.e. ambiguous qualification warnings that are printed to stderr
+        # TODO: remove this suppression once such warnings are better handled
+        # TODO: but before that change Pipe to devnull once devnull redirects work for JL_STDERR etc.
+        completions, partial, should_complete = redirect_stdio(;stderr=Pipe(), stdout=Pipe()) do
+            complete_line(st.p.complete, st, s.active_module; hint = true)::Tuple{Vector{String},String,Bool}
+        end
     catch
         @debug "error completing line for hint" exception=current_exceptions()
         return clear_hint(st)