move ans and err variables to MainInclude module

This hides them from `names()` unfortunately, but means we will not
accidentally discard a variable from the user in Main, either because
the import will fail or the assignment will fail. If we later have
world-versioned bindings, this would also mean you could toggle (between
worlds) between being module-local and a special import value.

Fix JuliaLang#43172
Fix JuliaLang#48299

base/client.jl

@@ -132,14 +132,14 @@ function eval_user_input(errio, @nospecialize(ast), show_value::Bool)
             end
             if lasterr !== nothing
                 lasterr = scrub_repl_backtrace(lasterr)
-                istrivialerror(lasterr) || setglobal!(Main, :err, lasterr)
+                istrivialerror(lasterr) || setglobal!(Base.MainInclude, :err, lasterr)
                 invokelatest(display_error, errio, lasterr)
                 errcount = 0
                 lasterr = nothing
             else
                 ast = Meta.lower(Main, ast)
                 value = Core.eval(Main, ast)
-                setglobal!(Main, :ans, value)
+                setglobal!(Base.MainInclude, :ans, value)
                 if !(value === nothing) && show_value
                     if have_color
                         print(answer_color())
@@ -159,7 +159,7 @@ function eval_user_input(errio, @nospecialize(ast), show_value::Bool)
             end
             errcount += 1
             lasterr = scrub_repl_backtrace(current_exceptions())
-            setglobal!(Main, :err, lasterr)
+            setglobal!(Base.MainInclude, :err, lasterr)
             if errcount > 2
                 @error "It is likely that something important is broken, and Julia will not be able to continue normally" errcount
                 break
@@ -478,6 +478,10 @@ function include(fname::AbstractString)
     Base._include(identity, Main, fname)
 end
 eval(x) = Core.eval(Main, x)
+
+# weakly exposes ans and err and Out variables to Main
+global ans, err, Out
+export ans, err, Out
 end
 
 """

base/sysimg.jl

@@ -5,6 +5,7 @@ Core.include(Main, "Base.jl")
 using .Base
 
 # Set up Main module
+using Base.MainInclude # ans, err, Out
 import Base.MainInclude: eval, include
 
 # Ensure this file is also tracked

stdlib/REPL/src/REPL.jl

@@ -151,7 +151,7 @@ function eval_user_input(@nospecialize(ast), backend::REPLBackend, mod::Module)
                 end
                 value = Core.eval(mod, ast)
                 backend.in_eval = false
-                setglobal!(mod, :ans, value)
+                setglobal!(Base.MainInclude, :ans, value)
                 put!(backend.response_channel, Pair{Any, Bool}(value, false))
             end
             break
@@ -293,7 +293,7 @@ function print_response(errio::IO, response, show_value::Bool, have_color::Bool,
             Base.sigatomic_end()
             if iserr
                 val = Base.scrub_repl_backtrace(val)
-                Base.istrivialerror(val) || setglobal!(Main, :err, val)
+                Base.istrivialerror(val) || setglobal!(Base.MainInclude, :err, val)
                 Base.invokelatest(Base.display_error, errio, val)
             else
                 if val !== nothing && show_value
@@ -316,7 +316,7 @@ function print_response(errio::IO, response, show_value::Bool, have_color::Bool,
                 println(errio, "SYSTEM (REPL): showing an error caused an error")
                 try
                     excs = Base.scrub_repl_backtrace(current_exceptions())
-                    setglobal!(Main, :err, excs)
+                    setglobal!(Base.MainInclude, :err, excs)
                     Base.invokelatest(Base.display_error, errio, excs)
                 catch e
                     # at this point, only print the name of the type as a Symbol to
@@ -1413,7 +1413,7 @@ end
 
 function capture_result(n::Ref{Int}, @nospecialize(x))
     n = n[]
-    mod = REPL.active_module()
+    mod = Base.MainInclude
     if !isdefined(mod, :Out)
         setglobal!(mod, :Out, Dict{Int, Any}())
     end