improve type stability of Base.active_module() (JuliaLang#45838)

Now this method is called from many `show`-related utilities, so it
would be better to make it robust against invalidations.

base/Base.jl

@@ -124,6 +124,10 @@ include("operators.jl")
 include("pointer.jl")
 include("refvalue.jl")
 include("refpointer.jl")
+
+# The REPL stdlib hooks into Base using this Ref
+const REPL_MODULE_REF = Ref{Module}()
+
 include("checked.jl")
 using .Checked
 

base/client.jl

@@ -370,9 +370,6 @@ function __atreplinit(repl)
 end
 _atreplinit(repl) = invokelatest(__atreplinit, repl)
 
-# The REPL stdlib hooks into Base using this Ref
-const REPL_MODULE_REF = Ref{Module}()
-
 function load_InteractiveUtils(mod::Module=Main)
     # load interactive-only libraries
     if !isdefined(mod, :InteractiveUtils)
@@ -390,10 +387,10 @@ function load_InteractiveUtils(mod::Module=Main)
     return getfield(mod, :InteractiveUtils)
 end
 
+global active_repl
+
 # run the requested sort of evaluation loop on stdio
 function run_main_repl(interactive::Bool, quiet::Bool, banner::Bool, history_file::Bool, color_set::Bool)
-    global active_repl
-
     load_InteractiveUtils()
 
     if interactive && isassigned(REPL_MODULE_REF)
@@ -402,17 +399,18 @@ function run_main_repl(interactive::Bool, quiet::Bool, banner::Bool, history_fil
             term = REPL.Terminals.TTYTerminal(term_env, stdin, stdout, stderr)
             banner && Base.banner(term)
             if term.term_type == "dumb"
-                active_repl = REPL.BasicREPL(term)
+                repl = REPL.BasicREPL(term)
                 quiet || @warn "Terminal not fully functional"
             else
-                active_repl = REPL.LineEditREPL(term, get(stdout, :color, false), true)
-                active_repl.history_file = history_file
+                repl = REPL.LineEditREPL(term, get(stdout, :color, false), true)
+                repl.history_file = history_file
             end
+            global active_repl = repl
             # Make sure any displays pushed in .julia/config/startup.jl ends up above the
             # REPLDisplay
-            pushdisplay(REPL.REPLDisplay(active_repl))
-            _atreplinit(active_repl)
-            REPL.run_repl(active_repl, backend->(global active_repl_backend = backend))
+            pushdisplay(REPL.REPLDisplay(repl))
+            _atreplinit(repl)
+            REPL.run_repl(repl, backend->(global active_repl_backend = backend))
         end
     else
         # otherwise provide a simple fallback

base/show.jl

@@ -482,9 +482,11 @@ function _show_default(io::IO, @nospecialize(x))
     print(io,')')
 end
 
-active_module()::Module = isdefined(Base, :active_repl) && isdefined(Base.active_repl, :mistate) && Base.active_repl.mistate !== nothing ?
-                      Base.active_repl.mistate.active_module :
-                      Main
+function active_module()
+    isassigned(REPL_MODULE_REF) || return Main
+    REPL = REPL_MODULE_REF[]
+    return REPL.active_module()::Module
+end
 
 # Check if a particular symbol is exported from a standard library module
 function is_exported_from_stdlib(name::Symbol, mod::Module)

stdlib/REPL/src/REPL.jl

@@ -491,21 +491,28 @@ REPLCompletionProvider() = REPLCompletionProvider(LineEdit.Modifiers())
 mutable struct ShellCompletionProvider <: CompletionProvider end
 struct LatexCompletions <: CompletionProvider end
 
-active_module(repl::LineEditREPL) = repl.mistate === nothing ? Main : repl.mistate.active_module
+function active_module() # this method is also called from Base
+    isdefined(Base, :active_repl) || return Main
+    return active_module(Base.active_repl::AbstractREPL)
+end
+active_module((; mistate)::LineEditREPL) = mistate === nothing ? Main : mistate.active_module
 active_module(::AbstractREPL) = Main
 active_module(d::REPLDisplay) = active_module(d.repl)
 
 setmodifiers!(c::REPLCompletionProvider, m::LineEdit.Modifiers) = c.modifiers = m
+
 """
     activate(mod::Module=Main)
 
 Set `mod` as the default contextual module in the REPL,
 both for evaluating expressions and printing them.
 """
 function activate(mod::Module=Main)
-    Base.active_repl.mistate.active_module = mod
+    mistate = (Base.active_repl::LineEditREPL).mistate
+    mistate === nothing && return nothing
+    mistate.active_module = mod
     Base.load_InteractiveUtils(mod)
-    nothing
+    return nothing
 end
 
 beforecursor(buf::IOBuffer) = String(buf.data[1:buf.ptr-1])