Don't print bad Expr(:toplevel) misleadingly

base/show.jl

@@ -2504,6 +2504,11 @@ function show_unquoted(io::IO, ex::Expr, indent::Int, prec::Int, quote_level::In
     elseif head === :meta && nargs == 2 && args[1] === :pop_loc
         print(io, "# meta: pop locations ($(args[2]::Int))")
     # print anything else as "Expr(head, args...)"
+    elseif head === :toplevel
+        # Reset SOURCE_SLOTNAMES. Raw SlotNumbers are not valid in Expr(:toplevel), but
+        # we want to show bad ASTs reasonably to make errors understandable.
+        lambda_io = IOContext(io, :SOURCE_SLOTNAMES => false)
+        show_unquoted_expr_fallback(lambda_io, ex, indent, quote_level)
     else
         unhandled = true
     end

test/show.jl

@@ -2683,3 +2683,14 @@ end
 using .Issue49382
 (::Type{Issue49382.Type49382})() = 1
 @test sprint(show, methods(Issue49382.Type49382)) isa String
+
+# Showing of bad SlotNumber in Expr(:toplevel)
+let lowered = Meta.lower(Main, Expr(:let, Expr(:block), Expr(:block, Expr(:toplevel, :(x = 1)), :(y = 1))))
+    ci = lowered.args[1]
+    @assert isa(ci, Core.CodeInfo)
+    @test !isempty(ci.slotnames)
+    @assert ci.code[1].head === :toplevel
+    ci.code[1].args[1] = :($(Core.SlotNumber(1)) = 1)
+    # Check that this gets printed as `_1 = 1` not `y = 1`
+    @test contains(sprint(show, ci), "_1 = 1")
+end