Re-enable tab completion of kwargs for large method tables (#58012)

while testing to ensure that ~~absurdly large method tables~~ tab
completing over an abstract function call doesn't tank the performance
of the REPL

Fixes #57836

(cherry picked from commit 6f12957)

Author: mbauman

stdlib/REPL/src/REPLCompletions.jl

@@ -883,7 +883,13 @@ function complete_keyword_argument!(suggestions::Vector{Completion},
     kwargs_flag == 2 && false # one of the previous kwargs is invalid
 
     methods = Completion[]
-    complete_methods!(methods, funct, Any[Vararg{Any}], kwargs_ex, shift ? -1 : MAX_METHOD_COMPLETIONS, kwargs_flag == 1)
+    # Limit kwarg completions to cases when function is concretely known; looking up
+    # matching methods for abstract functions — particularly `Any` or `Function` — can
+    # take many seconds to run over the thousands of possible methods. Note that
+    # isabstracttype would return naively return true for common constructor calls
+    # like Array, but the REPL's introspection here may know their Type{T}.
+    isconcretetype(funct) || return false
+    complete_methods!(methods, funct, Any[Vararg{Any}], kwargs_ex, -1, arg_pos == :kwargs)
     # TODO: use args_ex instead of Any[Vararg{Any}] and only provide kwarg completion for
     # method calls compatible with the current arguments.
 

stdlib/REPL/test/replcompletions.jl

@@ -2632,6 +2632,35 @@ const issue57780_orig = copy(issue57780)
 test_complete_context("empty!(issue57780).", Main)
 @test issue57780 == issue57780_orig
 
+function g54131 end
+for i in 1:498
+    @eval g54131(::Val{$i}) = i
+end
+g54131(::Val{499}; kwarg=true) = 499*kwarg
+struct F54131; end
+Base.getproperty(::F54131, ::Symbol) = Any[cos, sin, g54131][rand(1:3)]
+f54131 = F54131()
+@testset "performance of kwarg completion with large method tables" begin
+    # The goal here is to simply ensure we aren't hitting catestrophically bad
+    # behaviors when shift isn't pressed. The difference between good and bad
+    # is on the order of tens of milliseconds vs tens of seconds; using 1 sec as
+    # a very rough canary that is hopefully robust even in the noisy CI coalmines
+    s = "g54131(kwa"
+    a, b, c = completions(s, lastindex(s), @__MODULE__, #= shift =# false)
+    @test REPLCompletions.KeywordArgumentCompletion("kwarg") in a
+    @test (@elapsed completions(s, lastindex(s), @__MODULE__, false)) < 1
+
+    s = "f54131.x("
+    a, b, c = completions(s, lastindex(s), @__MODULE__, false)
+    @test only(a) isa REPLCompletions.TextCompletion
+    @test (@elapsed completions(s, lastindex(s), @__MODULE__, false)) < 1
+
+    s = "f54131.x(kwa"
+    a, b, c = completions(s, lastindex(s), @__MODULE__, false)
+    @test_broken REPLCompletions.KeywordArgumentCompletion("kwarg") in a
+    @test (@elapsed completions(s, lastindex(s), @__MODULE__, false)) < 1
+end
+
 # Completion inside string interpolation
 let s = "\"example: \$varflo"
     c, r = test_complete_foo(s)