Add bodyfunction(m::Method) (#36385)

This finds the function that implements the "body" of keyword-method `m`,
as generated by lowering.

This addresses the "discovery" part of #30908.
The renaming is done by LoweredCodeUtils.

base/reflection.jl

@@ -1280,6 +1280,51 @@ function hasmethod(@nospecialize(f), @nospecialize(t), kwnames::Tuple{Vararg{Sym
  return issubset(kwnames, kws)
 end
 
+"""
+ fbody = bodyfunction(basemethod::Method)
+
+Find the keyword "body function" (the function that contains the body of the method
+as written, called after all missing keyword-arguments have been assigned default values).
+`basemethod` is the method you obtain via [`which`](@ref) or [`methods`](@ref).
+"""
+function bodyfunction(basemethod::Method)
+ function getsym(arg)
+ isa(arg, Symbol) && return arg
+ isa(arg, GlobalRef) && return arg.name
+ return nothing
+ end
+
+ fmod = basemethod.module
+ # The lowered code for `basemethod` should look like
+ # %1 = mkw(kwvalues..., #self#, args...)
+ # return %1
+ # where `mkw` is the name of the "active" keyword body-function.
+ ast = Base.uncompressed_ast(basemethod)
+ f = nothing
+ if isa(ast, Core.CodeInfo) && length(ast.code) >= 2
+ callexpr = ast.code[end-1]
+ if isa(callexpr, Expr) && callexpr.head == :call
+ fsym = callexpr.args[1]
+ if isa(fsym, Symbol)
+ f = getfield(fmod, fsym)
+ elseif isa(fsym, GlobalRef)
+ newsym = nothing
+ if fsym.mod === Core && fsym.name === :_apply
+ newsym = getsym(callexpr.args[2])
+ elseif fsym.mod === Core && fsym.name === :_apply_iterate
+ newsym = getsym(callexpr.args[3])
+ end
+ if isa(newsym, Symbol)
+ f = getfield(basemethod.module, newsym)::Function
+ else
+ f = getfield(fsym.mod, fsym.name)::Function
+ end
+ end
+ end
+ end
+ return f
+end
+
 """
  Base.isambiguous(m1, m2; ambiguous_bottom=false) -> Bool
 

test/reflection.jl

@@ -909,3 +909,19 @@ end
 
  @test length(methods(g, ())) == 1
 end
+
+module BodyFunctionLookup
+f1(x, y; a=1) = error("oops")
+f2(f::Function, args...; kwargs...) = f1(args...; kwargs...)
+end
+
+@testset "bodyfunction" begin
+ m = first(methods(BodyFunctionLookup.f1))
+ f = Base.bodyfunction(m)
+ @test occursin("f1#", String(nameof(f)))
+ m = first(methods(BodyFunctionLookup.f2))
+ f = Base.bodyfunction(m)
+ @test f !== Core._apply_iterate
+ @test f !== Core._apply
+ @test occursin("f2#", String(nameof(f)))
+end