RFC: Introduce TypedCallable

TypedCallable provides a wrapper for callable objects, with the following benefits:
    1. Enforced type-stability (for concrete AT/RT types)
    2. Fast calling convention (frequently < 10 ns / call)
    3. Normal Julia dispatch semantics (sees new Methods, etc.) + invoke_latest
    4. Pre-compilation support (including `--trim` compatibility)

It can be used like this:
```julia
callbacks = @TypedCallable{(::Int,::Int)->Bool}[]

register_callback!(callbacks, f::F) where {F<:Function} =
    push!(callbacks, @TypedCallable f(::Int,::Int)::Bool)

register_callback!(callbacks, (x,y)->(x == y))
register_callback!(callbacks, (x,y)->(x != y))

@Btime callbacks[rand(1:2)](1,1)
```

This is very similar to the existing `FunctionWrappers.jl`, but there
are a few key differences:
  - Better type support: TypedCallable supports the full range of Julia
    types (incl. Varargs), and it has access to all of Julia's "internal"
    calling conventions so calls are fast (and allocation-free) for a
    wider range of input types
  - Improved dispatch handling: The `@cfunction` functionality used by
    FunctionWrappers has several dispatch bugs, which cause wrappers to
    occasionally not see new Methods. These bugs are fixed (or soon to
    be fixed) for TypedCallable.
  - Pre-compilation support including for `juliac` / `--trim` (#55047)

Many of the improvements here are actually thanks to the `OpaqueClosure`
introduced by @Keno - This type just builds on top of OpaqueClosure to
provide an interface with Julia's usual dispatch semantics.

base/opaque_closure.jl

@@ -110,3 +110,196 @@ function generate_opaque_closure(@nospecialize(sig), @nospecialize(rt_lb), @nosp
     return ccall(:jl_new_opaque_closure_from_code_info, Any, (Any, Any, Any, Any, Any, Cint, Any, Cint, Cint, Any, Cint, Cint),
         sig, rt_lb, rt_ub, mod, src, lineno, file, nargs, isva, env, do_compile, isinferred)
 end
+
+struct Slot{T} end
+struct Splat{T}
+    value::T
+end
+
+# Args is a Tuple{Vararg{Union{Slot{T},Some{T}}}} where Slot{T} represents
+# an uncurried argument slot, and Some{T} represents an argument to curry.
+@noinline @generated function Core.OpaqueClosure(Args::Tuple, ::Slot{RT}) where RT
+    AT = Any[]
+    call = Expr(:call)
+    extracted = Expr[]
+    closure_args = Expr(:tuple)
+    for (i, T) in enumerate(Args.parameters)
+        v = Symbol("arg", i)
+        is_splat = T <: Splat
+        if is_splat # TODO: check position
+            push!(call.args, :($v...))
+            T = T.parameters[1]
+        else
+            push!(call.args, v)
+        end
+        if T <: Some
+            push!(extracted, :($v = something(Args[$i])))
+        elseif T <: Slot
+            SlotT = T.parameters[1]
+            push!(AT, is_splat ? Vararg{SlotT} : SlotT)
+            push!(closure_args.args, call.args[end])
+        else @assert false end
+    end
+    AT = Tuple{AT...}
+    return Base.remove_linenums!(quote
+        $(extracted...)
+        @opaque allow_partial=false $AT->$RT $(closure_args)->@inline $(call)
+    end)
+end
+
+"""
+    TypedCallable{AT,RT}
+
+TypedCallable provides a wrapper for callable objects, with the following benefits:
+    1. Enforced type-stability (for concrete AT/RT types)
+    2. Fast calling convention (frequently < 10 ns / call)
+    3. Normal Julia dispatch semantics (sees new Methods, etc.) + invoke_latest
+
+This was specifically introduced to provide a form of Function-like that will be
+compatible with the newly-introduced `--trim` without requiring fully specializing
+on (singleton) `Function` types.
+
+This is similar to the existing FunctionWrappers.jl, with the advantages that
+TypedCallable supports the full range of Julia types/arguments, has a faster calling
+convention for a wider range of types, and has better pre-compilation support
+(including support for `--trim`).
+
+Examples:
+
+
+If `invoke_latest` is `false`, the provide TypedCallable has normal Julia semantics.
+Otherwise it always attempts to make the call in the current world.
+
+# Extended help
+
+### As an invalidation barrier
+
+TypedCallable can also be used as an "invalidation barrier", since the caller of a
+TypedCallable is not affected by any invalidations of its callee(s). This doesn't
+completely cure the original invalidation, but it stops it from propagating all the
+way through your code.
+
+This can be especially helpful, e.g., when calling back to user-provided functions
+whose invalidations you may have no control over.
+"""
+mutable struct TypedCallable{AT,RT}
+    @atomic oc::Base.RefValue{Core.OpaqueClosure{AT,RT}}
+    const task::Union{Task,Nothing}
+    const build_oc::Function
+end
+
+function Base.show(io::IO, tc::Base.Experimental.TypedCallable)
+    A, R = typeof(tc).parameters
+    Base.print(io, "@TypedCallable{")
+    Base.show_tuple_as_call(io, Symbol(""), A; hasfirst=false)
+    Base.print(io, "->◌::", R, "}()")
+end
+
+function rebuild_in_world!(@nospecialize(self::TypedCallable), world::UInt)
+    oc = Base.invoke_in_world(world, self.build_oc)
+    @atomic :release self.oc = Base.Ref(oc)
+    return oc
+end
+
+@inline function (self::TypedCallable{AT,RT})(args...) where {AT,RT}
+    invoke_world = if self.task === nothing
+        Base.get_world_counter() # Base.unsafe_load(cglobal(:jl_world_counter, UInt), :acquire) ?
+    elseif self.task === Base.current_task()
+        Base.tls_world_age()
+    else
+        error("TypedCallable{...} was called from a different task than it was created in.")
+    end
+    oc = (@atomic :acquire self.oc)[]
+    if oc.world != invoke_world
+        oc = @noinline rebuild_in_world!(self, invoke_world)::Core.OpaqueClosure{AT,RT}
+    end
+    return oc(args...)
+end
+
+function _TypedCallable_type(ex)
+    type_err = "Invalid @TypedCallable expression: $(ex)\nExpected \"@TypedCallable{(::T,::U,...)->RT}\""
+
+    # Unwrap {...}
+    (length(ex.args) != 1) && error(type_err)
+    ex = ex.args[1]
+
+    # Unwrap (...)->RT
+    !(Base.isexpr(ex, :->) && length(ex.args) == 2) && error(type_err)
+    tuple_, rt = ex.args
+    if !(Base.isexpr(tuple_, :tuple) && all((x)->Base.isexpr(x, :(::)), tuple_.args))
+        # note: (arg::T, ...) is specifically allowed (the "arg" part is unused)
+        error(type_err)
+    end
+    !Base.isexpr(rt, :block) && error(type_err)
+
+    # Remove any LineNumberNodes inserted by lowering
+    filter!((x)->!isa(x,Core.LineNumberNode), rt.args)
+    (length(rt.args) != 1) && error(type_err)
+
+    # Build args
+    AT = Expr[esc(last(x.args)) for x in tuple_.args]
+    RT = rt.args[1]
+
+    # Unwrap ◌::T to T
+    if Base.isexpr(RT, :(::)) && length(RT.args) == 2 && RT.args[1] == :◌
+        RT = RT.args[2]
+    end
+
+    return :($TypedCallable{Tuple{$(AT...)}, $(esc(RT))})
+end
+
+function _TypedCallable_closure(ex)
+    if Base.isexpr(ex, :call)
+        error("""
+              Invalid @TypedCallable expression: $(ex)
+              An explicit return type assert is required (e.g. "@TypedCallable f(...)::RT")
+              """)
+    end
+
+    call_, RT = ex.args
+    if !Base.isexpr(call_, :call)
+        error("""Invalid @TypedCallable expression: $(ex)
+                 The supported syntax is:
+                     @TypedCallable{(::T,::U,...)->RT} (to construct the type)
+                     @TypedCallable f(x,::T,...)::RT (to construct the TypedCallable)
+                 """)
+    end
+    oc_args = map(call_.args) do arg
+        is_splat = Base.isexpr(arg, :(...))
+        arg = is_splat ? arg.args[1] : arg
+        transformed = if Base.isexpr(arg, :(::))
+            if length(arg.args) == 1 # it's a "slot"
+                slot_ty = esc(only(arg.args))
+                :(Slot{$slot_ty}())
+            elseif length(arg.args) == 2
+                (arg, ty) = arg.args
+                :(Some{$(esc(ty))}($(esc(arg))))
+            else @assert false end
+        else
+            :(Some($(esc(arg))))
+        end
+        return is_splat ? Expr(:call, Splat, transformed) : transformed
+    end
+    # TODO: kwargs support
+    RT = :(Slot{$(esc(RT))}())
+    invoke_latest = true # expose as flag?
+    task = invoke_latest ? nothing : :(Base.current_task())
+    return quote
+        build_oc = ()->Core.OpaqueClosure(($(oc_args...),), $(RT))
+        $(TypedCallable)(Ref(build_oc()), $task, build_oc)
+    end
+end
+
+macro TypedCallable(ex)
+    if Base.isexpr(ex, :braces)
+        return _TypedCallable_type(ex)
+    elseif Base.isexpr(ex, :call) || (Base.isexpr(ex, :(::)) && length(ex.args) == 2)
+        return _TypedCallable_closure(ex)
+    else
+        error("""Invalid @TypedCallable expression: $(ex)
+                 The supported syntax is:
+                     @TypedCallable{(::T,::U,...)->RT} (to construct the type)
+                     @TypedCallable f(x,::T,...)::RT (to construct the TypedCallable)
+                 """)
+    end
+end