inference: form PartialStruct for extra type information propagation

This commit forms `PartialStruct` whenever there is any type-level
refinement available about a field, even if it's not "constant" information.

In Julia "definitions" are allowed to be abstract whereas "usages"
(i.e. callsites) are often concrete. The basic idea is to allow inference
to make more use of such precise callsite type information by encoding it
as `PartialStruct`.

This may increase optimization possibilities of "unidiomatic" Julia code,
which may contain poorly-typed definitions, like this very contrived example:
```julia
struct Problem
    n; s; c; t
end

function main(args...)
    prob = Problem(args...)
    s = 0
    for i in 1:prob.n
        m = mod(i, 3)
        s += m == 0 ? sin(prob.s) : m == 1 ? cos(prob.c) : tan(prob.t)
    end
    return prob, s
end

main(10000, 1, 2, 3)
```

One of the obvious limitation is that this extra type information can be
propagated inter-procedurally only as a const-propagation.
I'm not sure this kind of "just a type-level" refinement can often make
constant-prop' successful (i.e. shape-up a method body and allow it to
be inlined, encoding the extra type information into the generated code),
thus I didn't not modify any part of const-prop' heuristics.

So the improvements from this change is almost for local analysis,
and for very simple inter-procedural calls.

base/compiler/abstractinterpretation.jl

@@ -1542,22 +1542,26 @@ function abstract_eval_statement(interp::AbstractInterpreter, @nospecialize(e),
         if isconcretetype(t) && !ismutabletype(t)
             args = Vector{Any}(undef, length(e.args)-1)
             ats = Vector{Any}(undef, length(e.args)-1)
-            anyconst = false
-            allconst = true
+            local anyconst = anyrefine = false
+            local allconst = true
             for i = 2:length(e.args)
                 at = widenconditional(abstract_eval_value(interp, e.args[i], vtypes, sv))
                 if !anyconst
-                    anyconst = has_nontrivial_const_info(at)
+                    if has_nontrivial_const_info(at)
+                        anyconst = true
+                    elseif !anyrefine
+                        anyrefine = at ⋤ fieldtype(t, i - 1)
+                    end
                 end
                 ats[i-1] = at
                 if at === Bottom
                     t = Bottom
-                    allconst = anyconst = false
+                    anyconst = anyrefine = allconst = false
                     break
                 elseif at isa Const
                     if !(at.val isa fieldtype(t, i - 1))
                         t = Bottom
-                        allconst = anyconst = false
+                        anyconst = anyrefine = allconst = false
                         break
                     end
                     args[i-1] = at.val
@@ -1569,7 +1573,7 @@ function abstract_eval_statement(interp::AbstractInterpreter, @nospecialize(e),
             if t !== Bottom && fieldcount(t) == length(ats)
                 if allconst
                     t = Const(ccall(:jl_new_structv, Any, (Any, Ptr{Cvoid}, UInt32), t, args, length(args)))
-                elseif anyconst
+                elseif anyconst || anyrefine
                     t = PartialStruct(t, ats)
                 end
             end
@@ -1741,17 +1745,21 @@ function widenreturn(@nospecialize(rt), @nospecialize(bestguess), nslots::Int, s
     isa(rt, Type) && return rt
     if isa(rt, PartialStruct)
         fields = copy(rt.fields)
-        haveconst = false
+        anyconst = anyrefine = false
         for i in 1:length(fields)
             a = fields[i]
             a = isvarargtype(a) ? a : widenreturn(a, bestguess, nslots, slottypes, changes)
-            if !haveconst && has_const_info(a)
-                # TODO: consider adding && const_prop_profitable(a) here?
-                haveconst = true
+            if !anyconst
+                if has_const_info(a)
+                    # TODO: consider adding && const_prop_profitable(a) here?
+                    anyconst = true
+                elseif !anyrefine
+                    anyrefine = a ⋤ fieldtype(rt.typ, i)
+                end
             end
             fields[i] = a
         end
-        haveconst && return PartialStruct(rt.typ, fields)
+        (anyconst || anyrefine) && return PartialStruct(rt.typ, fields)
     end
     if isa(rt, PartialOpaque)
         return rt # XXX: this case was missed in #39512

base/compiler/typelattice.jl

@@ -239,6 +239,7 @@ function ⊑(@nospecialize(a), @nospecialize(b))
         return a === b
     end
 end
+⋤(@nospecialize(a), @nospecialize(b)) = !⊑(b, a)
 
 # Check if two lattice elements are partial order equivalent. This is basically
 # `a ⊑ b && b ⊑ a` but with extra performance optimizations.

test/compiler/inference.jl

@@ -3645,3 +3645,26 @@ end
 
 # issue #42646
 @test only(Base.return_types(getindex, (Array{undef}, Int))) >: Union{} # check that it does not throw
+
+# form PartialStruct for extra type information propagation
+struct FieldTypeRefinement{S,T}
+    s::S
+    t::T
+end
+@test Base.return_types((Int,)) do s
+    o = FieldTypeRefinement{Any,Int}(s, s)
+    o.s
+end |> only == Int
+@test Base.return_types((Int,)) do s
+    o = FieldTypeRefinement{Int,Any}(s, s)
+    o.t
+end |> only == Int
+@test Base.return_types((Int,)) do s
+    o = FieldTypeRefinement{Any,Any}(s, s)
+    o.s, o.t
+end |> only == Tuple{Int,Int}
+@test Base.return_types((Int,)) do a
+    s1 = Some{Any}(a)
+    s2 = Some{Any}(s1)
+    s2.value.value
+end |> only == Int