Implement single source of truth for structure

src/make_zero.jl

@@ -21,8 +21,10 @@ element of `xs` such that `yi == x1i`. If `copy_if_inactive == false`, this is d
 sharing, `yi = x1i`; if `copy_if_inactive == true`, it is done by copying,
 `yi = deepcopy(x1i)`.
 
-The first element of `xs` is also used to keep track of values that reference the same
-memory. This structure is reproduced in the return value `y`.
+Each element in `xs` is assumed to have the same structure as `x1 = first(xs)`, including
+which fields, if any, reference the same memory or are undefined. This structure will be
+mirrored in the return value `y`. If this assumption does not hold, errors or incorrect
+results may occur.
 
 A function `isleaftype` can be provided to customize which types are considered leafs:
 values of type `T` such that `isleaftype(T) == true` are not recursed into, but instead
@@ -67,10 +69,11 @@ end
     end
 
     nf = fieldcount(RT)
+    x1 = first(xs)
     if ismutabletype(RT)
         y = ccall(:jl_new_struct_uninit, Any, (Any,), RT)
         for i in 1:nf
-            if all(x -> isdefined(x, i), xs)
+            if isdefined(x1, i)
                 yi = newyi(i)
                 if Base.isconst(RT, i)
                     ccall(:jl_set_nth_field, Cvoid, (Any, Csize_t, Any), y, i - 1, yi)
@@ -82,15 +85,15 @@ end
         return y
     elseif nf == 0
         return f(xs...)::RT
-    elseif all(x -> isdefined(x, nf), xs)
+    elseif isdefined(x1, nf)
         # fast path when all fields are set
         return splatnew(RT, ntuple(newyi, Val(nf)))
     else
         flds = Vector{Any}(undef, nf)
         nset = nf
         for i in 1:nf
-            if all(x -> isdefined(x, i), xs)
-                flds[i] = newyi(i)
+            if isdefined(x1, i)
+                @inbounds flds[i] = newyi(i)
             else
                 nset = i - 1  # rest of tail must be undefined values
                 break
@@ -104,11 +107,12 @@ end
     ::Type{RT}, f::F, seen::IdDict, xs::NTuple{N,RT}, args...
 ) where {RT<:Array,F,N}
     y = RT(undef, size(first(xs)))
-    for I in eachindex(xs...)
-        if all(x -> isassigned(x, I), xs)
+    x1 = first(xs)
+    for I in eachindex(y, xs...)
+        @inbounds if isassigned(x1, I)
             xIs = ntuple(j -> xs[j][I], N)
             ST = Core.Typeof(first(xIs))
-            @inbounds y[I] = recursive_map(ST, f, seen, xIs, args...)
+            y[I] = recursive_map(ST, f, seen, xIs, args...)
         end
     end
     return y
@@ -125,6 +129,10 @@ equal `f(x1i, x2i, ..., xNi)`, where `x1i, x2i, ..., xNi` are the corresponding
 in the `xs`. Each subtree in `y` that can be proven by type to only contain
 non-differentiable values is left unchanged.
 
+Each element in `xs` is assumed to have the same structure as `y`, including which fields,
+if any, reference the same memory or are undefined. If this assumption does not hold, errors
+or incorrect results may occur.
+
 If every differentiable value in `y` is contained in a mutable object (i.e., `y` has
 inferred activity state Duplicated), this function performs a fully in-place update and
 returns `y`. If every differentiable value is held in immutable storage (i.e., `y`
@@ -187,7 +195,7 @@ end
         return nothing
     end
     for i = 1:nf
-        if isdefined(y, i) && all(x -> isdefined(x, i), xs)
+        if isdefined(y, i)
             yi = getfield(y, i)
             xis = ntuple(j -> getfield(xs[j], i), N)
             newyi = recursive_map!!(f, yi, seen, xis, isleaftype)
@@ -207,12 +215,12 @@ end
     f::F, y::Array{T,M}, seen, xs::NTuple{N,Array{T,M}}, isleaftype
 ) where {F,T,M,N}
     for I in eachindex(y, xs...)
-        if isassigned(y, I) && all(x -> isassigned(x, I), xs)
+        @inbounds if isassigned(y, I)
             yvalue = y[I]
             xvalues = ntuple(j -> xs[j][I], N)
             newyvalue = recursive_map!!(f, yvalue, seen, xvalues, isleaftype)
             if newyvalue !== yvalue
-                @inbounds y[I] = newyvalue
+                y[I] = newyvalue
             end
         end
     end
@@ -235,15 +243,15 @@ end
     nf = fieldcount(T)
     if nf == 0
         return f(xs...)::T
-    elseif isdefined(y, nf) && all(x -> isdefined(x, nf), xs)
+    elseif isdefined(y, nf)
         # fast path when all fields are set
         return splatnew(T, ntuple(newyi, Val(nf)))
     else
         flds = Vector{Any}(undef, nf)
         nset = nf
         for i = 1:nf
-            if isdefined(y, i) && all(x -> isdefined(x, i), xs)
-                flds[i] = newyi(i)
+            if isdefined(y, i)
+                @inbounds flds[i] = newyi(i)
             else
                 nset = i - 1  # rest of tail must be undefined values
                 break