Add fast path for when all struct fields are set

src/compiler.jl

@@ -1970,19 +1970,31 @@ end
         return y
     end
 
-    flds = Vector{Any}(undef, nf)
-    for i in 1:nf
-        if isdefined(prevs, i)
+
+    y = if all(p -> isdefined(p, nf), prevs)
+        # fast path when all fields are set
+        splatnew(RT, ntuple(Val(nf)) do i
+            Base.@_inline_meta
             xis = ntuple(j -> getfield(prevs[j], i), N)
             T = Core.Typeof(first(xis))
-            yi = recursive_map(T, f, seen, xis, basetype)
-            flds[i] = yi
-        else
-            nf = i - 1 # rest of tail must be undefined values
-            break
+            recursive_map(T, f, seen, xis, basetype)
+        end)
+    else
+        flds = Vector{Any}(undef, nf)
+        nset = nf
+        for i in 1:nf
+            if all(p -> isdefined(p, i), prevs)
+                xis = ntuple(j -> getfield(prevs[j], i), N)
+                T = Core.Typeof(first(xis))
+                yi = recursive_map(T, f, seen, xis, basetype)
+                flds[i] = yi
+            else
+                nset = i - 1 # rest of tail must be undefined values
+                break
+            end
         end
+        ccall(:jl_new_structv, Any, (Any, Ptr{Any}, UInt32), RT, flds, nset)
     end
-    y = ccall(:jl_new_structv, Any, (Any, Ptr{Any}, UInt32), RT, flds, nf)
     seen[prevs] = y
     return y
 end