libstore: bring back some pointer equality nonsense

the change to shareable thunks also removed a few cases of pointer
equality checks that allowed structures containing functions to be
considered equal to other sets containing the same functions, even
if the sets themselves were pointer-equal themselves. *so* busted.

Change-Id: If87fdab658f9037ce2a654f69a9e3da6ae2f53e5

doc/manual/rl-next/ptr-eq.md

@@ -0,0 +1,48 @@
+---
+synopsis: "Function equality semantics are more consistent, but still bad"
+cls: [4556, 4244]
+issues: []
+category: "Breaking Changes"
+credits: [horrors]
+---
+
+Lix has inherited a historic misfeature from CppNix in the form of pointer
+equality checks built into the `==` operator. These checks were originally
+meant to optimize comparison for large sets, but they have the unfortunate
+side effect of producing unexpected results when sets containing functions
+are compared. **Lix 2.93 and earlier** behave as shown in the repl session
+
+```
+Lix 2.93.3
+Type :? for help.
+nix-repl> f = x: x
+Added f.
+
+nix-repl> f == f
+false
+
+nix-repl> let s.f = f; in s.f == s.f
+false
+
+nix-repl> # however!
+          { inherit f; } == { inherit f; }
+true
+
+nix-repl> [ f ] == [ f ]
+true
+
+nix-repl> # and, in another twist:
+          [ f ] == map f [ f ]
+false
+```
+
+Nixpkgs relies on sets containing functions being comparable, so we cannot
+simply deprecate this behavior. Due to changes to the object model used by
+Lix ***all* comparisons above now evaluate to `true`**. This is considered
+a breaking change because eval results may differ, but we also consider it
+minor because the optimization is unsound (c.f. `let l = [NaN]; in l == l`
+evaluates to `true` even though floating point `NaN` is incomparable). Lix
+intends to remove this optimization altogether in the future, but until we
+can do that we instead make it slightly less broken to allow other, *real*
+optimizations. Function equality comparison remains **undefined behavior**
+and should not be relied upon in Nixlang code that intends to be portable.

lix/libexpr/eval.cc

@@ -2680,9 +2680,9 @@ bool EvalState::eqValues(Value & v1, Value & v2, const PosIdx pos, std::string_v
             return true;
         }
 
-        /* Functions are incomparable. */
+        /* Functions are incomparable, except for identity (see note above about this nonsense). */
         case nFunction:
-            return false;
+            return pointerEq();
 
         case nExternal:
             if (pointerEq()) return true;

tests/functional2/lang/eq/eval-okay-pointer-equality.out.exp

@@ -0,0 +1 @@
+{ functionsDirect = [ true false false true ]; functionsSelected = [ true false false true ]; lists = { bigList1 = { bigList1 = true; bigList1' = false; bigList2 = true; bigList2' = false; bigList3 = true; bigList3' = false; list1 = false; list1' = false; list2 = false; list2' = false; }; bigList1' = { bigList1 = false; bigList1' = true; bigList2 = false; bigList2' = true; bigList3 = false; bigList3' = true; list1 = false; list1' = false; list2 = false; list2' = false; }; bigList2 = { bigList1 = true; bigList1' = false; bigList2 = true; bigList2' = false; bigList3 = true; bigList3' = false; list1 = false; list1' = false; list2 = false; list2' = false; }; bigList2' = { bigList1 = false; bigList1' = true; bigList2 = false; bigList2' = true; bigList3 = false; bigList3' = true; list1 = false; list1' = false; list2 = false; list2' = false; }; bigList3 = { bigList1 = true; bigList1' = false; bigList2 = true; bigList2' = false; bigList3 = true; bigList3' = false; list1 = false; list1' = false; list2 = false; list2' = false; }; bigList3' = { bigList1 = false; bigList1' = true; bigList2 = false; bigList2' = true; bigList3 = false; bigList3' = true; list1 = false; list1' = false; list2 = false; list2' = false; }; list1 = { bigList1 = false; bigList1' = false; bigList2 = false; bigList2' = false; bigList3 = false; bigList3' = false; list1 = true; list1' = false; list2 = true; list2' = false; }; list1' = { bigList1 = false; bigList1' = false; bigList2 = false; bigList2' = false; bigList3 = false; bigList3' = false; list1 = false; list1' = true; list2 = false; list2' = true; }; list2 = { bigList1 = false; bigList1' = false; bigList2 = false; bigList2' = false; bigList3 = false; bigList3' = false; list1 = true; list1' = false; list2 = true; list2' = false; }; list2' = { bigList1 = false; bigList1' = false; bigList2 = false; bigList2' = false; bigList3 = false; bigList3' = false; list1 = false; list1' = true; list2 = false; list2' = true; }; }; nans = { list = { list = true; plain = false; set = false; }; plain = { list = false; plain = false; set = false; }; set = { list = false; plain = false; set = true; }; }; sets = { bigSet1 = { bigSet1 = true; bigSet1' = false; bigSet2 = true; bigSet2' = false; bigSet3 = true; bigSet3' = false; set1 = false; set1' = false; set2 = false; set2' = false; }; bigSet1' = { bigSet1 = false; bigSet1' = true; bigSet2 = false; bigSet2' = true; bigSet3 = false; bigSet3' = true; set1 = false; set1' = false; set2 = false; set2' = false; }; bigSet2 = { bigSet1 = true; bigSet1' = false; bigSet2 = true; bigSet2' = false; bigSet3 = true; bigSet3' = false; set1 = false; set1' = false; set2 = false; set2' = false; }; bigSet2' = { bigSet1 = false; bigSet1' = true; bigSet2 = false; bigSet2' = true; bigSet3 = false; bigSet3' = true; set1 = false; set1' = false; set2 = false; set2' = false; }; bigSet3 = { bigSet1 = true; bigSet1' = false; bigSet2 = true; bigSet2' = false; bigSet3 = true; bigSet3' = false; set1 = false; set1' = false; set2 = false; set2' = false; }; bigSet3' = { bigSet1 = false; bigSet1' = true; bigSet2 = false; bigSet2' = true; bigSet3 = false; bigSet3' = true; set1 = false; set1' = false; set2 = false; set2' = false; }; set1 = { bigSet1 = false; bigSet1' = false; bigSet2 = false; bigSet2' = false; bigSet3 = false; bigSet3' = false; set1 = true; set1' = false; set2 = true; set2' = false; }; set1' = { bigSet1 = false; bigSet1' = false; bigSet2 = false; bigSet2' = false; bigSet3 = false; bigSet3' = false; set1 = false; set1' = true; set2 = false; set2' = true; }; set2 = { bigSet1 = false; bigSet1' = false; bigSet2 = false; bigSet2' = false; bigSet3 = false; bigSet3' = false; set1 = true; set1' = false; set2 = true; set2' = false; }; set2' = { bigSet1 = false; bigSet1' = false; bigSet2 = false; bigSet2' = false; bigSet3 = false; bigSet3' = false; set1 = false; set1' = true; set2 = false; set2' = true; }; }; }

tests/functional2/lang/eq/in-pointer-equality.nix

@@ -0,0 +1,96 @@
+let
+  f1 = x: x;
+  f2 = x: x;
+
+  inf = let f = n: if n == 0 then 1.0 else 10 * f (n - 1); in f 1000;
+  nan = inf * 0;
+
+  # all of these sets and lists are assumed to have different *addresses* as long as CSE does not occur
+  sets = rec {
+    set1 = {
+      f = f1;
+    };
+    set1' = {
+      f = f2;
+    };
+    set2 = {
+      f = f1;
+    };
+    set2' = {
+      f = f2;
+    };
+    bigSet1 = {
+      f = f1;
+      meow = true;
+    };
+    bigSet1' = {
+      f = f2;
+      meow = true;
+    };
+    bigSet2 = bigSet1 // {
+      meow = true;
+    };
+    bigSet2' = bigSet1' // {
+      meow = true;
+    };
+    bigSet3 = {
+      f = f1;
+      meow = true;
+    };
+    bigSet3' = {
+      f = f2;
+      meow = true;
+    };
+  };
+
+  lists = rec {
+    list1 = [ f1 ];
+    list1' = [ f2 ];
+    list2 = [ f1 ];
+    list2' = [ f2 ];
+    bigList1 = [ f1 true ];
+    bigList1' = [ f2 true ];
+    bigList2 = map (e: if builtins.isBool e then true else e) bigList1;
+    bigList2'= map (e: if builtins.isBool e then true else e) bigList1';
+    bigList3 = [ f1 true ];
+    bigList3' = [ f2 true ];
+  };
+
+  nans = rec {
+    plain = nan;
+    list = [ nan ];
+    set = { inherit nan; };
+  };
+
+  compareAllPairs = cases:
+    let
+      names = builtins.attrNames cases;
+    in
+      builtins.listToAttrs (map
+        (n1: {
+          name = n1;
+          value = builtins.listToAttrs (map
+            (n2: { name = n2; value = cases.${n1} == cases.${n2}; })
+            names);
+        })
+        names);
+
+  results = {
+    functionsDirect = [
+      (f1 == f1)
+      (f1 == f2)
+      (f2 == f1)
+      (f2 == f2)
+    ];
+    functionsSelected = [
+      (sets.set1.f == sets.set1.f)
+      (sets.set1.f == sets.set1'.f)
+      (sets.set1'.f == sets.set1.f)
+      (sets.set1'.f == sets.set1'.f)
+    ];
+    sets = compareAllPairs sets;
+    lists = compareAllPairs lists;
+    nans = compareAllPairs nans;
+  };
+in
+builtins.deepSeq results results