libexpr: add hidden pos origin for builtin code

currently only used to hide the nix code of derivation from stack traces
and debug frames, but perhaps we'll find it useful for other things too.

Change-Id: Ie5667873d8858d25dd4113bdf454e800b59082d7

lix/libexpr/eval.cc

@@ -268,7 +268,6 @@ EvalState::EvalState(
     std::shared_ptr<Store> buildStore)
     : s(symbols)
     , repair(NoRepair)
-    , derivationInternal(CanonPath("/builtin/derivation.nix"))
     , store(store)
     , buildStore(buildStore ? buildStore : store)
     , regexCache(makeRegexCache())
@@ -1331,13 +1330,10 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
         state.forceValue(*vCurrent, (posCurrent ? posCurrent : this->pos));
 
     } catch (Error & e) {
-        if (posCurrent) {
-            auto pos2r = state.positions[posCurrent];
-            auto origin = std::get_if<SourcePath>(&pos2r.origin);
-            if (!(origin && *origin == state.derivationInternal))
-                e.addTrace(state.positions[posCurrent], "while evaluating the attribute '%1%'",
-                    showAttrPath(state, env, attrPath));
-        }
+        auto pos2r = state.positions[posCurrent];
+        if (pos2r && !std::get_if<Pos::Hidden>(&pos2r.origin))
+            e.addTrace(pos2r, "while evaluating the attribute '%1%'",
+                showAttrPath(state, env, attrPath));
         throw;
     }
 

lix/libexpr/eval.hh

@@ -284,8 +284,6 @@ public:
      */
     std::optional<PathSet> allowedPaths;
 
-    const SourcePath derivationInternal;
-
     /**
      * Store used to materialise .drv files.
      */

lix/libexpr/primops.cc

@@ -2852,7 +2852,7 @@ void EvalState::createBaseEnv()
     char code[] =
         #include "primops/derivation.nix.gen.hh"
         ;
-    eval(*parse(code, sizeof(code), derivationInternal, {CanonPath::root}, staticBaseEnv), *vDerivation);
+    eval(*parse(code, sizeof(code), Pos::Hidden{}, {CanonPath::root}, staticBaseEnv), *vDerivation);
 }
 
 

lix/libutil/position.cc

@@ -68,6 +68,9 @@ std::optional<std::string> Pos::getSource() const
             } catch (Error &) {
                 return std::nullopt;
             }
+        },
+        [](Hidden) -> std::optional<std::string> {
+            return std::nullopt;
         }
     }, origin);
 }
@@ -79,7 +82,8 @@ void Pos::print(std::ostream & out, bool showOrigin) const
             [&](const std::monostate &) { out << "«none»"; },
             [&](const Pos::Stdin &) { out << "«stdin»"; },
             [&](const Pos::String & s) { out << "«string»"; },
-            [&](const SourcePath & path) { out << path; }
+            [&](const SourcePath & path) { out << path; },
+            [&](Hidden) { out << "«internal»"; },
         }, origin);
         out << ":";
     }

lix/libutil/position.hh

@@ -38,8 +38,11 @@ struct Pos
         bool operator<(const String & rhs) const
         { return *source < *rhs.source; }
     };
+    struct Hidden {
+        auto operator<=>(const Hidden &) const = default;
+    };
 
-    typedef std::variant<std::monostate, Stdin, String, SourcePath> Origin;
+    typedef std::variant<std::monostate, Stdin, String, SourcePath, Hidden> Origin;
 
     Origin origin = std::monostate();