refactor(3p/nix/libexpr): Make nix::AttrName a std::variant r/1278

nix:AttrName was one of the few classes that relied on the default
constructor of nix::Symbol (which I am trying to remove in a separate
change).

The class essentially represents the name of an attribute in a set,
which is either just a string expression or a dynamically evaluated
expression (e.g. string interpolation).

Previously it would be constructed by only setting one of the fields
and defaulting the other, now it is an explicit std::variant.

Note that there are several code paths where not all eventualities are
handled and this code is bug-for-bug compatible with those, except
that unknown conditions (which should never work) are now throwing
instead of silently doing ... something.

The language tests pass with this change, and the depot derivations
that I tested with evaluated successfully.

Change-Id: Icf1ee60a5f8308f4ab18a82749e00cf37a938a8f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1138
Reviewed-by: edef <edef@edef.eu>
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

1 files changed, 12 insertions, 10 deletions

diff --git a/third_party/nix/src/libexpr/nixexpr.cc b/third_party/nix/src/libexpr/nixexpr.cc
index 28d8eee7a7f8..2de37f50e9b9 100644
--- a/third_party/nix/src/libexpr/nixexpr.cc
+++ b/third_party/nix/src/libexpr/nixexpr.cc
@@ -1,9 +1,11 @@
 #include "libexpr/nixexpr.hh"
 
 #include <cstdlib>
+#include <variant>
 
 #include "libstore/derivations.hh"
 #include "libutil/util.hh"
+#include "libutil/visitor.hh"
 
 namespace nix {
 
@@ -198,17 +200,17 @@ std::ostream& operator<<(std::ostream& str, const Pos& pos) {
 std::string showAttrPath(const AttrPath& attrPath) {
   std::ostringstream out;
   bool first = true;
-  for (auto& i : attrPath) {
+  for (auto& attr : attrPath) {
     if (!first) {
       out << '.';
     } else {
       first = false;
     }
-    if (i.symbol.set()) {
-      out << i.symbol;
-    } else {
-      out << "\"${" << *i.expr << "}\"";
-    }
+
+    std::visit(util::overloaded{
+                   [&](const Symbol& sym) { out << sym; },
+                   [&](const Expr* expr) { out << "\"${" << *expr << "}\""; }},
+               attr);
   }
   return out.str();
 }
@@ -268,8 +270,8 @@ void ExprSelect::bindVars(const StaticEnv& env) {
     def->bindVars(env);
   }
   for (auto& i : attrPath) {
-    if (!i.symbol.set()) {
-      i.expr->bindVars(env);
+    if (auto* expr = std::get_if<Expr*>(&i)) {
+      (*expr)->bindVars(env);
     }
   }
 }
@@ -277,8 +279,8 @@ void ExprSelect::bindVars(const StaticEnv& env) {
 void ExprOpHasAttr::bindVars(const StaticEnv& env) {
   e->bindVars(env);
   for (auto& i : attrPath) {
-    if (!i.symbol.set()) {
-      i.expr->bindVars(env);
+    if (auto* expr = std::get_if<Expr*>(&i)) {
+      (*expr)->bindVars(env);
     }
   }
 }